ANATOMY, VEGETABLEVEGETABLE ANATOMY.

All the plants which collectively form the vegetable kingdom have been arranged under two great divisions. Those which possess visible organs of reproduction, as stamens and pistils, have been termed phoenogamous, and constitute the first 23 classes in the sexual system of Linnaeus; while those in which the reproductive organs are either obscure or have not yet been discovered, have been termed cryptogamous, and form the 24th class of that system. Humboldt estimates the total number of known species at 44,000; of which 38,000 belong to the former division, and 6000 to the latter. Μ. Brogniart, jun. supposes the present known Flora to embrace 50,350 species, of which 10,200 may be considered as cryptogamous, and 40,150 species as belonging to the phaenogamous division. On the present occasion we propose to describe the structure of those plants only which are included in the division of phaenogamous plants.

Of the vast number of plants which cover the surface of our globe, it belongs to the botanist to describe the external forms in such manner as may serve to discriminate species, and assign to each its place in a methodical system of arrangement: it is the province of the anatomist to demonstrate, by dissection, their internal structure, so as to prepare the way for a rational explanation of their functions. In pursuing this object we may examine successively each of the parts as they severally present themselves to view; or we may, in the first place, study the elementary organs common to all plants, and then consider how their combinations form the different parts of vegetables. The former method is the analytic, and was necessarily that of the first observers; but, now that all the different parts are recognised as being formed out of the same elementary organs, we gain, both in conciseness and clearness, by adopting the latter or synthetic mode.

We shall accordingly describe, first, the elementary organs, and the primary textures which they form; and proceed afterwards to the consideration of the individual members and more complex organs of the plant. These elementary organs have been denominated vessels and cells; and they form, either singly or conjointly, what are called the vascular system and cellular tissue of plants. Their combination gives rise to certain textures, which appear in the well-known forms of skin, of bark, and of wood. On these we shall bestow the appellation of common textures, and exhibit a general view of their structure and disposition in the several parts of the vegetable body. A brief description of some minuter parts, as of hairs and glands, will terminate this division of the subject.

In the second part we shall begin with a description of the general structure of seeds, and afterwards treat more particularly of those bodies under the two great divisions of monocotyledonous and dicoyledonous seeds, tracing also the changes of form and of structure which they exhibit in their evolution and progress to the state of the mature plant. The structure of the mature plant itself will next claim our attention; and we shall accordingly exhibit the anatomy of its several members, as of the trunk, the branch, and the root, in their more remarkable varieties and forms. After this, the structure of the organs that spring from these several members, as buds and bulbs, leaves, flowers, and fruits, will be separately and distinctly examined; and having thus followed the progressive changes of form and of structure exhibited in the-several stages of vegetable existence, we shall terminate our descriptions by anatomical representations of the organs in which the seed was produced, and the series of appearances successively displayed in its formation.

Through the whole of the descriptive detail we shall adhere as closely as we can to the language of demonstration; supporting and illustrating our representations of structure by continual reference to figures, selected in great part from authors of repute, and in some instances from dissections made by ourselves. We are aware, that of many reputed anatomical facts very different representations have been given, all equally professing to rest on microscopical observation. In such circumstances, we can do no more than report concisely the statements of different observers; but shall dwell chiefly on those descriptions and representations which seem best entitled to credit, and appear most conformable to the analogies of other organized structures.

When we consider the immense number of species that compose the vegetable kingdom, and call to mind that in form, in size, and in structure, each species differs from every other through every period of its existence, it must appear altogether impracticable to describe and delineate any considerable number. Fortunately, however, these diversities arise not from differences in the elementary organs, but chiefly from their varied proportion, disposition, and texture. In numerous species the disposition of the internal organs is very similar, where the external form and texture widely differ. In other instances the arrangement and composition of the internal parts vary not less than that of the external figure. Of these varieties we shall exhibit different examples.

In describing individual parts or organs, we might have brought many concurring examples, and exhibited many similar representations, to confirm the views of structure under consideration; but, in general, we have dwelt only on one or two examples, and these we have selected from plants which are either important in themselves, or whose structure has been most satisfactorily displayed, or which seemed to afford the best illustration of the peculiarities we were engaged in describing. From one example clearly given, the reader will readily apprehend the nature of analogous structures, and escape the perplexity and fatigue which unnecessary repetitions might occasion.

Instead, also, of describing the vegetable at one or two stationary points of its existence, in some of which its size is so minute as to be scarcely capable of demonstration, we have followed it through the several stages of its growth. In this way we really study it as a living body, continually exerting its vegetative powers, and daily exhibiting the most striking variations in external form, and frequently in internal structure. We hope thus to have conferred an interest on the descriptive part, which may in some measure relieve the unavoidable dryness of anatomical details; to have exhibited, in some instances, clearer views of vegetable organization; and to have given a continuity to the subject which isolated dissections, at a few stated periods, could not alone have bestowed.

It remains only to add a few remarks on the nomenclature employed in the present article.

In the description of external parts we have adhered chiefly to the Linnaean nomenclature; but some of the terms employed by Linnaeus, in relation to anatomical structure, are exceedingly vague and inappropriate; others [7:3:42] are manifestly erroneous, and, however well suited to the purposes of botany, are not at all to be tolerated in any thing that aspires to correct anatomical description.

In the anatomy of seeds we have adopted many of the terms employed by Gaertner, in his excellent work De Fructibus et Seminibus Plantarum, most of which had previously been used by Malpighi and Grew.

Thus, in every instance, we have exercised our own judgment in the selection of terms, and, where it seemed necessary, have subjoined the synonyms of different writers. Though we presume not to say that we have uniformly chosen the best, we trust they will always clearly express the idea we designed to convey; and that, in general, they have been used in one and the same sense, and in no other. Except in one or two trivial instances, we have not ventured to introduce new terms, but have studiously sought to avoid it, retaining even an inappropriate expression, sanctioned by use, if it did not, at the same time, lead to ambiguity, or convey an idea evidently false; and we have in general resisted that torrent of new and barbarous terms, founded often on fancied refinements and pretended discoveries, with which several continental writers have of late attempted to deluge this branch of science.

PART I.

OF THE ANATOMY IN GENERAL OF THE ELEMENTARY ORGANS AND COMMON TEXTURES OF VEGETABLES.

CHAP. I.

OF THE ELEMENTARY ORGANS.

Before we proceed to describe the structure of the individual parts of vegetables, it may be useful to exhibit a general view of the elementary organs of which they seem to be composed. Such a view will prepare the reader for understanding more clearly the descriptive language hereafter to be employed, and will even much abridge the extent to which that description must otherwise be carried.

Every one is familiar with the natural division of plants into herbs and trees, and is aware that, how different soever they may appear in form and texture, they all possess, in common, certain parts or members which we name the root, the trunk, and the branches, from which proceed the leaves, the flowers, the fruits, and seeds. Infinitely varied as these several parts are, in figure, size, and texture, they all originate from a few constituent or elementary organs, whose situation, proportions, and combination, give rise to all the diversity that we see. “Upon the anatomical analysis of all the parts of a plant,” says Grew, “I have certainly found, that in all plants there are two, and only two, organical parts essentially distinct, viz. the pithy part and the ligneous part.” (Anatomy of Plants, p. 19.) “And as every part hath two, so the whole vegetable, taken together,” he adds, “is a composition of two only, and no more. All properly woody parts, strings, and fibres, are one body; all simple barks, piths, parenchymas, and pulps, and, as to their substantial nature, peels and skins, are all likewise but one body; the several parts of a vegetable differing from each other only by the various proportions and mixtures, and variated pores and structure, of these two bodies.” (Ibid. p. 47.) In the anatomical descriptions of Malpighi, the Compound structure is resolved, in like manner, into two constituent parts, called by him the ligneous and utricular portions. To these parts may be assigned the general appellations of the Vascular System and Cellular Tissue of plants, the description of which shall form the first subjects of consideration.

Section I.

Of the Vascular System. Art. I.— General Characters of the Vessels.

By the vascular system may be understood, in a general sense, all those parts of a plant which .do not exhibit the form either of membrane or of cells. It constitutes almost the entire bulk of the more solid parts of trees, and by Grew and Malpighi was denominated the ligneous body, in contradistinction to the cellular tissue which accompanies it, and which forms by far the largest portion of many herbaceous plants. To common observation, a piece of dry wood appears to be a mass of solid fibres, that is, a series of particles arranged in a filiform figure, and destitute of any continuous canal. Thus Tournefort and others considered the ligneous parts of a plant to be a mass of minute solid filaments, placed parallel to each other, like the threads in a skein of silk, between the interstices of which the sap ascended; but the anatomical researches of Grew conducted him to a different conclusion. “If it be asked,” says he, “what all that part of a plant, whether herb or tree, which is properly called the woody part—what all that is? I suppose that it is nothing else but a cluster of innumerable and most extraordinary small vessels or concave fibres.” (Anat. of Plants, p. 20.) Malpighi held similar opinions concerning the vascularity of plants, which was farther attested by the microscopical observations of Hooke and Lecuwenhoeck. Du Hamel, though he admits that, under maceration, the parts of plants seem capable of indefinite subdivision, yet, from many circumstances, avows his conviction of their vascularity; and Hedwig maintains that the oldest and most compact plant is but a congeries of vessels and cells, which have nothing of the character of a fibrous solid, except in the thin membranous coats by which they are formed. (De Fibroe Vegetab. Ortu, p.17.)

Few circumstances have contributed more to perplex and retard our knowledge of the structure of plants, than the vague and erroneous nomenclature that has been employed to designate their constituent organs, more particularly in relation to the vascular system. Thus the several terms filaments, fibres, strings, threads, and nerves, which, in their ordinary acceptation, are understood to express a solid substance, have been constantly made use of in describing the tubes or vessels of plants. The same organs, however, even by the same writers, are frequently called tubular bodies, ligneous tubes, concave fibres, ducts, canals, arteries, veins, and vessels. In our future descriptions we shall employ the term vessel in a generic sense, to express all the diversity of names just enumerated; and the different kinds or species of vessels we shall hereafter attempt to discriminate by appropriate appellations.

Vessels, as we have said, exist in almost every part of a plant. In the higher orders of animals the fluids contained in the vessels are conveyed to a central reservoir called the heart, from which they are again sent out to all parts of the body. Near to this reservoir the ves- [7:3:43]'sels are few in number and large in size; and they gradually lessen in size and increase in number as they recede from it. In plants there is no such reservoir, but the fluids which enter by innumerable mouths at the root are at once distributed equally through all parts of the vegetable that are fitted to receive them. Hence in plants there is little variation in the diameter of the vessels; and their general figure is therefore cylindrical.

From the extreme minuteness of the vessels, it is scarcely possible to compute their number with accuracy. By driving off their fluids without destroying their figure, as is done in the preparation of charcoal, Hooke numbered in a line, ∙⅛th part of an inch long, not fewer than 150 vessels; therefore, in a line an inch long, there must be 2700, and in a surface of a square inch, 7,290,000 vessels; “which would seem incredible,” says he, “were not every one left to believe his own eyes.” These facts he verified by other observations on decayed wood, in which the vessels were empty; and also on petrifactions of ligneous bodies, in which the places of the vessels were very conspicuous. In very close and dense wood, as that of guaiacum, the vessels were still more minute than in the examples just quoted. (JWicrographia, p. 101, 108.) In a piece of oak of the size of about ⅛th part of a square inch, Lecuwenhoeck reckoned 20,000 vessels; so that in an oak-tree of no more than one foot in circumference, or about four inches in diameter, there will be found, according to his computation, 200,000,000 of such vessels. (Select Works, translated by Hoole, vol. i. p. 3.) The largest vessel observed by Hedwig (De Vegetab. Ortu, p. 26) in the stem of the gourd appeared through his microscope about Ath of an inch in diameter; and as his instrument magnified 290 times, the true diameter must be reckoned the 3480th part of an inch, which would give for the square inch 12,110,400 vessels. In certain plants, however, the vessels are large enough to be discerned by the naked eye, and in some cases they acquire a large size.

The vessels of plants do not, like those of animais, exist single, but are collected into fasciculi or bundles, which, however, have often the appearance of single vessels. In the stems of herbs, and in roots, Grew discovered each small fasciculus to be composed of from 30 to 100, or sometimes many hundred vessels. (Anatomy of Plants, p. 20.) The direction of these fasciculi in the trunk is generally perpendicular, but in other parts their course is often oblique, and in their smaller ramifications they produce all sorts of figures. In herbs the fasciculi are more or less numerous, and placed often at considerable distances from each other, exhibiting the appearance of small columns dispersed through the cellular tissue: in other instances they are much more numerous, but destitute of any symmetrical arrangement; while, in trees, they are disposed regularly around the axis, presenting in their transverse section the well-known appearance of concentric circles in the wood.

In some parts, where the fasciculi stand at a distance from each other, some vessels often quit one parcel to unite with another, and return afterwards to that which they had previously left. In this manner they are said to ramify, and frequently, by their ramification, a reticulated appearance is produced, as occurs especially in the bark and leaves. In the wood of the trunk, where they stand collaterally in a perpendicular direction, they very seldom, if ever, run into one another, but keep, says Grew, like so many several vessels, all along distinct; as, by cutting, and so following any one fasciculus, may be observed. (Anatomy of Plants, p. 20.) In branches and roots, though the direction of the fasciculi be changed, they seem only to break into smaller parcels, and run side by side, never inosculating with each other, nor being ramified, so as to be successively propagated one from another, as in the vessels of animals. Neither, adds he, are they wound together like threads in a rope, but are only contiguous or simply tangent, like the several cords in the braces of a drum. (Anatomy of Plants, p. 66.) Even in the leaf, where the vessels seem to ramify out of greater into less, as in the arteries of animals, yet, if the skin and pulp of the leaf be removed, and the vessels laid bare, it will appear that they are all of the same size everywhere in the leaf, and all continued through it, as distinct tubes, like the several threads in a skein of silk. The distribution of the vessels is not effected, therefore, by their ramifying out of greater into less, but by the division of a greater fasciculus into several smaller fasciculi, till at last they come to be single. (Anatomy of Plants, p. 155.)

It may be doubted whether, when the vessels of different fasciculi come into contact, they ever actually unite and are lost in each other, forming that kind of connection which is called inosculation or anastomosis. Grew strenuously contends against any such connection of the vessels in any part of the plant. “On a superficial view, indeed, the vessels of the leaf,” says he, “seem to be inosculated, not only side to side, but the ends of some into the sides of others: but neither is this ever really done, the lesser fasciculi being only so far diducted as to stand at right angles with the greater; but they are never inosculated, except end to end, or mouth to mouth, after they are come at last to their final distribution.” (Anatomy of Plants, p. 155.) Malpighi, however, from the fact of the alternate separation and conjunction of the vessels, and from analogy with what occurs in the animal system, speaks always of the anastomosis of the vessels, but he nowhere gives us any thing like proof of the fact; and Du Hamel, from actual dissection of several fasciculi, regards them in their union as resembling more the nerves than the blood-vessels of animals. When, indeed, we consider the extraordinary minuteness of the vessels, and the circumstance of their possessing nearly the same size in every part, there is no room for that continual ramification out of one into another, and consequent diminution of diameter, that occur in the vessels of animals; and the immensity of their number, together with the endless separations and re-unions which their fasciculi make, seems calculated to fulfil the purposes of less general distribution in the plant, which successive division and perpetual anastomosis effect in the animal system.

Another general circumstance in the vessels of plants is, that we do not discover in them any structure which has the true nature and use of valves, similar to what is met with in the veins and absorbent vessels of animals. Dr Hooke could never observe in their canal any thing that had the appearance of valves. (AIicrographia, p. 116.) Did such a structure exist, the absorption of nutrient matter from the lobes of the seed, and its conveyance, in a backward course, to the embryo, could not, says Grew, have place; neither could the root, as it often does, grow upward and downward both at once. (Anatomy of Plants.) If the piece of a root of elm-tree be cut in autumn, the juices, says Du Hamel, are found to escape indifferently at either end, as the one or the other is alternately held downward; a circumstance, he observes, inconsistent with the opinion of Mariotte, who maintained the existence of valves. (Phys, des Arbres, tom. i. p. 56.) It is well known, also, that many plants may be made to grow in an inverted position, so as to put forth leaves and flowers from their roots; and large trees have been [7:3:44]nourished by juices received through the extremities of their ingrafted branches, after all connection between the earth and the roots had been cut off. In general, indeed, the extreme minuteness of the vessels seems almost to preclude the possible existence of valves in their canal; but in some instances, where the vessels in aged trees have become enlarged, membranous productions have been found to occupy their cavities, which some have alleged to perform the office of valves. They occur, however, only at an advanced period of growth, and form no necessary part of the structure of the vessel, and, instead of promoting, contribute only to obstruct the course of the fluids.

Thus far with regard to the general nature of the vessels of plants: let us next discriminate their several species or kinds.

Art. II.— Of the Common Sap-Vessels.

To ascertain the kinds and situation of the vessels of plants, various means have been employed. The plant has been dissected both in its dry and recent state; the natural qualities and movements of its fluids have been observed; and its vessels have been filled with coloured liquors, by causing it to vegetate in them. By the combined use of these several means, many important particulars have been ascertained; but it must be acknowledged that the question is still beset with doubts and difficulties, and that, with relation to it, great diversity of opinion continues to prevail. A concise statement of the facts ascertained with respect to the movements of the vegetable fluids may perhaps contribute to define the situation and kinds of the vessels that convey them.

It has been proved that, early in spring, before the leaves appear, a watery fluid rises abundantly in the woody part of the trunk of trees, and continues visibly to ascend to the very extremity of the branches, until the leaves are developed, when, to appearance, it ceases to flow, and can no longer be collected by perforating or tapping the tree. This fluid has been shown to ascend through the wood, and to rise, in general, most abundantly through its youngest or outmost circle; but in trees whose vessels have not been obstructed from age or other causes, it rises through every circle to the very pith, and, as far as can be judged, in all the vessels that compose those circles. At this early period of vegetation no fluid is found in the bark, nor between it and the wood, nor in the pith; but the vessels of the bark are perfectly dry. These facts are deducible from observations on the natural flow of the fluids by Grew, Du Hamel, Walker, and others; and are supported by various experiments of Μ. de la Baisse, Bonnet, Reichel, and others, made by causing plants and parts of plants to grow in coloured liquors, in which the vessels of the wood alone became filled; but no tinge of colour was communicated to those of the bark. To these vessels the several names of lymph-ducts, sap-vessels, ligneous tubes, ascending and adducent vessels, have been applied: we shall in future denominate them sap-vessels.

The vessels which thus form the mass of wood have by some writers been distributed into different kinds, and supposed to exercise very different functions. At certain periods of vegetation they appear empty; and hence Malpighi supposed two species of vessels to exist in the wood, one destined to carry sap, and the other to convey air: to these latter, from their supposed office, he gave the name of tracheae, and from their structure called them spiral vessels. (Anatom. Plantar. Idea, p. 3.) Grew also believed these empty tubes to be air-vessels, but admitted that at certain seasons they carried sap. At an early period, however, Ray maintained that the vessels thus supposed to convey air were truly sap-vessels; and Du Hamel, in common with Grew, admitted that they carried sap in spring. Hill considered them altogether as sap-vessels; and Reichel, Hedwig, and others, by filling them with coloured fluids, proved that such was their true office. On the other hand, no experiments, says Ludwig, have yet shown that there exist in vegetables vessels destined to convey only air; and in this opinion subsequent writers, with few exceptions, have acquiesced. It will, however, be convenient to treat of their general nature and form under the distinct appellations of sapvessels and spiral vessels, by which they are commonly known.

By Grew and Malpighi the common sap-vessels were regarded as entire tubes, having no apertures but in< the direction of their length. Grew represents a single vessel as having the appearance exhibited in fig. 1, Plate XXXy∏L, the aperture or canal of which is not visible unless highly magnified, as in fig. 2. According to Malpighi< these vessels send off numerous capillary filaments to the] cellular tissue, so that the cells are surrounded by a plexus of vessels, as is particularly seen in the pith of elder and some others; and these ramifications, he adds, spring probably from the perpendicular vessels both in the bark and wood. (Anatom. Plantar, p. 29. Lugd. Bat. 1688.) These lateral ramifications were observed also by Lecuwenhoeck in a piece of fir-wood newly felled. Of ∣ this wood he procured a longitudinal section, so extremely thin that he could see distinctly the particles of fluid moving in the vessels, as represented in the upper portion of fig. 3, Plate XXXVIII.; while lower down, on many parts of these vessels, small points or dots were visible, which he at length discovered to be round apertures; and as he did not see these apertures in any other parts than those in which he had separated the horizontal cellular tissue from the perpendicular vessels, he concluded that, at these points, these two organs were connected. He farther separated two of the vessels from the remainder, and through the microscope they appeared as in fig. 4; but the “engraver,” he adds, “said that he could not possibly draw all the jagged parts that he saw; and we both of us perceived, in the broken membrane or coat of the tube, many excessively minute vessels, which, by reason of their smallness, he was unable to express in the drawing.” (Select Works of Lceuwenhoeck, by Hoole, vol. i. p∙ 12 ∙)

It may however be said, that this communication between the vessels and cells is maintained not by ramifications from the vessels, but through apertures or pores in their sides; and, accordingly, many appearances have been remarked as existing on the sides of the vessels, of which different authors have given very different representations, and which some have regarded as pores. Thus, on the vessels of the fir Malpighi observed certain dotted appearances, which he describes as roundish tubercles, and which were so numerous that they appeared to cover the vessels. On the vessels of the elm, the beech, and the willow, Lecuwenhoeck saw similar particles, which resembled small globules. (SelectWorks,vo ∖ . ii. p.I.) Dr Hill describes the vessels of the alburnum, or newly formed wood of the willow, as connected with each other by a flocculent interstitial matter. When, by long maceration, this matter is detached, the vessels then exhibit a dotted appearance; and, if examined by a highly magnifying power, these dots, according to him, are so many oval swellings, and each has, as it were, a mouth. Through these mouths, which he represents as innumerable, and [7:3:45]existing on all parts of the vessels, he conceives the fluids to be discharged into the cellular tissue. ( On the Construction of Timber, p. 18.) In fig. 5, Plate XXXVΠI., is a representation of these vessels connected by flocculent matter, with their extremities collapsed from the escape of their juices, and their sides sprinkled with the little mouths which he mentions. These mouths, if they exist at all, are probably not pores in the sides of the vessels, but the little apertures seen by Lecuwenhoeck, and produced by the separation of the cellular tissue while the parts are still young and tender. The same author, speaking of the vessels of the mature wood of the pear, states them to be close canals, as in fig. 6, with no lateral apertures in them.

A still later writer, Μ. Mirbel, declares, that not fewer than fĵve species of vessels are to be found in the woody part of plants. These he denominates porous tubes, cleft tubes, tracheae, mixed tubes, and vessels en chapelet, from their supposed resemblance to a string of beads. Of these several species we have given representations in figures 7, 8, 9, 10, 11, and 12, Plate XXXVIII. The first species, or porous tubes, according to this writer, exist in every part of the plant where the sap moves with freedom. Their sides are covered with small eminences or projections, in the centre of which is to be found a small pore. (Exposition de la Théorie de l’ Organisation Végétale, p. 107.) Improving a little on Hill, he represents these pores not as promiscuously placed, but as ranged in transverse lines (fig. 7); and through them he conceives the fluids of the plant to percolate, not, however, into the cells only, but out of one layer of tubes into another, in a lateral direction. In this manner he conducts the fluids from the centre to the circumference of the wood, and at length, by a route not so easily followed, contrives to get them into the vessels of the bark, the sides of which he declares to be perfectly entire, and alike destitute of pores and clefts. (Ibid. p. 297.)

Several writers have sought to discover these alleged pores in the sides of the vessels. With Malpighi, some regard the appearances observed not as pores, but as elevations on the surface of the vessels; others, as particles contained within them. Under a very highly magnifying power, Kieser thinks he has, in some vessels, detected the existence of pores; while Μ. Dutrochet pronounces the same objects to be corpuscles, filled with nervous matter; and De Candolle suggests the probability of their being small glands, destined in some way to contribute to the nutrition of the plant. These different opinions, formed on viewing the same objects, sufficiently manifest the difficulty and uncertainty of microscopical observations made with highly magnifying powers, as was long since pointed out by Hooke, who first applied the microscope to the examination of the structure of plants. “Of such minute objects,” says he, “there is much more difficulty to discover the true shape by an instrument than by the naked eye; the same object quite differing in one position to the light, from what it really is and may be shown to be in another. In some objects,” he adds, “it is exceedingly difficult to distinguish between a prominency and a depression, between a shadow and a black stain, or a reflection and a whiteness in the colour; and the transparency of most objects renders them yet more difficultthan if they were opaque.” (Micrographia, Preface.)

Art. III.— Of the Spiral Vessels.

Various as have been the opinions of writers respecting the common sap-vessels of plants, they have differed yet more in their views concerning the position, number, size, structure, and uses of those which have been denominated spiral. The common fathers of Vegetable Anatomy, Grew and Malpighi, who at the same time, but in different countries, prosecuted their inquiries without any knowledge of or communication with each other, are nearly of one opinion on all the more important points in relation to these vessels. Later writers have differed alike from them, and from each other, on almost every point. As the subjectis of fundamental importance in the economy of vegetables, we shall endeavour to set before the reader the leading facts and opinions concerning it; to canvass their relative merits; and deduce from the whole such conclusions as seem most nearly to approach the truth.

Malpighi describes spiral vessels as existing in the ligneous parts of all plants. He called them spiral tubes, because, when extended, they were resolved, not into separate rings, but into a single zone, which might be drawn out to a great length. In general they form continuous tubes, but are sometimes contracted at regular distances, so as to resemble somewhat a series of oblong cells. One of these contracted spiral vessels, as delineated by Malpighi, is represented in fig. 17, Plate XXXVΠI., at the extremity of which the spiral filament is in part drawn out; and similar appearances of the spiral structure are exhibited in figures 9 and 10, by Mirbel. In herbs, according to Malpighi, these spiral vessels constantly accompany the common sap-vessels, and are ensheathed by them; in shrubs, they occur in every part of the wood, single or in clusters; and in trees, an intermixture of spiral vessels with the common sap-vessels is observed in every part of the wood. In the fir, they are found immediately beneath the bark, and are so numerous as to constitute the chief bulk of the wood. They exist with the sap-vessels in the petioles and ribs of the leaf, and likewise in the petals of the flower. In the roots they are also met with, and in some roots are so abundant, as to exceed in bulk all the other parts. They exist, he adds, in every part of the plant except the bark; and are annually formed with the alburnous vessels of trees. (Anatom. Plantar, passim.)

From the writings of Grew we collect also that spiral vessels exist in every part of a plant except the bark. In the root they are numerous, of very various size, and their bore is generally larger than that of the common sap-vessels. In the trunks both of herbs and trees they are equally visible, and in position, size, and number, subject to great variation. Sometimes they are collected into fasciculi, at other times they are disposed in rays, and in other instances they are arranged in a circular form: they stand sometimes next to the pith, in other instances next to the bark, and in other cases again they alternate with the common sap-vessels in every part of the wood. They have a more ample bore than the common vessels, and vary in size to at least twenty different degrees. In the leaf they always accompany the sap-vessels, and both in its petiole and ribs are constantly surrounded or ensheathed by them. They have a similar position in the petals of the flower, and in the vascular parts of the fruit. (Anatomy of Plants, passim.) Hence, then, it appears, from the dissections of Malpighi and Grew, that, in every plant in which vessels are to be seen, and almost in every part of it, spiral vessels abound; they exist, however, only in the ligneous portion, or that part in which the sap ascends, and are never to be found in the bark.

By Du Hamel, the spiral vessels, supposed to con-< vey air, are described as existing in the leaves and the 1 flowers, the petals of which are almost wholly composed of them. In the herbaceous portion of young branches [7:3:46]they are also well seen; which portion afterwards becomes ligneous; so that it cannot be doubted that they exist in the wood, though he could never discover them but in young branches. (Phys, des Arbres, tom. i. p. 42.) If, however, all the empty vessels seen in a transverse section of the wood be deemed air-vessels, and all air-vessels have a spiral structure, then, says this author, they would, in many plants, form a great part of the ligneous body. From these large vessels, however, he has seen, in autumn, fluids to escape; so that they are not properly air-vessels, or, as Grew observed, they sometimes carry sap. They are not to be found in the bark.

Under the common name of sap-vessels, Hill delineates all the varieties of vessels that constitute the wood; they are largest in the outer circles, smaller in the others; they contain, says he, in spring and at midsummer, a limpid liquor; but at all other seasons they appear empty, and have therefore been erroneously deemed air-vessels. He says nothing of their spiral construction; but describes the vessels which form the chief mass of the wood as possessing solid and firm coats, forming an arrangement of plain and simple tubes, as in fig. 6, Plate XXXVIII., resembling those of the alburnum, except that they have no mouths in their sides. ( On the Construction of Timber, p. 8 and 19.)

Μ. Reichel maintains the existence of spiral vessels in almost all parts of the plant. By causing plants to vegetate in coloured fluids, he traced them from the roots, through the trunk and branches, to the extremity of the leaves, and into all parts of the flower,—as the calyx, the petals, the style, the stamens, and the anthers. In fruits and in seeds, and in the radicle and plume of the latter, they were equally apparent. The coloured liquors, as they rose, communicated a tinge to the cellular tissue of the wood, as was previously observed by De la Baisse; but no trace of colour was ever observed either in the bark or pith, which therefore contain no spiral vesśels. He considered the spiral vessels as the organs everywhere conveying nutrient matter to the plant, and as having no title to the appellation of air-vessels. (En-cyclop. Méthodique, article Physiol. Végét. p. 288.) Similar experiments of Hedwig and others confirm these facts as to the general distribution of the spiral vessels, and their bearing to every part the fluids absorbed by the roots.

We have already enumerated the five different species of vessels which Mirbel regards as constituting the woody part of plants. According to him, true spiral Vessels (which he continues to denominate tracheae) are not to be found in the root, but only in the trunk and the parts which are produced from it. Even in the trunk they are to be found only around the pith, and never in the exterior ligneous layers. He admits that they exist in all soft and succulent parts, and that coloured fluids rise in them as well as in the other varieties of vessels; but they never are to be found either in the bark or pith. (Exposition de la Théorie de l’ Organisation Végétale, p. 74, 78.)

According to Μ. Kieser, spiral vessels are found in all the more perfect plants, from the earliest period of their existence; and in all parts of them, except the bark and pith. They vary much in size, and in aged plants are frequently obstructed by a species of vesicle, which originates from the interior sides of the vessels. According to him, nothing but air is found in their cavity, though, in the wood of guaiacum, he has seen all the spiral vessels, in common with the cellular tissue, completely filled with resinous matter. He does not know the anatomical relation subsisting between the spiral vessels and the other organs; but thinks it proved that no direct communication exists between them and the cells. (Mém. sur l’ Organisation des Plantes, 1814, p. 116, 117.) Lastly, Μ. de Candolle admits the existence of spiral vessels in the wood of the trunk, but not of the roots; and, with Kieser, regards them as conveying only air. ( Organographie Végétale, tome i. p.* 39.)

From the combined observations of all the preceding writers, with the exception of MM. Mirbel and de Candolle, we learn that spiral vessels exist not only in every part of the trunk, but in the root, and in every other part of the plant except the bark and the pith. From the actual presence of sap in these vessels early in spring, and again in autumn, and from the entrance of coloured fluids at all seasons, we also learn, that their true office is not to convey air, as Malpighi and others formerly, and Kieser and others more recently, maintain; but to carry sap. Are we therefore to regard them as a species distinct from the common sap-vessels, or are both to be held merely as varieties of one common kind? This question may receive some illustration from a little farther inquiry into the formation and structure of the sap and spiral vessels.

Art. IV.— Of the Structure of the Sap and Spiral Vessels.

Malpighi seems to have regarded the common sapvessels as being formed of a series of small vesicles or cells, mutually opening into each other. (Anat. Plantar. p. 28.) In this instance he appears either to have mistaken a series of elongated cells for vessels, or to have conceived that the contractions, occasionally formed in the vessels themselves, afforded evidence of their having been constructed originally by cells. Grew, on the other hand, considered these vessels to be composed of straight fibres, placed parallel to each other, so as to form a cylindrical tube. (Anat. of Plants, p. 112.) Lecuwenhoeck held them to be composed of two fine transparent coats, formed of other vessels excessively minute. ( Select Worhs, vol. i. p. 11.) And Du Hamel, by submitting thin layers of wood to long maceration, obtained bundles of longitudinal fibres of extreme minuteness, by which he considered the lymphatic vessels of the wood to be formed. (Phys, des Arbres, tome i. p. 32.) Hill describes the alburnous vessels as being formed of the same material as the cells, and to collapse when emptied of their fluids. In one instance, where a strong light was made to penetrate the vessel, it appeared as if composed of numerous cells; but on farther examination, these seeming divisions altered their places, and were found to proceed from portions of watery sap still retained in the vessel. This appearance, as he properly observes, may be a very necessary lesson against hasty judgments. (Construction q∕ Timber, p. 33.)

According to Μ. Mirbel, “the entire mass of the plant is nothing but cellular tissue, the cells of which differ only in form and dimensions.” The cells and vessels are thus considered to be formed out of one and the same membranous tissue. In forming cells, this membrane is supposed to dilate in every direction: in producing vessels, it increases only in length. (Exposition de la Théorie de l’ Organisation Végétale?) The manner in which he supposes the vessels to be formed out of the primitive membrane is very fanciful; and he is probably right in thinking that “no one before himself had formed a similar conception of vegetable organization.”

Concerning the structure of the spiral vessels, opinions have varied still more than in relation to those just considered. Malpighi describes them as composed of [7:3:47]-a thin pellucid plate, of a silvery colour, somewhat broad, and which, being placed spirally, and united at its edges, ' constructs a tube. When this tube is drawn out, it does not ^separate into distinct rings, but is resolved into a continuous spiral zone. At particular places these vessels are sometimes contracted, so as to exhibit the appearance of oblong cells opening into each other, as in fig. 17, Plate XXXVIII. In size they greatly surpass the ordinary sapvessels; and their canal is then frequently occupied by membranous vesicles, which nearly fill their cavity. (Anal. Plantar, p. 3-26.) These vesicles were also observed by Lecuwenhoeck, and their appearance, in the transverse section of an enlarged vessel, is represented in fig. 25.

According to Grew, the thin plate that forms the spiral vessel is not flat, and, instead of being single, consists of two or more round filaments or threads, placed collaterally, but perfectly distinct. These component filaments he regards as united by other smaller transverse filaments, and the thin plates which they form by their connection constitute the spiral vessel,—“as if we should imagine,” he adds, “a piece of fine narrow riband to be wound spirally, and edge to edge, round a stick, and the stick being drawn out, the riband to be left in the figure of a tube, answerable to a spiral vessel.” As, however, the riband is composed of numerous threads, placed parallel to each other, so is the plate that forms the spiral vessel; and it is according to the greater or less delicacy of the vessel examined, and the manner of its dissection, that it appears to be constituted either of a flat plate or a round filament. The spiration of the filaments he considered to be made in the root from right to left, and in the trunk from left to right. (Anat. of Plants, p. 73 and 117.)

- The opinion of Du Hamel with respect to the construction of these vessels was very similar to that of Grew, and he employed the same analogy of a riband , twisted round a stick to illustrate it. Hedwig gave a very different account of them. He considered the spiral vessel to be composed of two distinct parts; one a membranous canal conveying air, and the other a spiral tube rolled round it, by which the fluids were conveyed. The spires of the tube, in some instances, are represented as close; in others they are separated, and the intervening portions of the membranous canal exhibit a dotted appearance. He considered all the sap-vessels, from their first formation, to possess this compound structure, and, by a series of changes, which he professes to describe, to be ultimately transformed into the solid fibre of the wood. (De. Fibrae VegetaD. Ortu, p. 25.) Others, also, have believed the spiral vessel to be formed of a membranous tube, but have denied that it conveyed air; and the spiral tube of Hedwig they have regarded as a solid filament. From the different appearances which they exhibit, MM. Treviranus and Bernhardi distinguish several varieties of spiral vessels; as does also Μ. Kieser, whose account of these vessels is not only the most elaborate, but that probably which approaches nearest to truth. We shall therefore subjoin a brief abstract of his observations on the structure of these vessels, and more particularly on the series of transformations which they seem to undergo.

The construction of these vessels Μ. Kieser professes to have studied with the greatest care, and to have established incontestably the following points. Sometimes, says he, only one fibre, sometimes many, go to form the spire of a vessel. These fibres are commonly round, sometimes a little flattened, and are twisted spirally about an empty space, so as to form a tube. The spiral fibres in young plants, and sometimes in mature ones, are in close contact; in other instances they are separated, and the interstices are then occupied by a dotted or punctuated membrane, as is very frequent in trees; or sometimes they are connected by ramifications which proceed from the spiral fibres themselves. From the minuteness of the spiral fibre, it is difficult to decide whether it is a solid or tubular body, and often, from the same cause, to pronounce whether it is round or flattened.. It is transparent, has considerable consistence and tenacity, and in some plants appears to possess elasticity.

The number of fibres that form the spire of a vessel is very various; sometimes, as before remarked, there is only one, but more often several, which are twisted in the same plane and the same direction. He has seen nine fibres thus united; and when unrolled, the spires of the vessels then seem to form a kind of riband. When many fibres are employed to form a spire, they always run parallel, and do not cross; so that the side of the vessel has never more than the thickness of a single fibre. At their first formation the fibres are not united; it is only in more advanced age that they become united, either by ramifications from their edges, or by a peculiar membrane. The direction of the fibre in twisting is sometimes from right to left, and sometimes the contrary. The size also is very various. In young plants it is very small, so that a millimetre comprises the diameter of 2600 of these fibres; and in some instances they have little more than half that size. In older vessels they have a larger size. Between the knots of the trunk they are simple, and do not ramify; but in the knots they undergo great changes of form, and are variously ramified and combined.

Art. V.— Of the Transformations of the Spiral Vessels.

It was the opinion of Grew, that the “common sapvessels begin to be formed in spring, but the spiral vessels not till the end of summer, at least not till about that time do they appear.” (Anat. of Plants, p. 131.) Malpighi also describes these vessels as gradually appearing in the alburnum, and augmenting in size in every successive ring of wood; an observation made also by Grew, who describes the “spiral vessels as being amplified in each annual ring, so as to form a vessel of a wider bore.” These facts indicate that the spiral vessels, after their formation, undergo considerable changes in the progress of vegetation, or, to use the language of Grew, that ¢ ‘ they are postnate, and seem produced by some alteration in the quality, position, and texture of their fibres.”

In treating of the transformations or metamorphoses of the spiral vessels, Μ. Kieser illustrates his ideas by a series of dissections of the stem and root of the gourd (cucurbita pepo), made apparently with great care. These designs occupy several plates of his work; and being taken from different parts of the same plant, at different stages of its growth, and viewed with the same magnifying power, they show the progressive changes in form and appearance of these vessels as the plant increases in age. Μ. Kieser exhibits corresponding vertical and horizontal sections of the same parts; but our object will be answered by copying only certain portions of the vertical sections.

The ligneous portion of the stem of the gourd possesses ten bundles of vessels, symmetrically disposed around the pith, and standing at unequal distances between it and the bark. In fig. 13, Plate XXXVIIL, we have copied the representation of a vertical section of the vessels in one of these bundles, taken from a portion of the plant near the summit of its stem, and made with the aid of a microscope magnifying 130 times. The number of vessels in each bundle at this period is said to vary from three to five. They are constructed of one or more fibres, placed, [7:3:48] contiguous to each other, and twisted into a spiral form, producing a round cavity within. These are called simple spirals. They are found in every young plant, and in the newly formed parts of old ones. Their size is smaller than that of the other varieties to which they give origin. The number of fibres in each spire varies from one to nine; and they are the only variety met with in some of the inferior vegetables.

The nextfigure(fig. 14, Plate XXXVIII.) exhibits a similar section of the vessels of the same plant at the internodial space below. Of the four vessels contained in this bundle, the outer one, α, is larger than the rest, but its spires are contiguous, as in those of the former figure: the inner one, i, is formed of a series of rings, disposed in a perpendicular line. These rings are very like the spires of the preceding variety, and often combined with them in the same vessel. They are sometimes separated from each other only by a space equal to their own diameter; in other instances to eight or ten times that distance, forming then the substratum of the next variety. In their present form they constitute the annular spiral; and in herbaceous plants occupy frequently the same position as the simple spiral, that is, next to the pith.

From the two simple varieties of form just described, other more complex forms are produced, as the plant advances in age. In the next section, made still lower down on the same plant, the vessels exhibit the forms represented in fig. 15 of the same plate, which brings the third variety of spiral vessel into view. The exterior vessel, b i of this fasciculus, or that nearest to the bark, is of much larger size: its spires, represented by the white lines in the figure, are separated to nearly equal distances from each other, and the intervening spaces are filled up with a membrane, sprinkled over with small obscure points or dots, constituting the punctuated spiral vessel. In this variety the rings are never contiguous. Such vessels occur only in the more advanced age of herbaceous plants, but are said to be original formations in the alburnum of trees: they acquire a much greater size than the two former varieties, especially in the stem; but do not become so large in the root. In herbs they occupy a position exterior to that of the two preceding varieties; but in trees the largest vessels of the annual layers are nearer to the pith. The three other vessels in this figure are simple spirals, in part unrolled.

The spiral fibre in these vessels is of various size, being largest in the greatest vessels: it is sometimes so small as to be scarcely visible. It is often difficult to determine whether its form is spiral or annular: for the most part it is spiral in herbs, and annular in trees. The membrane which connects the spires or rings in this variety is not visible in young vessels, but appears in more mature age. It is at first transparent, but becomes opaque from age. In this punctuated variety of vessel the spires cannot be unrolled without tearing the connecting membrane. As to the points· or dots observed on this membrane, they have been taken by some for pores on its surface; by others for clefts produced by the joining of the spires. They are clearly not clefts, says Μ. Kieser, and it is against the supposition of their being pores that their position has a relation always to the spires, and not to the adjoining cells. In the vessels of some plants, as in those of the French bean, when magnified 400 times, these points, fig. 21, α, seem as if pierced by a hole exceedingly small; but in other instances they are quite dark, and similar to those found on the membranous productions that fill the cavity of aged vessels, where there is not the smallest reason for considering them as pores. There seems a great analogy between these connecting membranes and those which thus fill the cavity of aged vessels. In some instances the size of the dots is large, and they appear transparent at the centre; in others the dots are extremely small. Their form is oblong, and they are ranged generally in determined lines, parallel to the direction of the spires.

In the next vertical section of a fasciculus of vessels, taken from about the middle of the stem of the same plant, all the peculiarities of size and form are still more distinctly seen. Of the twenty-three vessels which appeared in the horizontal section of a fasciculus at this period of growth, six only were apparent in the vertical section, and are exhibited together in fig. 16, Plate XXXVIII. Of these, the first two, or outer ones, are by much the largest, and belong to the punctuated variety. A portion of the anterior wall of these two vessels has been removed by the sections at I, I, so that the inner surface of the posterior wall is brought into view; and in the former vessel, d, a portion of the posterior wall itself has also been carried away. In both vessels the spiral fibre, indicated by the white line, is prominent in the section; and in the second, e, it is unrolled at m, n, — where, at m, it is seen single, transparent, and round, while at n it is united with the punctuated membrane that connects the spires, and has a flat or riband-like form. The third vessel, f, of this fasciculus is also of the punctuated class, and the three last, g, A, «, are simple spirals; the last of all, i, being in the progress of formation, and its spires not yet brought into a state of contiguity.

There is a fourth variety of vessel that is said to have the same origin as the last, being formed in part by spires or rings, but the separations between which are filled up, not by membrane, but by small productions or ramifications from the spires or rings themselves; and these ramifications are often so implicated as to form a net-work; whence the names of ramified scad reticulated spiral vessels. Like the former variety, they do not exist in the young plant, but are formed gradually in a more mature age, and by the same series of actions; that is, the spires or rings, which were at first contiguous, separate at a later period, and the intervals are filled up by ramifications from the spires themselves. At first these ramifications are few, as in the vessel, α, fig. 21, when they are termed ramified spirals; and as these ramifications increase, the vessel becomes reticulated, as exhibited at the letter c, fig. 20. They are found but in few plants, and ap-∙ pear to differ from the punctuated variety chiefly in the position of the rings, which are more or less obliquely placed, and at different points send off one or more branches.

In his ninth plate, Μ. Kieser has given similar representations of the vessels of the same plant, as they appear from sections made near the root. The fasciculus at this part of the stem contained twenty-nine vessels, as seen in the horizontal section, of which only eight were visible in the vertical section. These retained nearly the characters of those given in the preceding section, fig. 16; but exceeded them in number and size. The large ones, situated next the bark, were of the punctuated class, while the smaller ones, next the pith, were simple spirals. In fig. 18 we have copied the three smaller vessels. In one of these, o, which belongs to the punctuated variety, the interior of the vessel is nearly filled up with small portions of punctuated membrane, produced from the inner wall of the vessel itself, as is still more clearly seen in horizontal sections of similar vessels in fig. 25. The two vessels p, q, of fig. 18, are the youngest, and consist only of the simple spiral fibre; the last, it is said, being composed of two such fibres, which, in consequence of the transpa [7:3:49]rency of the posterior wall of the vessel, appear to cross each other.

But Μ. Kieser has not confined his examination to the stem of this plant. In his tenth plate he has given similar sections of a portion of the root. The piece of root he examined presented, in the horizontal section, four fasciculi, each of which contained about thirty-seven vessels, varying in size; the larger ones being placed exteriorly, and the smaller ones towards the centre of the root. The cells of this part were small, and nearly of the same size and form throughout, differing much from the varying size and form of the cells in the stem. Of the thirty-seven vessels before mentioned, only ten were retained in the vertical section, all of which belonged to the punctuated variety. Of these, two had united so as to form but one cavity; and two others were filled up with membrane. In fig. 19 we have copied the appearance of three of these vessels. One of these, indicated by the letter r, is from the middle of the figure, and its size is intermediate between those on each side of it. A portion of its anterior wall has been carried away in three places, which brings the inner side of the posterior wall into view. The vessels s t of the same figure are the smallest, and situated at the centre of the root: all the three belong to the punctuated variety, no simple or annular spirals being found at this period in the root.

In the knots of plants the vessels undergo great changes of form, the larger vessels being contracted in different parts, and giving off productions or ramifications which form new vessels, and give to the whole a very irregular appearance, as may be seen in the upper part of fig. 20, taken from the appearance of the vessels in the knot of the balsam (Impatiens Balsamina L.). In this figure Μ. Kieser has also given an ideal representation of the various metamorphoses already described, viz. of the annular vessel intĢ the simple spiral, and of that into the other varieties: thus the letter a represents the rings of the annular vessel, which at b begin to form the spiral: at c the simple spiral passes into the ramified variety, and this again, at d, into the reticulated vessel: at e the vessel becomes contracted at different parts, as previously observed by Malpighi, fig. 17, and by Mir-bel, fig. 11 and 12. This latter author called this the vessel en chapelet, from its resemblance to a string of beads. From these vessels go off the bran che sfff, to form the ramified spirals of the knot. In fig. 12 of the same plate Μ. Mirbel has given a similar ideal representation of the five species delineated by him in figs. 7, 8, 9, 10, and 11; but they do not carry with them the same evidence of accurate observation as the several varieties indicated in the alleged transformations of Kieser.

Such is the account given by Μ. Kieser, of the development and appearances of the vessels in herbaceous plants, and of the successive changes in character which, m the progress of growth, they undergo. All the figures except the last are taken from dissections of the same plant, beginning with that part of the stem which was of latest growth, and proceeding in succession to the parts of earlier growth, or older formation. At first the vessels are comparatively few in number, of nearly uniform size, and belong to the varieties of annular and simple spiral: at a later period those first formed become augmented in size, and assume the characters of punctuated and rami? fied vessels; they are now also removed farther from the centre, and their place is supplied by new vessels of simpler forms. With the age of the plant, they all continue to increase in size and number, and to acquire the new characters which distinguish the Several varieties in the manner already described.

In the early growth of trees, the formation and development of vessels go on at the centre, adjacent to the pith, in the same manner as in herbs; but at a later period, when the additional growth is made at the circumference of the tree, the first formation of vessels must occur in the alburnum, and the subsequent changes of character be effected in the several layers of wood; which accordswith the fact that spiral vessels are rarely to be detected in the alburnum, but are found with all their different characters, and of all sizes, in the several layers of wood, as may be seen in all the transverse sections of wood delineated by Grew, Malpighi, and others. In truth, the descriptions of Kieser with respect to the structure and transformations of the spiral vessels accord, in most respects, with the opinions of Grew, as briefly stated, page 47 of this article, and as may be farther seen by referring to pages 73 and 177 of Grew’s Anatomy of Plants.

It is further to be remarked that, in these representations of Kieser, which exhibit all the vessels visible in all parts of the stem and root of this plant, only one kind of vessel is described, which at different periods of growth assumes different forms; that no vessel answering to the common or lymphatic vessel of authors is mentioned; and consequently, that all the vessels described, however differing in size and external character, are in truth but varieties of one common kind. Calling to mind also that, in the earliest production of leaves and of flowers, and even of stamens and pistils, vessels either possessing the spiral character, or capable of assuming it, are everywhere met with,—that they occur also in the stem adjacent to the pith in succulent plants, and in the alburnum of trees, and every successive ring of wood produced from it—in short, in every part and portion of the plant, except the bark and pith,—and believing, as we do, in those changes of form suggested by Grew, and subsequently demonstrated by Kieser,—we are led to the general conclusion that only one kind of vessel exists in the ligneous portion of other plants, as well as in that of the gourd; and that in different plants, and in different circumstances and conditions of vegetation, this vessel is capable of assuming the different forms, sizes, and characters, which have led many writers to describe these organs as constituting different species, and serving different uses.

Μ. de Candolle, in his late work on the Organography of Plants, seems also to reject the separate existence of the ordinary sap-vessels or lymph-ducts of authors, and to consider, with Kieser, all the vessels collectively to possess a spiral character, which appears under five different forms in the vessels severally named tracheae or simple spirals, annular spirals, punctuated and reticulated spirals, and the vessels en chapelet. (Organographie Végétale, tome i. p. 32.) Without deciding positively in favour of Kieser’s views, he admits that the different orders of vessels have among them very intimate relations; and in favour of the opinion that they are really modifications of each other, he alleges that al mb st all these orders exist together in certain classes of plants, and are wanting altogether in others. Still, though he has no theoretical objection to make to these alleged transitions of form, he has never been able to verify them by actual observation, and thinks the facts require further investigation. (Ibid. p. 48, 51.) With respect to the existence of pores in the spiral vessels, as alleged generally by Mirbel, and even occasionally by Kieser, but denied by a great number of observers, Μ. de Candolle has been led by his own observation to entertain doubts, and to believe that what has been regarded as a pore is a luminous point, such as is exhibited by bubbles of air in water, when placed under the microscope. (Ibid. p. 55.) [7:3:50]The late opinions of Μ. Dutrochet on the structure of plants seem to us to have drawn attention more from their novelty than their value. He criticises the opinions of Mirbel in relation to the structure of the vessels, and says that those denominated by that author porous tubes, are altogether destitute of pores, but have corpuscles adhering to, or rather implanted in their sides, on which account he calls them eorpusculiferous tubes. (Recherches Anatom, sur la Structure des Végétaux, <⅛e∙ p. 23.) In like manner the tracheae are said to have similar corpuscles attached to their sides, but which adhere only weakly, and form no necessary part of the organization. (Ibid.) With respect to the uses of these two kinds of vessels in the wood, he considers the corpusculiserous tubes to be the canals by which the sap ascends from the roots; while the spiral vessels, or tracheae, convey not air, but a diaphanous liquid, prepared in the leaves, and which descends, by the spiral vessels, to impart its vivifying influence to all parts of the plant. (7Z>iW.p. 29-31.) No direct communication, it is said, exists between the cells and vessels which compose the vegetable texture; they are only contiguous. Fluids, nevertheless, are transmitted from one to the other; not however through holes or passages expressly designed for that purpose, as Mirbel contends, but through intermole-cular spaces, left between the integrant globular molecules, of which the organic solids are composed. (Ibid. p. 47, 48.)

Such is an outline of the different opinions that have been held regarding the structure of the sap-vessels. Differences not less great still continue to exist with regard to their uses. Our reasons have been already given for coinciding in opinion with those who hold the vessels to be the proper channels in which the sap is conveyed. Μ. Kieser, however, contends that the spiral vessels of the wood are always empty, or contain only air, and considers them, with Malpighi, to be organs by which air is conveyed. To this opinion Μ. de Candolle also inclines, though he admits that in some cases they may convey lymph. (Organog. Végét. tome i. p. 61.)

Those physiologists who thus oppose the opinion that the vessels carry fluids, suppose the sap to be conveyed by certain minute channels, said to exist between the cells. The place of these alleged channels is marked out in a section of cellular tissue (fig. 29, Plate XXXVΠL) taken from Μ. Kieser, in which the black points at the angular junctions of the cells are said to denote them: from their position they have obtained the name of intercellular canals. By many, however, the existence of such spaces at the angles of the cells is denied; and, indeed, when we contemplate the various size and form of the cells, the different degrees in which they are filled with fluid or solid matters, and consequently the greater or less degree of compression which these alleged intercellular spaces must undergo, we can hardly admit the sufficiency of such minute, tortuous, and precarious channels to convey the ascending sap with that force and velocity which the experiments of Hales and others have proved it to possess. On the other hand, no direct communication between the spiral vessels of plants and the external atmosphere, similar to that which occurs between the tracheae of insects and the air, has been shown to exist; and the total absence of spiral vessels in the bark, where they would be near to the atmosphere, and their presence in the root, where they are almost beyond its influence, are alike repugnant to the notion that they exercise a respiratory function. Lastly, the absence of sap in these vessels at certain seasons, on which alone the notion of their being tracheae or air-vessels was founded, depends, as we have seen, on the period at which the examination is made; for at certain periods they have been found filled with sap; and at all periods, when in active vegetation, they will absorb and convey coloured fluids. It should also be borne in mind that the respiratory function in vegetables is performed, not by the trunk, but by the leaves.of the plant.

Dismissing then these notions concerning the uses of the spiral vessels, and adhering to the opinion, that in all their variety they serve the office of conveying sap through the plant, we shall conclude this branch of the subject with a brief notice of their modes of termination.

If we begin at the root, we may consider these vessels as terminating at that part, by continuation of canal, in the capillary absorbents of the rootlet. As, further, the vessels of plants, like those of animals, are the only organs which convey the materials out of which not only the fluid, but the solid parts of the plant are formed, it may, in a general sense, be said that their modes of termination are as numerous as the kinds of distinct parts or organs which the vegetable system contains. Hence, therefore, as the cellular tissue of plants contains various matters often precisely similar to those which exist in the vessels, it may be inferred that the vessels have a termination in those organs; and this inference may perhaps carry more weight with many than the attempts before related of Malpighi and Lecuwenhoeck to demonstrate it anatomically. Another termination of these vessels must be in certain minute and ill-defined organs called glands, which separate from the mass of fluids peculiar secretions; and a fourth mode may, in the leaves, be in other vessels which carry back the juices from those organs. The last and fifth mode of termination is into transpiring or exhalent organs, by which a certain portion of the contents of the vessels is discharged: so that in plants the sap-vessels terminate externally at one extremity in absorbents, by which fluids are received; and at the other in exhalents, by which these fluids are in part discharged.

Art. VI.— Of the Proper Vessels.

By observation of the natural flow of the sap, combined with the results of experiments made with coloured liquors, we have endeavoured to determine the situation and kinds of vessels by which it is conveyed in the wood. The same method will best assist us in ascertaining the nature and place of the vessels met with in the bark.

It was before stated, that, early in spring, the sap of plants rises through the wood alone, and that no fluid whatever is then to be found in the bark. At a later period, however, the case is completely reversed; for the vessels of the wood no longer appear to carry sap, and those of the bark then become abundantly supplied with it. This difference is very dearly and concisely stated by Grew. “The sap,” says he, “in many plants, as the vine, ascends visibly through the wood for a month, in March and April, and rises through every ring of wood to the very centre, yet at the same time there ariseth no sap at all out of the bark, nor between it and the wood.” “But late in spring,” he continues, “and in summer, the sap is no longer visible in the wood, but is abundant in the bark, in the inner margin adjacent to the wood.” Du Hamel, too, remarks, that when the lymph rises abundantly through the wood in spring, the bark is dry, and adheres to the wood, and no sap then issues from it, nor from between it and the wood; but, later in the season, he adds, the bark yields abundance of sap. These statements have been verified by the multiplied observations of various subsequent authors.

But why is the sap thus present only in the wood at its first rising in spring Why at a later period does [7:3:51]it cease to be visible in that part? And how and why does it afterwards find a passage into the bark? Some observations of Du Hamel, Hales, and Walker, point, we think, to the true cause. In spring, says Du Hamel, when the sap rises vigorously, the buds have not appeared; when they begin to open, the sap then flows less freely; and when the leaves are fully developed, then the flow of sap entirely ceases. Dr Hales also remarks, that, towards the end of April, when the young shoots come forth, and the surface of the vine is greatly increased by the expansion of the leaves, the sap then ceases to flow in a visible manner till the return of the next spring. All bleeding trees, he adds, cease to bleed as soon as the young leaves begin to expand enough to perspire plentifully, and draw off the redundant sap. The bark of oak, too, separates easily when lubricated with sap; but before the leaves appear and perspire, the bark will no longer run (as they term it), but adheres most firmly to the wood. ( Vegetable Statics, 3d edit. p. 126.) In like manner, in an experiment of Dr Walker, a birch-tree bled from every perforation in its trunk, and from every cut extremity of its branches, until vernation or budding began; then thcbleeding was almost immediately checked; and when the young leaves had pushed beyond the hy-bernaculmn, the bleeding entirely ceased. (Edin. Phil. Trans, vol. i. p. 31.)

It is however certain, that though the sap was no longer visible in the wood after the leaves were developed, it continued nevertheless to rise through it; for in no other way could the leaves obtain the large portion of fluid which it is known that they constantly discharge by transpiration; and coloured fluids manifest their presence in the vessels of the wood, as well after as before the development of the leaves. The leaves, therefore, must be regarded as the organs which, by their perspiration, draw off, as Dr Hales observes, the redundant sap; and hence, in an experiment where a notch was cut two or three feet above the lower end of a stem, though a great quantity of sap passed by the notch, yet was it perfectly dry; because, stfys he, “the attraction of the perspiring leaves was greater than the force of trusion from the column of water.” (Vegetable Statics, p. 111.)

Not only, however, does the development of the leaves render the sap no longer visible in the wood, but they also appear to be the organs by or through which it finds its way to the bark. In all the experiments just recited, the bark continued dry until the sap disappeared from the wood; in other words, until it was drawn off by the leaves; and then, and not till then, the bark became moist, and continued laden with sap through the rest of the summer. Now, as it has been before shown that no sap enters the bark by the roots, nor gets into it directly from the wood, there is no other known channel by which it can be conveyed, except through the leaves; and these, therefore, necessarily appear to be the organs by which it is apparently carried off from the wood, and by or through which it at the same time finds its way to the bark.

This inference appears to follow, not only from the fact of the bark continuing dry until the leaves are developed, but from the circumstance that it is again rendered dry after having become moist, if these same leaves be removed. If, says Du Hamel, we remove the leaves of a young tree when in full sap, and whose bark is easily detached, ill a few days after the same bark will adhere as closely to the wood as it commonly does during winter. This direct connection between the leaves and bark is also well illustrated in an experiment of Hales, employed by him for a very different purpose. From two thriving shoots of a pear-tree he cut, in several places, half an inch of the bark off all round.

All the ringlets of bark between these incisions had a leaf-bud upon them except one, and all but this one ringlet grew and swelled at their bottoms till August; and the larger and more thriving the leaf-bud was, so much the more did the adjoining bark swell. ( Veg. Statics, p. 149.) Mr Knight also found the bark of the vine to become shrivelled and dry when the leaves were stripped off; but in those parts in which it communicated directly with the leaves, it continued moist and flourishing. (Phil. Trans. 1801, p. 335.)

By connecting, therefore, the circumstances attending the flow of sap with the development of the leaves, we gain satisfactory reasons for all the apparent anomalies observed to attend its course. Early in spring, before the appearance of the leaves, no natural outlet for the escape of the rising sap exists, and therefore, when the vessels arc cut or perforated, they readily pour out their sap, or bleed; but late in spring, and in summer, when the leaves are developed, the more watery parts of the sap are thrown off by transpiration; and while this process proceeds, the fluids do not accumulate in such quantity in the minute vessels of the trunk as to be effused or bleed through their cut or perforated sides. A cold day, however, or a moist and still atmosphere, by checking transpiration from the leaves, restores more or less the propensity to bleeding from the trunk; and in autumn, when fructification begins, and vegetation makes a pause, the same disposition to bleeding recurs in the trunk, from the check imposed on the more active powers of growth.

It is further evident, that, when the vessels of the bark become supplied with fluid, they cannot have derived it immediately from those of the wood, since, in these different parts, the fluids have frequently no sort of agreement in properties. Thus, Grew remarks, that almost all plants, late in spring, and in summer, bleed from their bark; and the sap has either a sour, sweet, hot, bitter, or other taste. At this period the bark of the vine yields a sour sap; but, “what is vulgarly called bleeding in the vine is,” he adds, “quite another thing, both as to the liquor which issueth, and the place whence it issues; that is, it is neither a sweet nor a sour, but a tasteless sap, issuing, not from any vessels in the bark, but from the air-vessels of the wood.” (Anatomy of Plants, p. 125.) Malpighi, also, was well aware of the difference in the qualities of the sap, and thought every plant possessed its peculiar sap; but he has not so accurately defined the situation of the vessels that contain it. Du Hamel, however, points out distinctly the difference of quality in the sap of the bark and wood. In the bark of some plants it is white, in others red, and in others yellow. It is in some instances milky, in others resinous, and in others gummy. In many plants it has a sweet taste, in some it is caustic, and in others insipid. It has sometimes much odour as well as flavour, and frequently it is destitute of both. (Phys, des Arbres, tome i. p. 68.)

But if the qualities of the sap in the bark be thus different from those of the sap in the wood; if these peculiar qualities are detected in it only after the development of the leaves, and the leaves be the organs by which alone the sap can be conveyed from the one part to the other; then it seems to follow that the sap must acquire these new properties in the leaves during its transmission through them. Malpighi remarked the existence of this altered sap in the leaves, and held them to be the organs which prepared nutrient matter for the plant; and Dr Darwin, by immersing plants of spurge in coloured liquors, not only saw, as others had previously done, the red fluid ascend through the leaf, but another fluid, of a [7:3:52]white colour, returning at the same time from the extremities of the leaf, and descending into the petiole. (Botanic Garden, vol. i. notes, p. 450-53.) This same returning fluid Mr Knight observed in similar experiments on branches of the apple and horse-chesnut trees, and even traced it through the petiole into the inner bark, by the vessels of which it seemed to be conveyed to the extremities of the roots. (Phil. Trans. 1801.) The motion, therefore, of the sap in the bark is not that of ascent, as Grew and Malpighi and many others have believed, but of descent, as the observations and experiments of Μ. de Sarrabat, Du Hamel, Knight, and others, abundantly prove. To these vessels of the bark Grew assigned particular names, according to the apparent quality of the fluid they conveyed. Malpighi gave them the general appellation of vasapeculiaria, from their containing a fluid different from the common sap. By others they have been called cortical vessels, a term, however, not applicable to many tribes of vegetables which are entirely destitute of bark. Lastly, Du Hamel and others denominated them proper vessels, which term differs but slightly from the appellation of Malpighi, and, though not very precise, is that we shall continue to employ.

From regarding the newly formed vessels of the wood as part of the bark, Grew uniformly represents the bark 'as possessing two distinct species of vessels, in which he has been since followed by several other writers; but the vessels which he calls lymphoeducts belong rather to the wood, so that we may regard him as describing only one species of proper vessels. In herbs, these vessels stand sometimes in distinct parcels or columns; sometimes they are disposed in a ring; sometimes they have a radiated position; and sometimes they are more intermixed with the sap-vessels, and seem to alternate with them. In trees, the vessels of the bark are more distinct, and have a much more regular appearance. They are commonly postured near the inner margin of the bark, and, when viewed in a longitudinal direction, seem collected into fasciculi, which are more or less numerous, and the component vessels of which continually diverge and join with others, so as to form a reticulated appearance, as in fig. 24, Plate XXXVIII. Of these reticulated fasciculi, many layers’ exist in an old tree; and to these layers the thickness of the bark is chiefly owing. As they proceed inward, the direction of the fasciculi is less oblique, so that near to the wood they are almost straight. Hence the spaces formed by the reticulations are very unequal, often large in the exterior part of the bark, and diminishing in size towards the wood: they are. everywhere filled with cellular tissue. Such is a general description of the situation of the proper vessels of the bark, as given by Grew, Malpighi, and Du Hamel.

The vessels which thus form the vascular portion of the bark appear to differ but little in structure from the more simple vessels of the wood. Grew considered them to possess a similar structure, from believing them to be formed by the inner bark at the same time with the vessels of the wood; and therefore, he adds, “they may be reasonably thought similar in the bark and wood.” (Anat. of Plants, p. 112.) Malpighi regarded them as simple tubes, containing sometimes peculiar juices; but advances nothing particular respecting their structure. (Anat. Plantar, p. 3.) Μ. Mirbel’s opinion deserves some notice, inasmuch as he declares the structure of these vessels to differ entirely from all those of the wood. Their sides, says he, are perfectly entire; they have neither pores nor clefts, and may therefore be deemed simple tubes. (Exposit. de la Théorie de l’ Organisât. Végét. p. 109.) A single vessel of this kind is represented in Plate XXXVIII. fig. 22; and in fig. 23 a fasciculus of the same vessels magnified is given as delineated by Μ. Mirbel. A re-, mark also of Hill, if it be deemed to rest on correct observation, is entitled to great attention. “The vessels of the bark that form the fasciculi are not,” says he, “united to each other, but are connected with the cellular tissue at numerous places; and, when separated from it, there appear on the sides of the vessels small oval depressions, dotted as it were with pin-holes.” ( On the Construction of Timber, p. 28.) These appearances he regards as of a glandular nature, but their description corresponds better with that of Lecuwenhoeck respecting the lateral ramifications from the vessels of the wood; and they may probably be the points at which communication is effected between the vessels and cells.

The cause of the reticulated appearance· which the vessels of the bark exhibit in trees, is doubtless to be attributed, as we think Du Hamel somewhere remarks, to their peculiar mode of growth. A new layer of cortical vessels is every year added to. the inner surface of the bark, as well as a new layer of ligneous vessels to the outer surface of the wood; so that, as Grew observes, the new matter of the tree is every year distributed two contrary ways,—one part falling outward towards the bark, and the other part retaining its situation inward to constitute the wood. At first the newly formed cortical vessels are straight, and stand parallel, likç those of the wood; but, by the continual growth of the new parts formed between the bark and the wood, the older vessels of the bark are gradually forced, outward; and being thus every year disposed around a large cylinder, are necessarily more and more separated from each other, and produce at length that-net-like form which we .observe them to possess. The newly formed vessels of. the wood, on the other hand, retain their original position,· and therefore preserve their parallelism, seldom or never exhibiting those flexures and reticulations so common to the vessels of the bark.

In the bark, as well as in the wood, the vessels are I found to possess different sizes. In the pine, vessels containing turpentine are represented by Grew, which are very much larger than the common sap-vessels, and are surrounded by smaller ones, exactly as the large spiral tubes of the wood are said to be ensheathed by the common sap-vessels. The milky juice of a species of sumach is contained in very large vessels, disposed so as to form a ring, and each large vessel is surrounded by many smaller ones. (Anat. of Plants, tab. 20.) The appearance of these large vessels in one species of pine is well represented by Hill, and the account he gives of their formation is probably correct. In this tree (pinus orientalis), some of these vessels form oval openings, large enough to admit a straw; these openings occupy the centre of the bark, and are surrounded by a ring of smaller vessels. As their contents are soluble in alcohol, it is easy to obtain them empty. In fig.26, Plate XXXVIII., the vessels of this tree, as they appear in the bark, are displayed; the woody portion of the tree having been scooped away, so that the longitudinal aspect, as well as the transverse sections of them, is exhibited. From a strict inquiry into their nature, Dr Hill concluded that these larger vessels were originally the same as those of the smaller fasciculi in the bark of the same tree; “so that if we conceive one of these smaller fasciculi opened in its centre, and the vessels driven every way outward, till they are stopped by the substance of the bark, we shall have an idea of the structure of this large vessel, which is nothing more than a great cylindrical hollow formed in the centre of such a fasciculus.” (Hill on the Construction of Timber, [7:3:53] p. 29.) It is in trees that have copious and viscid juices that these enlarged vessels are formed; and where the juices do not concrete, it is probable that, as the vessels annually recede from the centre, they suffer a reduction in size from the continued effects of desiccation and compression to which they are exposed.

The foregoing varieties appear alone entitled to the appellation of the proper vessels of the bark. In many tribes of vegetables, however, as will afterwards be shown, no distinction of bark and wood exists; but one uniform distribution of vessels extends from the centre to the circumference of the plant. Such plants have also their proper vessels, but the place and disposition of these vessels are not so precisely ascertained. From the mode in which their growth is accomplished, as well as from observation of their structure, it may be inferred that their proper vessels are distributed through the whole substance of the plant, accompanying in every part the sap-vessels. In some plants possessing this arrangement of parts, such as different species of. wheat,∙Malpighi (Anat. Plant. tab. 4, fig. 15) describes and delineates a vas proprium as forming a part of each fasciculus of vessels; and a similar intermixture, of the two kinds must, we conceive, exist in all similaŗ structures.

Even in many plants possessing a distinct bark, vessels containing proper juices are found in the wood. In every circle of wood, from the inmost that surrounds the pith, to the outmost in contact with the bark, vessels containing a gum, turpentine, or some other concrete or coloured juice, hiay frequently be found. Malpighi conceived 4hem to exist in all plants, though, from the nature of their fluids, they could not always be distinguished; and he believed them to afford a highly perfect juice for the nutrftion-oî.the plant. According to Grew, “the turpentine vessels that are scattered up and down the wood of the pine and fir are the self-same which did once appertain to the bark; but being pinched up by the wood, they are become much smaller pipes.” (Anat. of Plants, p. 115.) Du Hamel also regarded them as similar to those of the· bark, but rendered much smaller by compression. In the ,pine and fir they are disposed circularly around the axis, much like, the sap-vessels, and alternate with them. (Phys, des Arbres, tħme i. p. 41.)

In Piscidia erythrina, the proper juices are of a scarlet colour, and the vessels that contain them are therefore readily discerned wherever they exist. This plant has been selected by Hill to demonstrate the position of these vessels. In the bark they are collected into fasciculi, all the vessels of which contain coloured juices, and are disposed in a ring on the inner margin of the bark. Within this ring stands the alburnum, through the substance of which many smaller red vessels are distributed; and similar red vessels are more sparingly seen in every layer of wood, particularly in that which envelopes the pith. ( Construction of Timber.)

Now the red vessels thus observed in the wood of the above-mentioned plant must either have been formed in the situations they occupy, or transported from some other place. The latter supposition is inadmissible, inasmuch as the wood of trees is formed by layers of new vessels superimposed on one another; and no removal of the old vessels, nor reproduction of new vessels within the old layers, ever takes place; consequently no actual transposition of vessels could occur, nor could new vessels be developed in the wood after it had been completely formed. Ifi however, an alternate deposition and absorption of matter go on into and from the cells, it is possible that the vessels might in this way become filled with a matter different from that which they originally possessed; but in the case before us, a readier explanation presents itself. The new matter of the wood is formed at the same time, and in the same place, as that of the bark; and through this new woody matter the red vessels were dispersed as well as in the bark. Consequently every addition made to the ligneous layers would furnish some vessels that contained these proper juices; and this, being annually repeated, would exhibit that intermixture of proper vessels with sap-vessels which is observed in all the ligneous layers of this and many other trees. Hence these proper vessels of the wood must be held to retain the position in which they were originally produced, and cannot be said to have approached nearer to the centre, but only, by the addition of new layers exterior to them, to be placed farther from the circumference of the tree. As Grew, therefore, held the alburnum to be a part of the bark, he might correctly say that these vessels “did once appertain to it.”

Art. VII.— Of Collections of the Proper Juices in the Cellular Tissue.

Besides this accumulation of the proper juices in certain vessels of the wood, it frequently happens that depositions of similar matter occur in all parts of the cellular tissue of plants. In the bark of the oak and poplar, and of other trees, resinous concretions are often found in the cells; they are situated irregularly, and, according to Malpighi, are observed even in very young bark. Sometimes the cells containing milky and resinous juices are so postured in the bark, says Grew, as to form cylindrical channels, which are neither parallel nor anywhere inosculated, but run, with some little obliquities, distinct from one another. They appear to be formed out of the cells, and are not bounded by any walls or sides proper to themselves, but only by those of the cells. (Anat. of Plants, p. 112.) They are often short and tortuous, always isolated; and are sometimes placed irregularly, at others disposed in a circular form. They occur sometimes in the pith, and possess very different sizes and figures. They have frequently been deemed a species of proper vessels, from the mere circumstance of their containing similar juices, and from possessing sometimes an elongated form; but they are organs which, neither in form nor in function, bear any resemblance to vessels. Mirbel proposes to name them secretory canals, and Μ. Link cellular reservoirs — the term certainly most generally applicable, and involving no hypothesis respecting either their formation or functions.

The manner in which these cellular reservoirs, may be produced in the bark or pith is readily explained, on the supposition that a communication everywhere subsists between the vessels and the cells. One set of vessels has been shown to receive and carry sap to the leaves, and another to bring it back from them to the bark; and these two sets of vessels are everywhere in their course surrounded by cellular tissue. Hence the cells in every part may receive a portion of the fluid which the vessels are employed to convey. Thus in herbs, the cells both of the bark and pith are filled with fluid, which led Grew to believe that the sap was actually transmitted through those organs; but at the same time he delivers facts which, even in his own opinion, prove that it is derived directly from the vessels. Not only in herbs, but “in every annual growth, whether of a sprout from a seed, of a sucker from a root, or of a scion from a branch, the pith is always found the first yeaŗ full of sap; but in the second year the same individual pith always becomes dry, and so it continues ever after. One cause whereof is, that the [7:3:54] lymphaeducts of the bark being the first year adjacent to the pith, they do all that time transfuse part of their sap into it, and so keep it always succulent. But the same lymphaeducts the following year are turned into wood, and the vessels which are then generated and carry the sap stand beyond them in the bark; so that the sap, being now more remote from the pith, and intercepted by the new wood, cannot be transfused with that sufficient force and plenty as before into the pith, which, therefore, from the first year always continues dry.” (Anat. of Plants, p. 124.)

All that is here said respecting the transfusion of the common sap from the vessels into the cells as it ascends through the wood, is equally applicable to the proper juices as they descend through the bark. During the first year of growth, both the sap and proper vessels are adjacent to the pith as well as to the bark, and each order may· therefore transfuse its fluids into the cells of either. The common sap, from retaining its fluidity, is frequently removed by absorption, and the cells that contained it appear empty and dry; but where the proper juices arc transfused, and become viscid or concrete, they are retained, and appear in different quantities and forms both in the bark and pith, according to the nature and properties of the fluids from which they are derived, and the texture and situation of the tissue into which they are poured.

To these collections of the proper juices in the cells of the cellular tissue may be referred the opinions of those who describe them as existing sometimes in vacuities between the cells. Grew had remarked the occasional presence of these juices in cylindrical channels formed by these vacuities, to which Μ. Treviranus has given the name of intercellular canals. According to him, these canals not only contain the proper juices, but convey them to all the cellular parts of plants, and are the true proper vessels of the bark. We before saw that Μ. Kieser, after making the wood to consist almost entirely of spiral vessels, destined only to convey air, supposed the sap to be carried by these same intercellular canals. (Kieser, Mém. sur l’ Organisât, des Plantes, p. 36.) With Μ. Treviranus, he also considers them as the real proper vessels of the bark, by which alone the juices are conveyed. (Ibid. p. 86.)

Μ. de Candolle considers the proper juices to possess no special organ for their conveyance, but to nestle in the cavities that surround them, and form sacs or reservoirs of various size and form. He distinguishes several kinds of these reservoirs, which vary with the figure of the containing sac. Thus, what others call “proper vessels,” he designates by the names of tubular and fascicular reservoirs. In this, however, he appears to confound the “proper juices,” destined immediately to the functions of nutrition and growth, with various fluid or solid matters, either simply deposited in the cellular tissue, or that have undergone subsequent alteration by the agency of a secreting organ. The reservoirs of the proper juices, and the juices themselves, are, he says, readily seen in many families of dicoyledons; but have not been observed with certainty, either in monocotyledonous or acotyledonous tribes. ( Organog. Veg. tome i. chap. 11.) The reason of this seems to be, that in dicoyledons the bark, in which the proper juices are seen, is distinct from the wood, in which the common sap flows; and the colour, consistence, and qualities of the two' fluids, therefore, at once point out their respective places; but in plants belonging to the two latter tribes there is no proper distinction of bark and wood; and the two orders of vessels, as shown by Malpighi in the instance of wheat, are associated together through the whole substance of the plant, which renders the discrimination of their common and proper juices more difficult.

Besides two orders of vessels existing in the ligneous part of trees, Μ. Dutrochet describes a particular form of cell, which, in the wood, serves only as a receptacle for fluids; but to which, in the bark, he assigns a vascular function. This is the elongated cell observed by Malpighi, and represented by him in fig. 18, Plate XXXIX. Μ. Dutrochet, from the resemblance which its figure is said to bear to a spindle, has given it the name of clostre; and his representation of it is given in fig. 17, Plate XXXIX. These elongated cells form what Kieser and others have called the ligneous fibres. By Μ. Du-trochet they are regarded as reservoirs of the nutrient juices, and as containing secretions which impart solidity to the wood. In the bark, however, these same clostres are declared to be the organs in which the elaborated sap is contained, and by which įt is transmitted to the roots to furnish the materials of their growth. The clostres, which thus convey the nutrient matter, ought not, he adds, to be confounded with the “proper vessels,” which are the true secretory organs. These vessels are tubes of greater diameter than the clostres, and contain various excrementitious substances, as resin, &c. in different plants. (Recherches Anatomiques, &c. p. 36, 37.) According to Μ. Dutrochet, therefore, the elongated cells and vessels which exist in the bark and wood exercise functions precisely opposite; for whilst in the wood the corpusculiterous vessels convey the sap, and the cells are reservoirs for containing it, in the bark the cells are the organs which transmit the elaborated sap, and the vessels those in which the excrementitious matters are contained.

In our view, these notions are the reverse of fact, since we consider the vessels, both in the wood and the bark, as the organs by which the fluids are conveyed, and the various forms of the cellular tissue as the general receptacles into which they are in part deposited: they may, however, in some instances be retained in the vessels themselves, as well as in the cells.

Section II.

Of the Absorbent and Exhalent Systems. Art. I.— Of the Absorbent System.

Connected with the vessels that distribute the fluids through organized bodies, is another system of vessels, by which extraneous matter is taken -up to support the growth of parts, and supply the waste occasioned by the exercise of the various functions. To these vessels anatomists have given the name of Absorbents. The function which they perform is carried on either from the external surface, or from some internal part of the body; and its exercise, in animal bodies, may be distinguished into three kinds or stages. The first is that in which new or extraneous matter is taken up and added to the system, as in the absorption of substances from the skin, or of chyle from the intestines; the second is that in which substances previously separated from the fluids by secretion, but without becoming organized, are again taken up by the absorbents, and reconveyed into the blood-vessels, as in the absorption of milk, bile, and fat; the third kind is that in which the secreting organs themselves, and successively all the solid parts of the body, are removed by the action of the absorbent vessels. By plants this function is exercised to less extent, and seems to comprise only the first two kinds or degrees of it, viz. the primary absorption of extraneous matter, and the reabsorption of certain secreted matters; but in them there [7:3:55]does not appear any power of re-absorbing what has once formed an organized part of the system. Hence the formation of organized parts in plants is not accompanied, as in animals, by the unceasing removal of old particles; but the particles which have once become organized continue permanent until removed by some cause or process foreign to the living powers of the plant. When once, therefore, the organs of plants have become mature, they are exposed to decay only from the operation of foreign causes, and cannot be mined from within by that gradual loss of balance between the secreting and absorbing functions which the advances of age bring on the animal system. Neither, in the vegetable system, can the removal of organized parts under disease, any more than in health, have place; and, consequently, the several modes or stages of ulcerative absorption, so finely illustrated by the researches of Mr Hunter, belong not to the economy of plants.

As the function of absorption is thus of more limited extent in plants than in animals, so may we expect to find the arrangement of organs destined to its exercise. In the more perfect animals, the absorbent vessels are quite distinct from those which carry blood, are provided everywhere with glands, and, like the veins, are furnished with valves. From their beginnings, in all parts of the body, to their termination, they continually unite with one another, until, after a long course; they form at length two trunks, which deliver their contents into the large veins near the heart. As no common reservoir exists in plants, there is no such single point to which their absorbed fluids require to be carried. Hence their absorbents seem everywhere to have a very short course, to form no union with one another, but to deliver their contents at once into the sap-vessels adjacent to them. They appear to be destitute alike of glands and valves, and indeed, in an anatomical view, they can scarcely be considered distinct from the sap-vessels, but may rather be deemed ramifications from them; so that although we grant, with Grew, that the ordinary sap-vessels do not ramify one out of another, yet they certainly send off those fine ramifications which, from their office, we denominate the absorbents of plants.

In the root, the absorbents are capable of being demonstrated. When a plant is immersed in coloured fluids, many of its capillary absorbents become tinged through their whole course to their termination in the sap-vessels, proving them to be simple ramifications from these vessels themselves. A further proof of the identity of these two systems is derived from the fact, that any part of a sapvessel is not only capable of emitting capillary absorbents from its sides, but of exercising itself an absorbent function, whenever its cut extremity is brought into contact with a fluid. These absorbents are formed very speedily and in great multitudes on the roots of annual plants; and even in perennial plants they appear, like the leaves, to suffer an annual decay, and to be reproduced with the return of vegetation.

Μ. de Candolle describes a structure as existing at the extremities of the roots and of the pistil, and also on the outer coat of seeds, which possesses a peculiar aptitude for absorption. To this structure he gives the name of spon-giole: it is composed of small round cells, and in roots is situated at all their fibrous extremities. The entire body of each radical fibril is indeed made up of an analogous cellular tissue; but it is the extremity alone which actually absorbs, and which therefore possesses a peculiar absorbent or hygroscopic power. The pistillary spongioles are in like manner situated at the extremity of that organ, and absorb the fecundating liquor. The third variety, or seminal spongioles (spongiolae seminales), are placed on the surface of the grain, apparently with great regularity; and by them the absorption of moisture is effected. Not only water, but coloured liquors, are absorbed by these spongioles, though the molecules of such liquids never enter the ordinary pores of plants, which pores must be infinitely greater than the apertures with which the spongiolcs may be furnished. This is the more remarkable, says Μ. de Candolle, when we call to mind that these colouring molecules traverse the compact and almost stony coats of seeds, and yet do not enter leaves whose tissue is so lax, and which are visibly furnished with pores that have the power of absorbing water when in contact with it. ( Organog. Végétale, tome i. chap. 7.)

But plants absorb fluids by other parts of their surface as well as by the roots. This absorbing power extends, in some of the lower tribes, over the entire surface of the vegetable, which is destitute of any organ analogous to a root; in other instances, where the roots are small and the soil arid, the plant derives almost all its moisture by the absorption of dews through the leaves. The experiments of Bonnet show that all leaves, both those of herbs and of trees, when brought into contact with water, are capable of absorbing it, and that the moisture thus absorbed is communicated through the vascular system of the leaf. The leaves of herbs he found to absorb nearly alike from either surface; but those of trees absorbed best by the lower surface. The petiole and larger riblets appeared to absorb much less than the other parts of the leaf. So great and general is this absorbing power, that vegetables, says he, may be said to be planted in the air, nearly as they are in the earth, the leaves being to the branches what the capillary rootlets are to the roots. (Recherches sur ι Usage des Feuilles, p. 22, 47.)

What then are the organs by which this function is carried on in the leaf? Μ. Bonnet imagined the vessels of the leaves to receive their fluids through the pores adjacent to them; and that the leaves, which had only few pores, possessed but little absorbent power. (Ibid. p. 20, 22.) He thought, also, that the hairs frequently distributed over the leaf attracted moisture, and might even act as absorbents; though he admits that many leaves which have only slight inequalities on their surface, without hairs, exercise an absorbent function. (Ibid. p. 47.)

This subject has been since investigated by Μ. de Candolle, whose researches appear to confirm the account of Bonnet as to the absorbing power of the pores. According to him, these pores are found on all parts of the leaf except the riblets, which have none, but are covered with hairs. At the mouth of the pore a vascular net-work is always to be found, which he regards as a production from the larger vessels of the leaf. He asserts that pores are found only in those parts where vessels go to terminate, and not in others; and that in trees this structure occurs chiefly on the lower surface of the leaf, while in herbs it is equally seen on both surfaces. The stem in general has few or no pores, except where it is soft and herbaceous; and even then the pores occur only in the deeper green furrows, not on the prominent lines which bound them, and are usually covered with hairs. No pores are to be found on roots or bulbs or fleshy fruits, but most of the organs above ground are more or less furnished with them. Exposure to the air seems necessary to their formation; for plants, or parts of plants, that live beneath water or earth, are destitute of pores, but acquire them if brought into the free air. Exclusion of light prevents also the formation of pores; and hence etiolated plants are not furnished with them. (Mém. de l’lnstit. Nat. tome i. p. 351.) In general, hįs anatomical researches respecting the ex [7:3:56]istence of pores in leaves agreed with the results of Bonnet’s experiments on the absorbing power of their surfaces; and when we consider that all plants and parts of plants secluded from the air are at the same time destitute of pores, and of the power of absorbing by their surfaces, it may be inferred that the organs of absorption on the external surfaces of plants are the minute vascular productions which, in tender and succulent parts exposed to the air and light, everywhere perforate the cuticle, and form in it those innumerable orifices which we denominate pores.

But the function of absorption in plants is not confined to the taking up of extraneous matters. Many facts prove that, in every part where active vegetation exists, internal absorption is continually going on. The organs by which this absorption is immediately performed cannot, perhaps, from their extreme delicacy and minuteness, be rendered capable of anatomical demonstration; but certain facts which occur in plants, coupled with certain analogies derived from other organized textures, must, we think, carry complete conviction of their existence. In almost every part, and on every surface of animal bodies, the vessels which exercise absorption may be traced; but the mouths or orifices by which they actually absorb are scarcely ever to be seen. In one instance only, viz. in the intestines, have they been followed to their beginnings, and discovered in the act of exercising their appropriate function. (Gordon’s System of Human Anat. vol. i. p. 70.) But though their orifices remain in other parts undiscovered, no anatomist hesitates to admit their existence when he sees the canals of the vessels themselves laden with blood, or milk, or bile; and seeing them thus to convey fluids whose colour manifests their presence, he equally believes them capable of absorbing and conveying other fluids, though they may not be visible to the eye. Believing further, that no solid part of the body, nor even fluid part that has been deposited in close cavities, can be removed in a natural manner from its place, but by the agency of these vessels, he comes to regard the simple fact of the disappearance of such solid or fluid part as sufficient evidence of its absorption.

Now the facts and analogies on which internal absorption rests in the vegetable system are precisely of the same nature, and the evidence of its existence is scarcely less complete. In every part of the cellular tissue of plants various substances have been found which must have been primarily derived from the vessels, the only organs which furnish new materials to the plant. These substances, however, often disappear from the cells, and are again to be detected in the vascular system. Thus, in the seed, as will afterwards be shown, the cells of the cotyledon contain a solid unorganized matter, which could have been originally deposited in them only by means of the vessels. During germination, this solid matter is rendered fluid, disappears from the cells, and is again to be traced in the vessels on its way to afford nutriment to the radicle and plume. We say, then, that this unorganized matter must have been taken up from the cells of the cotyledon, and conveyed into the vascular system, by the agency of absorbent vessels, which, it is probable, are distributed everywhere on the inner surface of these cells; just as, in the animal system, absorbent vessels are considered to take up the fat from the inner surface of the cells in which it is deposited, and convey it into the vessels of the animal.

This alternate absorption and deposition of the nutrient matter of seeds is sometimes strikingly displayed in the growth of potatoes. It frequently happens that potatoes lying in a damp cellar put forth shoots which grow to a considerable size, without the access of any foreign agents except heat, water, and air. On these shoots young bulbs, as large as the eggs of pigeons, are sometimes to be found, and the substance of the old bulb has in great part disappeared. In such cases, the matter from the cells of the old bulb must be considered as removed by absorption, and conveyed into the vessels of the shoot, where it was in part employed in forming the new organs of the young bulb, and in part deposited, to experience, perhaps, in some future growth, similar successions of removal and deposition. In the living parts of perennial plants, also, nutrient matter appears to be alternately deposited and absorbed from the cells during the active periods of vegetation; and in the cellular tissue of herbaceous plants a similar deposition and absorption of fluids seems to be frequently taking place; so that, in all the vegetating parts of plants, these alternate functions of secretion and absorption are more or less constantly exercised.

A good illustration of the manner in which these functions are alternately exercised is afforded by an experiment of Μ. de Candolle. The parasitic plant called misletoe draws its nourishment, as is well known, from the tree on which it grows. Μ. de Candolle placed a branch of an apple-tree, bearing a stalk of misletoe, in an infusion of cochineal for five days. He then dissected it, and observed the coloured liquor to have risen through the wood and alburnum of the apple-branch, and reached the place of junction between it and the misletoe, which it strongly reddened; and from thence it penetrated into the woody part of the misletoe. There did not, however, appear to be a true anastomosis between the vessels of these different plants; but, at the base of the misletoe, where the parts were so deeply reddened, a minute cellular structure was observed. Into these cells the vascular system of the apple appeared to deposit its sap, and from them the capillary absorbents of the misletoe, distributed upon the cells, seemed, like the ordinary absorbents of roots, to take it up. (Mém. de l’lnstit. Nat. tome i. p. 370.) From these and many similar facts it may be inferred, that absorbents, communicating with the vessels of the plant, exist in every part, and that the removal of all secreted matters from the cells and other close cavities of the vegetable, when effected by the living powers of the plant, is accomplished, as in animal bodies, by the exercise of an absorbent function.

Art. IL— Of the Exhalent System.

But from their external surface, and from the same parts as we have seen to exercise an absorbent function, plants, in certain circumstances, give off a large quantity of fluid by transpiration; and the organs by which this function is performed seem, from many considerations, to be the same as those by which, under other circumstances, absorption is accomplished. This function of transpiration is common to all terrestrial plants, and, with few exceptions, all are more or less furnished with pores; hut it does not occur in aquatic plants, according to De Candolle, which are destitute of pores. Fleshy leaves and fruits, and the petals of flowers, which have but few pores, transpire little; and etiolated plants, which are destitute of pores, do not transpire at all. On the contrary, herbaceous stems and plants, which have numerous pores, throw off most fluid by transpiration. This general agreement between the existence of pores and the exercise of the transpiratory function leads to the presumption that they are the orifices through which the fluids are discharged; and if it be admitted that these pores are situated at the extremity of the fine ramifications that come [7:3:57]’ off from the vessels, their fitness for such an office cannot be denied. Comparing these facts regarding transpiration with those previously stated concerning absorption, Μ. de Candolle is led to conclude that the pores on the surfaces of plants are the organs by which these functions are alternately carried on, according to the existing condition of dryness or humidity in the surrounding atmosphere-

Repulsive as this conclusion may at first seem to our ordinary conceptions of organized bodies, yet there are many circumstances in the structure and habits of plants that give to it great probability; so much so, that we ourselves had long since reached the same point by a route different from that pursued by Μ. de Candolle. It is highly probable that the exhalents of the leaves are simple ramifications from the larger vessels, like the capillary rootlets; and as they have no valves in their canal, there is no mechanical impediment to their exercising an inverted action. The sap-vessels themselves readily absorb even coloured fluids when inverted; and though their exhalent terminations are too fine to receive such fluids, yet why may they not, like the trunks from which they spring, be capable of taking up ordinary fluids in that manner? The fluids absorbed through the leaves must at once enter the sap-vesśels, for there is no common reservoir to which they can first be carried; and it is extremely improbable that, from the same parts of the same vessels, exhalents and absorbents, capable of exercising only opposite functions, should at the same time arise. In the animal system the exhalents spring from arteries, and the absorbents terminate in veins; but in the less complex structure of plants it seems demonstrable that both orders of vessels must at once communicate with the same sap-vessels. It is therefore more probable to suppose that, instead of two distinct orders of vessels, as in animals,, one only should be provided, capable, under different circumstances, of exercising different functions. This vicarious office of the organs under consideration leads to no confusion in its exercise; for the condition of the atmosphere, which favours transpiration, is that which removes from the leaves the power of absorption; and, on the contrary, absorption occurs only in a humid atmosphere, when, as Hales has shown, little or no transpiration takes place.

Besides exhalation carried on through visible pores, plants, like other bodies, lose their humidity by simple evaporation. This evaporation seems to go on in a moderate degree under favourable circumstances by day and by night, and is quite distinct from the copious transpiration that is carried on by the agency of solar light through the pores. That the pores are exhaling organs, was an opinion first advanced by Hedwig, and subsequently illustrated *by De Candolle, Sprengel, Link, and Rudolphi. (Organog. Végétale, tome i. p. 86.) In like manner, though absorption may be carried on in certain circumstances through the pores of leaves, yet there are plants which largely imbibe water, but in which few or no pores have been detected. Thus, fleshy leaves which have lost their moisture quickly regain it either when plunged in water or exposed to rain, though they possess but few pores; and’; The aquatic algae, says Μ. de Candolle, evidently imbibe water over their whole surface, though they are wholly destitute of pores. He supposes that, beside the ordinary pores which may be readily seen, plants may be furnished with others that are invisible from their extreme minuteness. By such pores we might account for the loss of weight which plants destitute of visible pores gradually undergo when exposed to the free air, and also for the increase of weight which mosses, and other plants equally destitute of pores, rapidly acquire when plunged in water. (Organog. Végét. tome i. p. 88.)

The view thus presented of the external absorbent and exhalent vessels may probably be extended to the' minute vascular productions which seem everywhere toļ spring from the vessels internally. If secreting and absorbing vessels be held to exist in every part of the plant, ∣ they must everywhere communicate with the vascular system; for it is from the vessels of this system that the matter of their secretions is primarily derived, and it is into the same vessels that, in many cases, these secretions are subsequently returned. Nor does the exercise of the two functions of secretion and absorption in plants present. any apparent obstacle to the supposition of their being performed at different times by the same organs. Thus, when nutrient matter is deposited in the cellular tissue of the seed, it is destined only for a future use; and the purpose of nature would be defeated were an absorbent function to be at the same time employed for its removal. On the other hand, when this matter is again taken up during the germination of the seed, no secreting function seems then to be exercised in that part; for the organ itself, in many seeds, gradually wastes, and no fresh matter is deposited in it. Even when the, cotyledon to a certain extent augments in size, its nutrient matter is continually drawn off for the support of the, radicle and plume, and no fresh matter of the same kind seems to be then deposited; so that the same vessels which formerly exercised the function of secretion may, without disturbing the economy of the plant, be now employed in the exercise of absorption. As thus the two functions do not require to be performed in the same part at the same time, they may, if nothing else forbid, be exercised at different times by the same organs. In the animal system, where the organs themselves are removed, secreting and absorbing vessels must necessarily co-exist; and to maintain the integrity of parts, their functions must proceed at the same time, and to a certain extent balance each other; but as no similar operations appear to be carried on in the vegetable system, no such complex organization is required to sustain them.

Section III.

Of the Cellular Tissue. Art. I.— Description and Structure of the Cells.

The elementary organ denominated cellular tissue may: be said to consist of a membranous substance, disposed into a great number of small circumscribed cavities, connected with each other, and arranged in rows or suites, generally in a direction opposed to that of the tubes, which represent the vascular system. From Grew it received the appellation of parenchyma, a term still often used in describing different parts of this tissue; by Malpighi, it was called the utricular substance ; and it owes, we believe, its present name to Μ. du Hamel.

The cavities which distinguish its construction were: called indifferently bags or bladders, pores and cells, by Grew; by Malpighi, utricles; by others, vesicles; and more commonly cells. The form of these cells varies so much, not only in different plants, but in different parts of the same plant, as to have authorized, in. some degree, these different appellations. The tissue which they constitute enters into the composition of every organ in the more perfect plants. Of many herbaceous plants it forms the chief portion, and some of the lower tribes of vegetables, as the algae, lichens, mushrooms, &c. are said to be wholly composed of it; in other words, no vessels can be [7:3:58]actually demonstrated in them. In most cases it contributes greatly to modify the form of organs, and adds always to their bulk and strength. Nothing can exceed the diversity of appearance in figure, bulk, and texture, which it exhibits in the several parts, circumstances, and conditions in which it is placed. It represents sometimes a lax cellular substance, all the parts of which are succulent and transparent; in other instances it is compressed into a solid, opaque body, retaining but faint traces of its former cellularity; and in others, again, it is spread out into a most thin and delicate membrane, in which the cellular character is wholly lost. It everywhere envelopes and holds together the vascular system, and seems to be the général receptacle of almost all the vegetable secretions.

The figure of the component cells of this tissue is exceedingly various. Sometimes they have nearly a globular or spheroidal shape; in other instances they are angular, and exhibit in their section a greater or lesser number of sides and angles, being in a few examples triangular; in others square, but more commonly hexagonal, the figure which collections of soft cells, mutually impressing each other, seem naturally disposed to assume. This form is represented in the transverse section of cells, fig. 28, Plate XXXVIII.; they seem in this figure, and in most of those given in different works, to possess double sides; but, as Μ. Kieser has remarked (Mém. sur l’ Organisât. des Plantes, p. 91), this appearance is produced by the borders of the subjacent cells being seen through the transparent sides of the superior layer. In Plate XXXIX. fig. 16, is a representation of a transverse slice of the cellular part of sugar-cane, drawn from nature, and so thin as to exhibit only one layer of cells, in which the sides appear distinctly single; but, in a thicker slice of the same plant, fig. 21, comprehending more than one layer, the double appearance becomes very evident.

The size of the cells varies not less than their figure in different plants and in different parts of the same plant. In one of the plates of Grew they are represented as possessing twenty different sizes, from that of a minute pore, to the size of a common pea. Hooke examined them in cork, and in the pith of many plants. In cork he reckoned several lines of these cells or pores, as he calls them, and found there were about sixty placed endwise in one eighteenth part of an inch, or somewhat more than a thousand in the length of an inch; and, therefore, in a square inch above a million, and in a cubic inch above 1200 millions. (Micrographia, p. 114.) In this substance the cells are not visible by the naked eye, but become very distinct when highly magnified. In most plants, however, they are readily visible, and their appearance is familiar to every one.

When viewed in a longitudinal section, their hexagonal form is much less distinct, and is sometimes wholly lost. In fig. 22, Plate XXXIX., is a series of single columns of the cells of sugar-cane, in which each cell is, to appearance, bounded only by four sides. Similar representations are given by Hooke of the cells in cork, and by Kieser in most of the figures which exhibit longitudinal sections of the cellular tissue; but in some instances the hexagonal form is visible even in these sections. In fig. 23 of the 'βame plate we have given, in outline, the appearance of two series of columns of these transparent cells, in which one series is seen behind the other, and gives somewhat of the confused double appearance exhibited in the transverse section, fig. 21.

The nature of the matter contained in the . cells of this tissue varies according to the part in which it exists, and the peculiar powers of the plant. Both Hooke and Grew remarked, that in the pith and bark of succulent plants the cells were often filled with aqueous juices, and in the same plant, at other periods, they appeared empty, or filled only with air. In the seed, the cells of the cotyledons contain minute unorganized particles, which, at a future period, serve as nutriment for the young plantule. Other particles of still smaller size, of a resinous nature and a green colour, exist in other parts of this tissue, and bestow on the plant its verdure. In every part of the plant these cells are also the occasional receptacles of the peculiar fluids which both the sap-vessels and the proper vessels convey; and hence various gummy and resinous substances, corresponding in quality to the fluids previously existing in the vascular system, are frequently detected in them. In the pulp of fruits, the various acid, saccharine, and austere juices that we meet with, are contained in different modifications of this tissue; and it is into its cells that the osseous secretions, which constitute their shells and stones, are made. These facts prove not only the great importance of this tissue in the construction of the vegetable organs, but the active share it bears in the economy of their functions; and demonstrate likewise an universal communication between the vessels and the cells.

The sides of these cells, when emptied of their contents, and viewed through the microscope, appear to be formed by a very fine transparent membrane, which some maintain to be everywhere entire, and others to be perforated with pores. The same sources of error exist here as before noticed in similar microscopical observations on the vascular system; and, accordingly, the respective disputants maintain, with equal confidence, the same opinions with regard to the porosity or non-porosity of the cells, as they had previously held concerning the vessels. We must therefore call in the aid of other means besides those of the microscope, for determining the important fact whether the cells have or have not any direct communication with each other.

Dr Hooke examined the films or sides of the cells of cork, of the pith of elder, and of many other plants, for' the very purpose of discovering whether any direct communication existed between them; but “each cavern or cell,” says he, “is distinctly separate from the rest, without any kind of hole in the encompassing films nor could he, with his microscope, nor by his breath, nor by any other way that he tried, “discover a passage out of one of those cavities into another.” {Micrographia, p. 116.) Dr Grew describes the little cells or bladders that compose the bark of roots as possessing a spheroidal shape in most plants. When viewed with the microscope, their sides are as transparent as water; and “none of them,” he adds, “are visibly pervious from one into another, but each is bounded within itself.” (Anat. of Plants, p. 64.) Both Hooke and Grew, however, believed a communication to exist between the cells, from the fact of their containing liquor; and Malpighi held the same opinion from similar considerations; but they nowhere describe the mode or structure by which they conceived it to be accomplished.

Later writers have not only adopted this opinion, but professed to demonstrate the. structure by which the communication is maintained. Μ. Mirbel describes the sides of the cells as composed of an extremely thin, colourless, and transparent membrane, which is commonly perforated with pores, the diameter of whose aperture is not perhaps the 300th part of a millimetre. These pores are ranged generally in transverse series, and through them, it is said, the cellular tissue both receives fluids. from the vessels, and transmits them very slowly [7:3:59]through its cells. (Exposition de la Théorie, &c. p. 105.) Μ. Sprengel and some others adopt this view of the porosity of the cells; but it is denied by Link, Treviranus, and Kieser. The latter author declares, that notwithstanding all that has been said concerning the pores in the sides of the cells, his observations, made with the greatest care and exactness, have not enabled him to discover the slightest trace of them. The sides of the cells, he adds, are always formed by a membrane extremely thin, but altogether smooth and uniform; and the cells themselves have never an open communication with each other. (Kieser sur l’ Organisât. des Plantes, p. 94.)

If, then, no pores exist in the sides of the cells for the reception and transmission of the fluids they contain, some other means must be provided for the accomplishment of these objects. Μ. Link, accordingly, supposes the juices to pass from one cell to another by transudation. Μ. Rudolphi thinks that a decomposition of the fluid is effected by the cells themselves, during which it is transmitted through their sides; and Μ. Dutrochet calls in the aid of electrical agency. To us there occur no probable means of accomplishing these operations, consistently with the integrity of the cellular structure, but the exercise of those alternate functions of secretion and absorption which, from so many other considerations, we have supposed to be carried on in every living part of the vegetable system.

Another question of importance in relation to the sides of the cells is, whether they are single or double; that is, whether each cell has a side of its own, or whether one side is in every position common to two cells. Mirbel •asserts the former, and Kieser maintains the latter opinion. In fig. 29, Plate XXXVIII., is an outline representation of these double sides as given by Kieser. From the extreme thinness of the membrane, it is very difficult, he says, to distinguish this double structure; but where the cells are large, and a glass that magnifies highly is employed, each partition that separates two cells is distinctly seen to be composed of two membranes, which are sometimes separated about the middle of the partition, and united towards the angular points. (Mém. sur ĭ Organisai. des Plantes, p. 91.) MM. Amici, Dutrochet, De Candolle, and others, support this opinion. (See Organog. Veg. tome i. p. 21.) The existence of this double structure receives some countenance from the fact observed in the construction of the honeycomb by the late Dr Barclay, who says that each side of every cell in the comb is composed of two plates, or is double. (Wernerian Transactions, vol. ii.) It may still, however, be more properly said, that each side of every cell is truly single, and is rendered double only by coming into contact with the corresponding side of an adjacent cell.

When the cells have a regular hexagonal figure, and arc equally distended with their appropriate juices, there is no reason to suppose that any vacuities are left between their sides or angles. Mathematicians have long since demonstrated a regular hexagon to be one of those figures that completely fill up a given space; and that no vacuities can exist either about its sides or its angles. Where, however, the cells deviate from this regular figure, and more or less approach to a spherical form, vacuities or interstices may readily be conceived to occur. These vacuities are said to have been first noticed by Grew and Lecuwenhoeck, and afterwards by Μ. Trevira-ríus, who describes them as interstices left between the cells in their mutual approximations towards each other. He gave them the name of intercellular canals, as already stated.

On the'other hand,' MM. Mirbel and Rudolphi altogether deny their existence; but Μ. Kieser contends strenuously for it. -He describes them as small interstices situated at the angles of the hexagonal cells, and formed not by any sides of their own, but by the mutual approach of three contiguous cells, and possessing, therefore, a prismatic form. These interstices he conceives to exist at every angle, and thus every cell to be surrounded by them. In fig. 29, Plate XXXVIIL, the black angular points denote their place. By their conjunction with each other they form a canal, which, when the hexagonal figure is perfect, and the cells are ranged horizontally, extends both in a longitudinal and transverse direction; and when the cells are placed obliquely, the canals have a similar direction. Their size varies according to that of the cells, by the sides of which they are constructed; they contain and convey the proper juices in the bark, but in the pith are often dry; and their course is said to terminate only with that of the cells themselves, at the surface of the plant. (Mém. sur l , Organisat. des Plantes, p. 104.) Such are the organs which, as we have seen, Μ. Kieser considers to convey both the sap and the proper juices in plants. That in some circumstances they may exist, and become reservoirs of the sap or other juices, seems highly probable; but of the impropriety of ascribing to such casual productions the performance of the primary functions of the vegetable system, we have already spoken.

The cellular tissue, as described above, is that form of it which must be regarded as the most perfect. From various causes, however, it is subject to great alterations. In herbs, and in the pith and succulent parts of trees, the cells preserve their original form and appearance for a considerable time; but by the growth of the other parts, and consequent extension and compression they experience, they acquire in the bark and wood an elongated figure, and this both in a transverse and longitudinal direction. In the latter case they surround and connect the layers of vessels with each other, constituting what has been named the parenchyma of the bark and wood. In this form their size is often greatly reduced, their cavities sometimes obliterated, and their cellular character altogether effaced. In other instances traces of a cellular structure are occasionally visible, appearing in detached portions among the perpendicular vessels. In some plants the cells become elongated in a longitudinal direction, and yet preserve their capacity nearly unchanged. It was probably this form of cell that led Malpighi to regard sometimes as a vessel, what, in reality, appears to be only a series of elongated cells. To his representation of these cells, as given fig. 18, Plate XXXIX., we have before referred. Μ. Kieser considers these elongated cells as forming the ligneous fibres of the wood, and that the bark is almost entirely formed of them. (Mém. sur l’ Organisat∙des Plantes, p. 99, 101.) These cells, he says, were originally round; but in trees they become so much lengthened as to exhibit the form of a tube. It is easy, however, he adds, to detect transverse partitions in these seeming tubes, which, having often a diagonal direction, give to these cells the form of a double pyramid with sharp points. The membrane which forms these cells is, in all trees except the Coniferae and some others, smooth, without pores, and transparent. (Ibid. p. 299.) To these organs Treviranus gave the name of fibrous utricles; by De Candolle they are called tubulated cells; and Dutrochet, from their spindle shape, denominates them, as we have seen, clostres. His representation of them, fig. 17, Plate XXXIX., corresponds with the foregoing description of Kieser.

From suffering compression in a transverse direction, the cells have frequently their longer diameter thrown [7:3:60]into that position, and thus extend from the centre to the circumference of the plant. This position, as will afterwards be shown, was fully noticed by Grew and Malpighi. Lecuwenhoeck also observed it, but mistook the cells thus elongated for vessels, and considered their partitions as valves,—errors which Μ. Kieser, as well as others, duly points out. In this transverse direction the tissue forms partitions more or less large between the vessels, as will afterwards be shown; and by the obliteration of its cells it is frequently reduced to the condition of a solid membrane.

Besides these more constant and necessary changes in the figure and character of the cellular tissue, it often suffers others of a more casual and accidental nature. In the pith, as the plant grows up, divers ruptures, says Grew, occur, oftentimes very regularly, and observed constantly in the same species of plant. These ruptures are sometimes prolonged, so as to form a tube of considerable length. (Anatomy of Plants, p. 120.) Others have observed similar canals in the pith, formed not by sides of their own, but by those of the adjacent cells, and very various in size and form. They have been called lacunae, or reservoirs, contain a variety of substances, and sometimes, especially in aquatic plants, only air. As we have seen the cavities of the larger spiral vessels to be filled with vesicles, so the larger cells of the pith, according to Grew, frequently contain smaller ones, or are divided by cross membranes. A similar observation is made by Kieser, who likewise remarks that, in the empty cells of Calla Aethiopica, he has sometimes seen small round-headed bodies, supported on little peduncles, which spring from the sides, and point towards the centre of the cells. Small crystallized bodies are also occasionally found in the cavities of the cells, and within the intercellular canals. (Mém. sur l’ Organisât, des Plantes, p. 94.) Of those changes in the character of the cellular tissue, by which its cells are converted into receptacles and reservoirs of the proper juices of the plant, we before discoursed when treating of the proper vessels. To such an extent does this change sometimes proceed, that, in aged oaks, and, according to Kieser, in guaiacum, and probably in many other plants, the whole cellular tissue becomes filled with these secreted matters, and the distinctive characters of the cells, and almost of the vessels themselves, are obliterated and lost.

CHAP. II.

THE ANATOMY IN GENERAL OF THE COMMON TEXTURES OF VEGETABLES.

Preliminary Observations.

The elementary organs, whose description has so long occupied our attention, form, either singly or by their combination, all the other parts of plants. Some of the lower tribes of vegetables consist entirely of cellular tissue, in which no vessels are at any period to be seen; and, even in the higher orders, many parts exhibit no appearance of a vascular structure. There can be little doubt, however, of the existence of such a structure, since, physiologically speaking, we can form no just conception of the growth of an organized body, without associating with it the existence of a vascular system. In all plants the pith consists of a cellular tissue alone. In herbaceous plants this tissue forms their greater portion; but in trees the number of vessels is so great as to constitute the chief bulk of the plant. To certain forms of these elementary organs, whether existing singly or in combination, we have given the name of common textures, because they are very generally to be found in all plants, and in almost all parts of them, howsoever varied in quantity, proportion, and arrangement. These textures are familiarly known under the names of cuticle or skin, of bark, of wood, and of pith; to which may be assigned the general appellations of the cuticular, the cortical, the ligneous, and medullary textures.

All the several textures just enumerated are readily distinguished by their different places and characters in the section of most arborescent plants, in which they commonly appear well defined, and perfectly distinct from each other. In many plants, however, both herbs and trees, this distinction of parts is not preserved; but, with the exception of the cuticle, all the other textures are blended together through the entire substance of the plant, as was long since noticed both by Malpighi and Grew. “In the stalk of maize or Indian wheat,” says Grew, “the work of nature appears less diversified; in which, although there are the same parenchymous and ligneous parts as in all other plants, yet is there neither bark nor pith, the vessels being dispersed and mixed with the parenchyma, from the circumference to the centre of the stalk.” “The like structure,” he adds, “may also be seen in the sugar-cane and some other plants.” (Anat. of Plants, p. 104.) Similar observations were made by Malpighi, not only on different species of wheat and sugarcane, but on ferns and palms. “In ferns,” says he, “the vascular fasciculi are numerous, but placed without order, and are everywhere sustained by the intervening cellular tissue, the cells of which are sometimes much smaller than the orifices of the vessels themselves.” Anat. Plantar. p. 24, 25.) This structure is represented in the transverse section of the sugar-cane, Plate XXXIX. fig. 25; and in a similar section of the palm, fig. 26 of the same plate.

This variety of structure, thus clearly described, and distinctly delineated in the works of Malpighi anel Grew, has likewise been noticed by Μ. Desfontaines. Vegetables, according to him, may be distinguished into two divisions: Is/, Those which have no distinct concentric layers, whose solidity decreases from the circumference towards the centre, and whose pith is interposed among the vessels, and does not extend in divergent rays: 2d , Vegetables which have distinct concentric layers, whose solidity decreases from the centre towards the circumference, and whose pith is contained in a longitudinal canal, and extends in divergent rays. The former structure he considers as peculiar to plants whose seeds are monocotyledonous, and the latter as belonging to those which have dicoyledonous seeds. (Mém. de l’lnstitut. Nat. tome i. p. 478.)

This opinion, though partly correct, is not universally applicable. That many plants which spring from monocotyledonous seeds are destitute of concentric layers, and have’ no distinct bark or pith, is certain; but it is not less certain that many herbaceous plants, which are produced from dicoyledonous seeds, are pretty much in the same condition, being equally destitute of concentric layers and of divergent rays, and in which the bark and the pith must be regarded as one continuous structure. On the other hand, some monocotyledonous plants, as Μ. Desfontaines admits, may deviate a little from the prescribed conditions. In a paper on the organization of such plants, Μ. Mirbel, who regards this doctrine as the most important step made of late years in Vegetable Anatomy, says, nevertheless, it would be erroneous to assert that they have never a bark. In several species of plants he produces examples to the contrary; and adds, that in Some instances their diametral growth goes on at the cir [7:3:61]cumference, which would seem to approximate them to dicoyledons. As, however, there is no appearance of divergent rays, or of concentric layers, these examples are considered by him rather to confirm than overturn the theory of Μ. Desfontaines. (Annales du Mus. d ’ Hist. Nat. tome xiii. p. 67.) But if in this theory its second division embrace only those plants in which the concentric layers are perfect, and divergent rays exist, then it excludes a great number of herbaceous plants, whose seeds have two cotyledons; and if the absence of these regular layers, and of divergent rays, serve as a passport to the first division, then many of these same plants must be admitted among those whose seeds, have but one cotyledon. The theory of Μ. Desfontaines, therefore, rests on too partial an observation of the structure of plants.

Section I.

Of the Pith or Medullary Texture.

The pith (medulla) of plants, when present, occupies the centre of the stem, where it is commonly surrounded by a circle of vessels which construct for it an appropriate canal. In the succulent shoots of trees, its proportion to the other parts is generally large; but it diminishes as the tree advances in age, and is frequently entirely obliterated. Where the vessels of the wood are few in number, as in herbs, only a few fasciculi are seen to surround the pith, and the intervening spaces are occupied by a boundary of thickened cellular tissue. In some plants, again, no pith whatever exists, but the stem is hollow or tubular. In other instances, and especially in roots, the centre of the stem is occupied by vessels; and in others, both cells and vessels, promiscuously blended together, constitute the centre of the stem.

In those plants where the pith is present, and possesses its most perfect form, it is seen to be composed entirely of cellular tissue, possessing often very different shades of colour, but, in its anatomical characters, resembling exactly the description already given of that tissue. Its bulk in different plants is exceedingly different, as are also the form and size of its cells. It is frequently entirely insulated by the surrounding vessels, but is often continuous with the cellular tissue of the bark. Its cells contain, especially in the early age of the plant, aqueous fluids, which afterwards disappear, and then the cells become fi∏cd only with air. The “proper juices” of the plant may also be sometimes detected in the cells of the pith. Of the ruptures produced in it by desiccation and other causes, we have already spoken in discoursing of the cellular tissue: they occur particularly in succulent plants, where the cells are large, and their sides thin; so that as the plant advances to maturity, the pith breaks and shrinks up, making the trunk a pipe. (Grew’s Anatomy of Plants. p. 129.) We have also noticed the fact, that, within the cavities of the larger cells of the pith, new vesicular productions are sometimes found.

Grew speaks of the existence of vessels in the pith of certain plants, as in that of the fig and the pine; but he adds, that they are usually so postured as to form a ring about its margin. (Anatomy of Plants, p. 119.) They are doubtless to be considered as enlarged proper vessels, which made a part of the first ligneous circle, and have retained therefore nearly the situation in which they were originally formed. Hence, as he observes, they are of divers kinds, answerable to those of the bark, containing in the fig a milky juice, and in the pine a resinous substance. Similar vessels, containing a “proper juice,” were observed also in the pith of elder by Malpighi, who seems to regard such appearances as common where .the. contained juice concretes or possesses a dark colour. (Anat. Plantar, p. 4.) It is probable, however, that the organs here considered to be vessels may in some cases be cells, into which these juices have been poured; but where real vessels of this kind are found, they are not to be considered as a part of the original structure of the pith, but occurring only in consequence of the changes which the vegetable body undergoes in the progress of its growth.

The general ∙ nature of the pith is thus clearly announced by Grew. “Although,” says he, “it have a different name from the parenchyma in the bark and the insertions in the wood, yet, as to its substance, it is the very same with them both; whereof there is a double evidence, viz. their continuity and the sameness of their texture so that all these parts are “one entire piece of work, being only filled up in divers manners with the vessels.” (Anatomy of Plants, p. 119.) This continuity of the pith with the cellular tissue, of the bark, by means of the insertions or transverse ranges of utriculi, as he calls them, is also adduced by Malpighi as evidence of the similarity of their nature, and of the pith being, as it were, an intercepted portion of the bark (Anat. Plantar, p. 4, 30); an opinion which seems abundantly confirmed by the intermixture of the medullary and cortical textures in many plants, in which, as already remarked, the distinctive characters of bark and pith are alike lost, and the entire stem exhibits only one uniform appearance of structure.

The term medulla, employed by the ancients to note this texture, derived its origin, no doubt, from the resemblance which the pith in the centre of trees bore to the marrow in the bones of animals; and as the same term, in animal anatomy, was incorrectly employed to express alike the marrow in the bones and the nervous substance in the vertebral column, so the same latitude of signification has been extended to the vegetable system. Hence, as Malpighi remarks, the medulla in vegetables was regarded as analogous in its nature to the brain of animals,—a doctrine which even later writers have continued to espouse. It is not our present intention to describe the uses of the pith, but only to remove erroneous opinions concerning its nature, and restore to it that just anatomical character long since assigned it by Malpighi and Grew, and which some writers have of late put forth as a considerable novelty.

Section II.

Of the Wood or Ligneous Texture.

Immediately surrounding and enveloping the pith is the part called the wood (lignum vel lignea portio of Malpighi). It is essentially composed of vessels and of cellular tissue, but combined in such an infinite variety of proportions, and exhibiting such a boundless diversity of forms, that it is difficult to seize even its more general features, without the risk of extending our description beyond the limits which our plan necessarily prescribes.

Except in those vegetables in which no .vessels have been hitherto demonstrated, but in which they must nevertheless be presumed to exist, this texture may be considered to form a part not only of every plant, but of all its organs; for into whatever part fluids are conveyed, vessels must be sůpposed to extend; and wherever vessels are present, cellular tissue is to be found: hence, in its distribution, it may be considered the most universal of all the textures. In trees, the vessels, as we have frequently remarked, are very numerous, and, when viewed in a transverse section, are seen to be disposed in layers or concentric circles around the axis, and to stand also in [7:3:62]lines or radii, diverging from the centre of the tree. (See fig. 4, Plate XL.) Between each line or ray of vessels a thin partition of cellular tissue is interposed, which extends in the direction of the ray, through the entire substance of the wood. At certain distances, varying in different trees, thicker transverse portions of the same substance are placed, and are readily distinguishable in almost every species of wood. Between each layer that is annually added to the wood, and each of the smaller layers that go to the formation of the larger one, cellular tissue seems also in some trees to be longitudinally interposed; so that it is probable that in both directions each fasciculus of vessels is intercepted by cellular tissue, and that in such trees no two fasciculi are on any side in immediate contact with each other. It is even probable that the individual vessels which contribute to form the fasciculi are themselves connected by intervening cellular tissue, which acts like the neurilema that holds together the filaments of the fasciculi in the nerves, or the cellular substance that connects the primary filaments in the muscular fibres of animals. In this manner, the whole vascular system of the plant is everywhere connected and held together by cellular tissue. Of this tissue, and the different figures its cells acquire, from the different modes and degrees of compression to which they are exposed, we have already spoken.

In many trees, however, as palms, the vascular fasciculi, though numerous, are much less abundant than in the examples just referred to. They are consequently placed at a greater distance from each other, and, not being disposed in regular lines, do not constitute that radiant appearance so common in ordinary trees, but are promiscuously dispersed through the cellular tissue. (See fig. 26, Plate XXXIX.) As this tissue itself is not, from the same causes, compressed either in the direction or to the extent before described, the smaller membranous partitions that divide the vascular radii from each other are not produced; neither, for similar reasons, is there any distinct appearance of the larger partitions that, at certain distances, intersect the diameter of other trees. The cellular tissue, therefore, in such plants, retains more of its primitive character, and appears everywhere to surround the vascular fasciculi, but nowhere to be so compressed as to form solid partitions between them. In some plants which possess this structure, as the sugar-cane (fig. 25, Plate XXXIX.), the cells indeed retain their perfect forms; and even the fasciculi of vessels, though standing at considerable distances from each other, have towards the centre of the plant a symmetrical arrangement. This latter circumstance is observable in many other plants, which have even fewer vessels than the sugar-cane; so that it is probable that, in the first instance, it takes place in all; and that the irregular position of the vessels in the palm and similar trees, particularly towards their circumference, proceeds from the peculiar modes of their growth, and are not a primary condition of their structure. This intermixture of the vessels and cells in the plants now under consideration extends from the circumference to the centre, so as to constitute their entire bulk, to the exclusion of bark and pith; unless we choose rather to say that in such plants the medullary, ligneous, and cortical textures, are all blended together.

* In other examples the vessels form a still smaller portion of the ligneous texture, consisting only of a few fasciculi, which stand at considerable distances from each other, the intervening spaces being occupied by'cellular tissue, which forms the chief bulk of the plant. (Fig. 15, Plate XXXIX.) Though few in number, the vessels, however,' are symmètrically disposed, and iń the same species preserve always the same position, the fasciculi being placed at the same relative distance from each other and from the common centre of the pith. Sometimes, instead of a few solitary fasciculi, they consist of several ranges, forming an imperfect sort of concentric layers; and in such examples the ligneous texture is commonly separated by distinct but irregular marks from the two other textures. In these plants the cellular tissue preserves its characters, and exhibits no appearance of divergent rays.

The three modes of arrangement above described appear to constitute the chief varieties of structure in the ligneous texture; but in each variety, and through every stage by which they graduate into one another, the greatest diversity of forms prevails. Each species of plant has its peculiar internal structure, as well as its external form; and this seems to be in a great measure determined by the number of vessels that enter into its composition, and their peculiar mode of arrangement. If the vessels are few, the cellular tissue is large in proportion, and its characters are distinct and well preserved ;—if they are numerous, and disposed in rays, the tissue is compressed in various directions, loses more or less completely its cellular character, and forms alike those divergent rays or transverse partitions already so often noticed, and those thin membranous expansions or fasciae which, both in the bark and wood, are seen sometimes to cover the vessels in a longitudinal direction. Though the whole of the ligneous texture is thus made up of vessels and cells, variously formed and blended together, yet many writers speak of ligneous fibres, which some describe as consisting of vessels, others of cells, and others of an assemblage both of cells and vessels.

The manner, too, or rather the place, in which the vessels are developed, in perennial plants, will greatly contribute to vary the appearance of the ligneous texture. In those trees whose diameter is annually increased by the formation of new vessels around the cylinder of older wood, the new parts must necessarily present in their longitudinal section the appearance of annual layers superimposed on one another, and, in their transverse section, that of concentric circles; but in palms and similar trees, where the development of new parts seems to be accomplished in a different manner, their appearance, under similar sections, may be expected to be different.

In a longitudinal section of the palm, says Μ. Desfon-i taines, we discover an assemblage of large ligneous fibres 1 (that is, vascular fasciculi), solid, smooth, and flexible; and these are composed of others still smaller, which are firmly united together: they mostly run parallel to the axis of the trunk, from the base to the summit, without interruption; but some proceed obliquely, and cross the others at angles more or less sharp. (See fig. 2^, Plate XXXIX.) In a transverse section of the same stem, continues the author, we remark neither concentric circles nor transverse partitions; but the fasciculi of vessels, placed without order by the side of each other, are enveloped by the cellular tissue, which fills up all the intervals: they sensibly approach each other, harden and diminish in size in proceeding from the centre to the circumference (see fig. 26, Plate XXXIX.); so that the stem has much more strength and solidity near the surface than in the interior,—an organization quite distinct from that of ordinary trees.

The cause of this diversity of structure seems to be amply accounted for by the different modes in which! the growth of these trees is accomplished. When the seed of a palm is sown, the leaves, says Μ. Desfontaines, successively develope and augment in number for four or five years; the neck of the root augments in the same proportion; the bulbous part, formed by the re-union of [7:3:63]the petioles of the leaves, increases insensibly; its solidity augments, and at length the stem rises above the earth with all the size it ever acquires. Its figure is cylindrical from the base to the summit; and if the diameter be measured at different epochs, it will be seen, as Kaempfer had already remarked, not to increase. The palm, therefore, is a regular column, whose summit is crowned with leaves, disposed above each other circularly: those which grow in spring shoot always from the top; the older ones, placed below, dry, and when they fall leave circular impressions, which furrow the surface of the stem, and mark its years until it has ceased to grow.

If next we examine the interior, we discover, as Μ. Desfontaines thinks, the true reason why the stem rises in a column, and does not, like other trees, yearly augment in size. This was done by Μ. Daubenton, who states that every leaf of the date-palm, in proceeding from the bud, is formed by a prolongation of the vascular fasciculi and cellular tissue which exist in the trunk of the tree, as is apparent in the petiole of the recent leaves, and of the dried ones that adhere to the trunk. The elongation of the trunk is produced, therefore, by the leaves which annually proceed from it; and as the parts which form these leaves spring from the centre, they always, as they shoot, force the older leaves outwards. Hence, therefore, as the augmentation of these trees originates at the centre, all the parts capable of displacement must be pushed outwards, just as the new layers of bark and wood, formed annually in ordinary trees, force outward the older layers of bark exterior to them.

In these latter trees, continues Μ. Daubenton, the recession of the bark has no limits so long as new parts continue to be formed beneath it, because the new cortical layers are flexible, and the older ones readily break and give way; but in the palm the substance of the trunk has more compactness as we approach the circumference, and, at a certain point of density, it no longer yields to the central force of the interior parts; so that when this point is reached, no further enlargement takes place; and hence the date-palm scarcely exceeds ten inches diameter. It is for similar reasons that the trunk of the date-palm is of the same size through its entire length; for, in proportion as the tree rises, the exterior parts of the trunk lose successively their flexibility; and when they have acquired a certain degree of density they no longer yield to the force from within; and as this is equally the case in all parts, the trunk is necessarily of the same size throughout. (Mėm. de l’lnstit. Nat. tome i. p. 482, &c.)

It is further evident that, in this mode of growth, no appearance of concentric circles, similar to those of ordinary trees, can have place; for, by the growth at the centre, the exterior vessels are continually displaced from their original positions, are more and more compressed as they are forced towards the circumference, and present in their transverse section that irregular distribution which they have been described to possess. Hence the cylindrical figure and the absence of concentric layers are as necessary consequences of the mode of growth in these trees, as the presence of those layers and the conical figure are of the mode of growth in ordinary trees. The greater solidity of the parts at the circumference is clearly to be ascribed to the same cause; and even the want of regular transverse partitions must in part also have, a similar origin, and be ascribed, perhaps, in part to the smaller number of vessels which these plants possess, as well as to their irregular distribution. From this difference in their mode of growth, these two great divisions have received different names: those which, like our ordinary trees, augment in size by the deposition of new matter near the surface, are named Exogenous; and those which, like palms, grow from within, ' Endogenous.

Section HI.

Of the Bark or Cortical Texture.

This texture, in its component parts, resembles that of the wood, being made up, like it, of vessels and cellular tissue intimately connected with each other. Its structure, as a distinct texture, is best characterized in the bark of ordinary trees, as it is there separated, in a great measure, from the ligneous texture. As in these plants a new layer of vessels is annually made to the wood, so a similar but much thinner layer is yearly added to the bark, to which the name of liber has commonly been applied. These vessels are at first straight, and run parallel to the axis of the trunk; but, by the successive formation of new layers beneath them, they are gradually forced outward, become separated more and more from each other, and, touching in a few points only, exhibit at length a reticulated figure (see fig. 24, Plate XXXVΠL), the meshes of which yearly augment in size, from the greater space over which they are continually spread.

Between the vessels thus annually formed, a considerable portion of cellular tissue is interposed, which, in the young and succulent state of parts, contributes chiefly to the thickness of the bark. This tissue is variously compressed by the vessels, so as to form transverse partitions between them, which, in the vine, the oak, and many other trees, as both Grew and Malpighi remarked, are seen to be continuous with those of the wood; and in this way the two textures are united together. In the expressive language of Grew, the bark, therefore, does not “merely surround the wood as a scabbard does a sword, or a glove the hand, but is truly continuous with it, as the skin of the body is with the flesh.” In the willow and other trees, when full of sap, the bark is so easily separated that it seems to have no connection with the wood; but this is supposed by Grew to arise merely from the extreme fineness and tenderness of the vessels that are annually formed in that part, and which on that account oppose no obstacle to the separation. (Anat. of Plants, p. 129.) It is probable, however, that the cellular tissue forms the only direct connection between the cortical and ligneous textures; and that, if a vascular communication exist, it is only, as in all other cases, through the medium of that tissue.

Besides the transverse compression which the cellular tissue experiences from the vessels, it is compressed in: the opposite direction by the formation of the new layers of bark and wood beneath the older bark. These in the progress of their growth exert an expansive force outwards, so that the cells of the tissue are made to assume an oblong or flattened form in the direction of the vessels of the trunk, or sometimes to form a thin fascia upon the vessels, in which the cellular character is nearly or entirely obliterated. It is by the continued exertion of this force acting on the exterior and desiccated layers that these, latter ultimately crack, producing figures of different sizes, which have frequently the shape of rhombs, the fissures of which represent, according to Grew, the position and track of the vessels in their reticulations. (Anat. of Plants, p. 129.) The spaces or meshes formed by these reticulations are always filled up by cellular tissue, which, in the opinion of Malpighi, originates from the vessels themselves.

Both in the vessels and cells of this texture, collections [7:3:64]of the “proper juices” frequently occur; especially in plants in which these juices are of a viscid nature, and disposed to concrete; as was before remarked, when treating of the proper vessels. Similar collections of matter, originally existing in the bark, are likewise met with in every layer of wood, and even in the pith of certain trees. It is only in plants in which the “proper juices” are coļoured, or disposed to concrete, that this intermixture of the cortical and ligneous textures has been traced through the whole substance of the tree; but it is probable, from the conjunction of the vessels of the bark and wood at the period of their formation, that it is common to other plants, the nature and properties of whose juices afford no clew to its detection.

In most herbaceous plants, the cortical texture is not so clearly distinguished from the ligneous, since in them the greatest variety obtains both in regard to the number and relative position of the sap and “proper vessels ;” and very frequently the cellular tissue is quite continuous and of uniform appearance through the entire substance of the plant. In general, however, the sap-vessels occupy the inner place, and are surrounded by the “proper vessels,” disposed either in rings or distinct fasciculi, more or fewer in number. Sometimes the sap-vessels seem to be placed exterior to the others; and hence it is difficult to discover the true place of the “proper vessels” in such plants, unless the nature of their juices conduct to it. From the position of the vessels, therefore, it is often difficult to define the boundaries of the cortical and ligneous textures; and, for the reason already assigned, the cellular tissue rarely affords much assistance. According, however, to the number and disposition of the vessels, this tissue, even in these plants, suffers a certain degree of compression, so as to form thickened boundaries around the fasciculi, or sometimes large transverse partitions, which communicate with those of the wood, in which, though the figure of the cells is more or less altered, the cellular character is usually preserved.

In other plants, the intermixture of “proper vessels” with those that carry sap seems to be general through the whole plant; and consequently no distinction can be made between the ligneous and cortical textures. In such plants the “proper juices” must be considered to exist in every part; and accordingly Malpighi, as we before remarked, points out a vas proprium, or “proper vessel,” as accompanying every fasciculus of vessels in different species of wheat. (Anat. Plantar, p. 24.) From the similarity of structure in palms, and especially from their mode of growth, there can be little doubt that a similar intermixture of the two kinds of vessels prevails everywhere in them; and with respect to these plants, what has already been said of the construction of the ligneous texture, is equally applicable to that of the bark.

We have thus given a very brief and general view of the principal textures that enter into the construction of plants, and pointed out the more prominent diversities of character and appearance which they exhibit, as well in their simple as in their more complex forms, and as they exist either separately or variously intermingled together. Our descriptions have been confined entirely to the trunk or stem; but, with slight variation, they are applicable; equally to the root and branch, in which a similar combination of the elementary organs obtains. In the root, however, they commonly exist in a more compressed and compacted form, so that the ligneous texture is seen chiefly to predominate, frequently to the entire exclusion of the pith, and often in great part also to that of the cellular portion of the bark. This, however, is not universal, especially in annual plants, some of which, as the carrot and others, particularly in a cultivated state, are distinguished by the very large portion of cellular tissue which enters into the construction of the cortical texture. Of the modifications of these several textures, as they exist in leaves and other organs, we shall have occasion to speak when we come to treat of them individually.

Section IV.

Of the Connection subsisting between the Vessels and Cells in the several Textures.

In our discussion of these several textures, we have noticed only in a general way the direct means by which the vessels and cells that construct them are connected with each other; but, when treating of the sapvessels, of the absorbent vessels, and of the cellular tissue, we endeavoured to show that an universal communication obtains between these elementary organs, and consequently inferred that some mode of connection, by which it can be accomplished, must have place. Grew considered the vascular and cellular parts to be connected with each other, not only by the transverse partitions of cellular substance that intercept the vessels, but “ļìer mini-mas partes organicas ; that is to say, the parenchymous fibres are wrapped round about the vessels, or at least interwoven with them, and with every fibre of every vessel, as in very white ash or fir wood may be observed.” (Anat. of Plants, p. 121.) These fibres Grew elsewhere considers as vessels, and consequently must be regarded as maintaining a vascular connection between these organs. In a description of the young branches of chesnut and oak, Malpighi delineates minute filaments springing on every side from the vessels of the wood, and continued into the adjacent cellular tissue (Anat. Plantar, p. 27); but he does not state whether they are to be regarded as vessels or simple fibres. He elsewhere describes minute vessels as surrounding the cells of the trunk like a network, and thinks it probable that the cells of the transverse septa are similarly furnished with vessels. In the cells of the pith of elder (sambucus), and of some other plants, the vessels are very abundant, and in every case are probably derived from the straight ligneous fibres (vessels) both of the bark and wood. (Ibid. p. 29.) Indeed he held it probable that the nutrient fluids moving through the vessels were in all parts poured into the cells, and there undergoing a certain preparation, were afterwards mixed with more recent juices, and with them taken up and applied to the support of the young buds and leaves. (Ibid. p. 30.) This doctrine has since been held by Darwin and Knight, and it necessarily supposes a vascular connection between the vessels and the cells, by which the functions both of secretion and absorption can be performed. The microscopical observations of Lecu-wenhoeck, already noticed, supply further evidence in support of this opinion.

In the hypothesis of Mirbel both cells and vessels are considered as formed out of one and the same membrane. He rejects, therefore, the aid of all intermediate organs as necessary to connect them together, and supposes a communication to be everywhere maintained between the vessels and cells, by the medium of pores in their sides. As, however, these pores are nowhere proved to have existence but in the imagination of the author, we may altogether reject their agency in maintaining a communication between the vessels and cells of plants. In the opinion of Μ. Kieser, the conjunction of the cells with the vessels is extremely simple, the sides of the cells, says he, being contiguous to the sides of the vessels. (Mém. sur ι Organisât, des Plantes, p. 94.) But mere contiguity of parts does not amount to connection, much less does it afford any information concerning the actual commuhica-tion[7:3:65] that exists between these organs. In addition, therefore, to connection by cellular substance, it seems abso- 'lutely necessary to suppose also the existence of a vascular structure, which shall at once serve as a medium both of connection and communication.

Before concluding this branch of the subject, we may observe that the structure of the cellular tissue, and its relation to the vascular system in plants, appear, in many points, to resemble that of the adipose cells, and their relation to the vascular system of animals. These cells are described as minute close cavities, possessing no apparent communication with each other; and within them adipose matter is alternately deposited and removed. Now the deposition of this matter could only be accomplished by secreting vessels which terminated in the cells, and its removal be effected by absorbents which originated from them; and accordingly both blood-vessels and absorbents are found to be present in this texture; but neither the secreting nor absorbing orifices have ever been actually observed. Within the cells of the cellular tissue of plants the alternate deposition and removal of various matters are not less certain; and in the germinating seed the matter that actually existed in the cells is found afterwards in the vessels. We are led, therefore, or rather we arc driven, not only by the direct exclusion of all other alleged means of communication, but by a close analogy in the exercise of these animal and vegetable functions, to conclude, that secreting and absorbing vessels must be employed to deposit and remove the secreted matter from the cells of plants, in the same way as they are considered to effect similar depositions and removals of adipose matter from the cells of animals: and as this alternate function seems to go on in every part of the plant capable of active vegetation, it may further be inferred that a vascular communication exists between the vessels and cells in all parts of the vegetable system.

By means of this general communication between the vessels and the cells, we are enabled to assign satisfactory reasons for some puzzling phenomena, which have occurred in relation to the movements of the sap. It is by this alternate action of secretion and absorption that in young plants we must suppose the cells of the pith, during the first year, to be filled with fluid, and to be rendered dry for the most part ever after. In like manner, the surface of the bark in contact with the wood appears in some trees, as the birch, to be rendered moist during the rise of the sap in spring; which led Dr Walker and others to suppose that the sap rose in part between the bark and the wood,—an opinion not at all probable in itself, and certainly not supported by what is observed in most other trees. The fact, however, is easily explicable, on the supposition that the sap was transfused from the alburnous vessels of the wood, in the same manner as, at a later period, it is secreted in the same part, but in a different form, by the vessels of the bark, to form the new matter that is annually added to the tree.

That the sap of plants was capable of moving in a lateral direction, was inferred by Malpighi, from the fact that parts lived and grew when the perpendicular vessels that supplied them with nutriment had been destroyed. (Anat. Plantar, p. 13.) The experiments of Hales afford more decisive evidence regarding this lateral movement of the sap. He cut two large gaps in the opposite sides of an oak-branch, at four inches distance from each other, carrying the incisions down to the pith: the branch nevertheless absorbed and perspired water, but only in half the quantity that another similar but uncut branch did. In a branch of cherry-tree he made four similar cnts down to the pith, at four inches distance from each other, and opposed to the four points of the compass: the branch notwithstanding absorbed, in forty-eight hours, twenty-four ounces of water. ( Veg. Statics, p. 128, 3d edit.) And when similar incisions were made on branches while still attached to the tree; their leaves continued green nearly as long as those of other branches in a natural state; whence he justly inferred that, at these gaps made in the branch, a lateral movement of the sap must have taken place. Experiments of a similar nature have been made, and like results obtained, by Mr Knight (Phil. Trans. 1808); so that it seems clear that in certain circumstances a lateral movement of the sap must have place.

In what manner, then, must we suppose this movement to be accomplished? Grew supposed the cellular tissue, that stretches from the circumference to the centre of the plant, to be the organ by which such a communication could be maintained; but the impermeability of this tissue to fluids opposes such an opinion. Malpighi thought the lateral communication to be made by an anastomosis of vessels; but in the vessels of plants no such mode of communication appears to exist. From the ascent of the sap in branches in which the vessels had been thus previously cut through, Mr Knight infers that this fluid does not rise in the vessels at all, but is conveyed through the cellular tissue. This opinion necessarily implies the permeability of this tissue by fluids, which, as we have shown, is contradicted by direct experiment, as well as by microscopical observation. Since, therefore, this lateral movement of the sap cannot be accomplished, either by simple percolation through the cells or vessels, or by direct anastomosis of the vessels with one another, no other known means of effecting it remain, but those of alternate deposition and absorption by the vessels into and from the cells. And if, as we have seen, the sap-vessels of plants deposit coloured fluids in the cells, which the capillary absorbents of parasitic plants are able to take up, there seems no reason for denying to the vascular productions, which have been supposed everywhere to spring from the perpendicular vessels, a like capacity of absorbing fluids from the adjacent cells. These fluids must, however, in all cases, have been deposited before they could be absorbed; and, by the alternate exercise of these functions, there is no difficulty in conceiving how a lateral movement of the sap might be accomplished in parts where, by the incision of the vessels, a stop was necessarily put to its perpendicular ascent.

Section V.

Of the Skin or Cuticular Texture, and its Appendages. Art. I.— Description and Structure of the Skin.

The skin, rind, cuticle, or epidermis, as it has been variously named, is the last of the common textures that remains to be described. It is the general envelope which invests all parts of the plant and all its productions, being equally common to the trunk and branches, the root, the leaves, the flowers, and the fruit; but in these different parts, and even in similar parts of different plants, it exhibits the greatest diversity of appearance and form.

In herbaceous plants, and in the young shoots of those; which are arborescent, it resembles a thin membrane, but∣ is generally thicker on the stem than on' the roots or leaves. In some leaves, however, it is thick and dense, as is the case also in several fruits, and is thereby fitted ι to resist the effects of too rapid desiccation. On the upper surface of some leaves, on many fruits, and on roots, it is an entire membrane, destitute of any apertures [7:3:66]or pores; but on many stems on the under surfaces of leaves, and sometimes on the upper, it is frequently furnished with numerous pores, often visible to the naked eye, and with other luminous points of smaller dimensions, which Du Hamel also regards as apertures. It is readily separable from the bark in recent and succulent parts, or after maceration in water; and in certain leaves it is very completely separated by a species of caterpillar, named by Reaumur the miner. It appears then to be a thin transparent membrane, often destitute of colour, and deriving, therefore, its appearance from the colour of the parts beneath; but both in leaves and flowers it is often itself coloured. It is frequently seen to extend in all its dimensions, in common with the parts it covers. Very often, too, as will be noticed hereafter, its surface is covered with hairs; and sometimes small follicles or utricles are met with, which exercise a glandular function.

The characters above enumerated belong chiefly to the cuticle in its young and succulent state. In perennial plants it commonly possesses others that are quite dissimilar. It is of a different colour not only on different trees, but on different parts of the same tree. It is white and shining on the trunk of the birch, and browner on the branches; greyish on the plum-tree; red and silvery on the cherry; green on the young branches of the peach; and ash-coloured on the larger branches. In these and many other instances it does not, says Du Hamel, merely participate in the colour of the body it covers, but contributes itself to give colour to the exterior bark; for when it is stripped off, the substance below has frequently a different colour. (Phys, des Arbres, tome i. p. 10.) By the gradual enlargement of the trunk it is stretched and dried, and at length loses its vitality, and, as well as the bark beneath, is variously cracked and broken. Before this happens, however, it often undergoes considerable extension in all its dimensions, enlarging in breadth, and stretching longitudinally over the young shoots. This expanded state is particularly remarkable in certain fruits, in which, when they enlarge slowly, the cuticle is extended without rupture to a very large size; but if the expansion be very rapid, as after considerable rains, the cuticle then gives way. In certain trees the cuticle is more susceptible of expansion than in others; and in very vigorous trees it breaks more slowly than in those whose growth is languishing, although these latter push forward more slowly than the former. (Ibid. p. 11.) In some vigorous trees of this description it altogether resists rupture; and in this state the tree is often said to be hide-bound or barkbound.

In most instances the cuticle, when taken from young branches, appears to consist of a single layer; but on the branches of many species, says Du Hamel, after one plate or layer has been removed, another may be seen beneath, which resembles the former in its texture, but is much thinner and more green and succulent. From the birchtree he has removed more than six layers, very thin and very distinct from each other, and is of opinion that more might still be separated. Sometimes the original cuticle seems to be entirely thrown off, and the exterior covering is formed by a portion of the cellular tissue of the bark. Grew thinks that this substitution takes place annually, the older skin being cast off, like the skin of an adder, by the generation of a new one beneath. (Anat. of Plants, p. 114.) Du Hamel describes also the existence of small leaflets or scales, which are continually detached from the cuticle of some trees; and these he considers to be as constantly replaced, by the formation of new ones beneath them.

• Concerning the regeneration of the. cuticle on parts from which it has been removed, Du Hamel observes, that when the wound is covered with waxed cloth, a new cuticle is promptly formed without any separation of a portion of the bark beneath. When the exterior portion of the bark is removed with the skin, the inner part of it is equally capable of regenerating a cuticle; but if the wound be not protected from the air, a certain degree of exfoliation first occurs, and, under the decayed parts, a new skin forms. Even where the bark of a cherry-tree was entirely removed from the trunk, he found that the wood was capable of regenerating a new bark and cuticle, if the parts were properly protected from the air. This cuticle did not originate from that which remained on the roots and branches, but was reproduced in isolated portions on different parts of the trunk; it continued, however, after the lapse of fifteen years, always different from that of the natural growth. In other instances, he adds, the cuticle does not seem to be regenerated at all. He remarks certain analogies to exist between the cuticle in some plants and in animals. In both, he adds, it seems in certain circumstances capable of great extension; inboth it is easily regenerated, and that too in isolated portions, and not by continuity of organs, as is common in other instances; and in both, lastly, it is perpetually obliterated, and continually and imperceptibly renewed. (Phys, des Arbres, tome i. p. 11.)

With respect to the nature of the cuticle, very different opinions have been advanced, and still continue to prevail. Are we to regard it as a peculiar organ, formed immediately by the proper exercise of the vegetative functions, or is it produced in a sort of secondary manner, by some changes induced on some previously constructed organ? Grew asserted it to be sometimes original, and in some instances produced out of the exterior layer of the cortical texture beneath it; and this first view of its origin seems to be generally supported by the descriptive character which has been assigned to it. Its co-existence with the first traces of vegetable organization, its continued growth and expansion, and its subsequent regeneration after removal, all seem to favour its primary and independent origin, which is also supported by investigations into its minute structure. Mirbel, however, and some other writers, after Hill, have regarded it, in all cases, not as an original membrane, but formed by the exterior sides of the common tissue of the plant; and where there is no separation of these sides in the form of a membrane, such plants are held to be destitute of a cuticle. (Exposit. de l’ Organisât. Végét. p. 103.)

Another question relating to this organ is, whether it must be considered a simple membrane of uniform structure, or a compound of two distinct parts, like the true skin and the cuticle in animals. Grew seems to have regarded it as a simple body, but constructed both of vessels and cells, the cells being continuous with those of the bark. (Anat. of Plants, p. 62.) Such, too, seems to have been nearly the opinion of Malpighi, who describes it as constructed of horizontal ranges of cells, but often delineates reticulations of vessels as constituting a part of its structure. (Anat. Plantar, p. 2, 19.) In the birch, the plum, the cherry-tree, and others, Du Hamel declares the component fibres of the cuticle to possess a direction transverse to that of the trunk; but this is not general. In the birch-tree the fibres seemed to be placed parallel to each other, and to be connected together by lateral fibres; but he could see nothing of the vesicular structure of Malpighi and Grew, and therefore regards the structure of this texture to be altogether fibrous. (Phys, des Arbres, tome i. p. 8 and 9.) Μ. Desfontaines, on the other hand, describes it as a membrane resembling in ap: [7:3:67]pearance a thin plate of parchment, and perforated by imperceptible pores, which give issue to the insensible transpiration. Its structure he regards as unknown, but considers it capable of regeneration. (Jfe⅝. ¢fe l’lnstit. Nat. tome i. p. 481.)

Μ. Kιeser, who professes to have studied this texture with great attention, adopts nearly the opinion of Grew, pronouncing the cuticle to be constructed of a very fine cellular tissue, and of extremely minute vessels which run through its whole extent. These vessels form an exceedingly delicate and subtile net-work, the meshes of which possess very different forms, and their vessels terminate at the orifice of a pore. His observations were made on the cuticle of leaves. On the inferior surface of the leaf of Amaryllis formosissima (fig. 14, A, Plate XLI.), magnified 260 times, these vascular meshes of the cuticle have an elongated hexagonal form; and four of their vessels proceed always to terminate at the orifice of the little oblong aperture or pore situated at their junction. In Canna Indica, the vessels of the meshes on the lower part of the leaf, which thus terminate in the pores, are said to originate from a fasciculus of the spiral vessels that ramify through the leaf, as is represented at a' in fig. 14, B, Plate XLI.; and within the areas of the larger meshes a still finer net-work of vessels is seen. On the inferior surface of the leaf of a species of fern, the vessels of the cuticle, instead of forming meshes of different figures, exhibit the appearance of sinuous lines, which run in every direction through the cuticle. See fig. 15, Plate XLI. which represents the central part of the little adjoining leaf, magnified 130 times. These sinuous vessels often join, and, after making a half-circle, terminate by one extremity in the minute pores everywhere spread over the leaf, and by the other in the larger vascular fasciculi that ramify through it. At the letter b', in this figure, the hexagonal cells that construct the parenchyma of the leaf are distinctly visible through the vascular sinuosities of the cuticle. It was by the examination of this leaf that Μ. Kieser was first enabled to discover the origin and termination of the vessels that construct the cuticle, having in all his previous investigations examined the cuticle in its separated state, after it was detached from its connection with the other organs; but the researches made on this leaf rendered every thing clear. (Mém. sur l’ Organisât, des Plantes, p. 141-2.)

The vascular net-work of the cuticle, thus described by Μ. Kieser and others, had been regarded as a deception by Μ. Krocker, who considered these reticulated figures as no part of the real structure of the cuticle, but merely as the sides of the subjacent cells; in which opinion Sprengel, Link, Jurine, and Mirbel, concurred; but Μ. Kieser, in opposition to this opinion, maintains that, in the fern and other leaves, the real cellular structure of the parenchyma is seen entire through the vascular reticulations of the cuticle, with the meshes of which the sides of the subjacent cells do not anywhere coincide. He observes that these cells are commonly much smaller than the vascular meshes which cover them; and that the vessels of these meshes may be traced, as before remarked, to the larger fasciculi that construct the leaf. In the fern the vascular structure of the cuticle is the same on both sides of the leaf, but the superior side does not possess pores.

A very different view of the structure of the cuticle was taken by the late celebrated Μ. de Saussure. He regarded it not as a simple, but a compound texture, consisting of a very delicate external pellicle or membrane, beneath which was placed a net-work of very fine vessels. The external membrane he describes as perforated by pores of unequal figure, between which he observed some opaque and tortuous filaments, disposed in a reticulated manner, each mesh being formed by six filaments, four of which terminated at each pore. To this arrangement of .filaments he gave the name of cortical net-work, and regarded it as quite distinct from the cuticle that covered it. The meshes of this net-work differ much in size and figure in different leaves; and, when minutely examined, they are often seen to form junctions, but never to cross each other; whence he was led to regard them as vessels derived from those of the expanded petiole, and thus constituting a very fine vascular net-work. A similar structure was observed in the petals of the flower. (Encyclop. Method, tome i. p. 67.)

Μ. de Candolle considers the cuticle in certain parts to be a simple and proper membrane; in other parts to be formed by layers of cellular tissue. The cuticle of leaves, and probably of all annual shoots, appears to be formed of a layer of flattened cells, differing in form and in other conditions from those of the parenchyma of the leaf. It may be regarded in this state as a proper membrane. When stripped off and examined by the microscope, it exhibits small spaces, bounded by lines or rays, or a sort of net-work, the meshes of which have different figures in different plants. The lines, says he, appear in the form of single or double filaments, and may be considered hollow, and as forming a system of cuticular vessels which terminate in the pores. In more advanced age, and especially in the trunks of old trees, this primitive cuticle gradually changes its character, and finally disappears. It is then succeeded by a thicker membrane, which no longer exhibits the net-like figure of the former, and appears evidently to be formed by the exterior cells of the cellular tissue, which, by the combined effects of distention and desiccation, assume a membranous appearance, to which the term epidermis is properly applicable. This epidermis may be considered single when composed of one layer of cells, and double, triple, or multiple, when formed of two or more successive layers. It is thus that Ulloa, in describing a tree in Peru, speaks of having detached more than 150 epidermoidal layers, when he lost all patience in counting them, seeing that he had not then reached to half the thickness of the bark. A similar appearance is exhibited in the numerous layers that form the epidermis of the white birch, amounting sometimes to 15 or 18 in advanced age, and which ends by breaking into that cracked condition of the bark that presents only discontinuous portions of white epidermis on the remnants of its cellular envelope. At an early period the cuticle is most easily torn in a longitudinal direction, which is that of its growth; but at a later period, when the diametral growth has increased, the cells of the tissue are drawn out more in that direction, and therefore more readily break transversely than in length. (Organog. Végét. tome i. chap. 5.)

Art. II.— Of the Pores of the Skin.

The pores of the cuticle, called sometimes cortical or miliary glands, sometimes exhaling pores, sometimes cortical pores, and more lately, by Link and De Candolle, stomata, were first noticed by Grew, who describes many orifices as existing on the leaves of different plants, which vary in size, number, shape, and position. In the white lily they are of an oval shape, of a white colour; and each is surrounded by a slender border. When viewed through a good glass they appear as if standing about one sixth or one eighth of an inch apart all over the leaf, but not arranged in any regular order. In the pine, also, [7:3:68]they have an oval shape, but have no rising border, and are arranged in lines from one end of the leaf to the 'other. (Anat. of Plants, p. 153.) Hedwig considered the borders mentioned by Grew as produced by a ring of one or more vessels, which terminated in the pore. Ιhe number of pores he represents as exceedingly great. In the square of a line of the cuticle of a bulbous lily he reckoned 577 pores.

The characters and position of these pores or stomata have been further examined by Μ. de Candolle on more than GOO plants. They occur most frequently on the leaves, occupying both surfaces in some herbs, and in trees chiefly the inferior surface. Stems in general have no pores, except, as in the Gramineoe, where they are succulent, and have the character of leaves; or where the plant is altogether destitute of leaves, as the Cactus. On the prominent lines or vessels of the leaves and stems no pores are to be seen, but only in the grooves or depressed surfaces of the parenchyma. They are never observed on the root, not even on bulbous roots, where the scales of the bulb are true leaves. The small leaflets called stipulae and bracteoe sometimes have and sometimes have not pores. The calices of the flower in general have pores, but the petals have not. Pericarps of a foliaceous consistence have pores: when fleshy, they are destitute of them. The envelopes of the seed are destitute of pores, but they are found on all seminal leaves that rise above the ground. The lower tribes of vegetables, as the fuci, musci, hepatic(e, fungi, &c. are destitute of pores.

The occurrence of pores in those plants where they are found seems to be much influenced by external circum- > stances. They are never met with but on vegetables, and those parts of vegetables that are exposed to the air; and therefore the internal surfaces of many leaves that embrace the stem are without pores, though on the external surfaces of the same leaves they are abundant. No plant that is completely aquatic, nor any part of it that is habitually under water, is provided with these organs; but the parts which rise above the water are furnished with them. In ranunculus aquaticus the leaves that are constantly under water are destitute of pores, while those that float on the surface are provided with them, but only on their superior face. Even leaves which do not naturally possess pores when under water, acquire them if they are made to grow in air; and land plants, on the contrary, when made to grow under water, may, by such treatment, be deprived of their pores. Thus, the leaf of green mint, when growing in air, possesses not fewer than 1800 pores on its lower surface; but if kept for a month under water, its leaves fall, and the new ones that succeed are destitute of pores.

1 Light seems to be necessary also to the production of pores, for etiolated plants do not possess them. When grown by the light of lamps, the leaves possess a few pores; and, in all cases, the parts secluded from light and air are destitute of these organs, but acquire them if they are duly exposed. (Mém. de īlnstit. Nat. tome i. p. 351.)

This general account of the pores of plants is confirmed by the researches of Μ. Rudolphi. In most herbaceous plants he found the pores to occupy both sides of the leaf, but in trees only the inferior surface. They were not often met with on the parts of the flowers, or on fruits; they were never seen on roots, nor on the trunks of trees; nor ever on aquatic plants, except on such parts as were raised above the water. The lower tribes of vegetables seemed to be universally destitute of them; the leaves also of those plants that were covered thickly with hairs on both sides had no pores. The form of the pores was commonly oval or elliptical, but in a few instances square or rhomboidal. In size they very much varied in different plants, but in the same plant the size was uniform. The largest pores were seen on the leaf of the white lily,’ the smallest on that of the French bean. (Kieser’s Mém. sur l’ Organisât, des Plantes, p. 144.)

Although the pores are generally dispersed over the parenchyma of leaves at nearly equal distances, yet where the vessels run parallel, they are disposed in one or more lines between them. Sometimes, again, they are collected into little clusters, as in the leaves of crassula cordata, where the roundish dots or points, visible by the naked eye, are clusters of pores or stomata. This appearance suggested to Μ. de Candolle the idea that the stomata might be the orifices of vessels, as each of the little points mentioned above is the termination of a fibre, which is itself a bundle of vessels. This idea is further strengthened by the fact that these pores are not found in plants that are destitute of vessels; and though he could never trace the continuity of a vessel with a pore, yet Μ. Com-paretti assures us he has seen the vessels terminate in them. Mirbel, on the other hand, considers the pores to be the orifices of cells; and Kieser regards them as connected with the intercellular canals. (Organog. Végét. tome i. chap. 6.)

Some have considered the pores as organs, by which the resinous or waxy matter found on certain leaves is exhaled; others, as organs for the absorption and transpiration of air or gas; and others, with more probability, as those by which moisture is thrown off by the leaf. In support of this last opinion, it may be stated that pores exist in all leafy parts that transpire, and are more numerous in membranous leaves that transpire most, than in fleshy leaves which transpire little. They are wanting altogether in aquatic leaves and in etiolated leaves; also in fleshy fruits, in roots, and in the petals of flowers, which do not transpire in any degree analogous to leaves. In darkness too, when transpiration ceases, the pores are closed; and they again open and transpire when light reappears, and especially in bright sunshine. It is always necessary to distinguish the evaporation, more or less great, that goes on through the tissue in all organs by day and by night, from the active transpiration which occurs in sunshine, but only in organs furnished with pores, and which seems to be executed by them.

Besides transpiration, Μ. de Candolle believes that, in certain cases, the pores may also serve for absorption; but this he considers to be rarely the case, and out of the natural course of vegetation; and that the experiments which favour this opinion may be explained by the hygroscopic powers of the vegetable tissue. He thinks the results obtained by Bonnet, who kept leaves a long time in life by laying their porous surfaces on water, may be explained by supposing this treatment to prevent decay by checking transpiration, rather than by promoting absorption. (Organog. Végét. tome i. p. 87.) This suggestion will not, however, explain the phenomena of an experiment continued for many years in the conservatories of the Royal Botanic Garden here, under the care of Mr Macnab. He has kept a species of fig (ficus elastică) for several years fastened against the wall, with its roots, stem, and branches entirely exposed to the air; and in that situation it has not only lived, but actually grown in every part, so as to have increased greatly in size, only by being thus kept in a warm and moist atmosphere, and occasionally sprinkled with water.

In addition to these visible pores, it is probable, says! Μ. de Candolle, that the surface of vegetables is furnished! with others that are invisible, but so small that with the strongest microscopes we are unable to recognise them: [7:3:69] their existence, therefore, can only be presumed from physiological phenomena. Thus, a part of a vegetable destitute of pores loses weight when exposed to the air, by the escape of the fluid it contained; or a portion of moss equally destitute of pores, if placed in water, speedily acquires additional weight. Whether these results are obtained through the medium of minute pores, or of unorganized apertures, such as are admitted to exist between the molecules of all matters,—and whether through the same pores or apertures a passage is afforded to gases or other fluids, or to the oily or waxy secretions that cover certain surfaces,—are questions, adds Μ. de Candolle, to which no answer can at present be given. (Oryanog. Vég. tome i. p. 88.)

Art. III.— Of Hairs.

From the surface of the cuticle, in many parts of herbaceous plants, and in the succulent parts of arborescent ones, hairs (pili) are seen to spring. They possess very different forms, and vary likewise greatly in texture. In a strict sense, they may be defined small filaments possessing considerable stiffness, which project from the surface, and stand out pretty erect. When they are very numerous, a little soft, and less erect, they take the name of villi ; when still softer and less numerous they are termed down (pubes). Sometimes this down is composed of long hairs nearly resembling wool, at other times it approaches more to the character of cotton. When the hairs are stiff and ranged along the edge of a surface, like the lashes of the eye, they are named cilia ; and if, with these characters, they are produced to a greater length, as in the beard or awn of wheat, they acquire the name of barba or arista. Sometimes they resemble the bristles of the hog, and arc then called setae. Many other varieties are enumerated by botanists, who further distinguish them by various names, according as they terminate in a single point, or are hooked, or forked, or branched, or feathered, &c. In some instances, instead of appearing like one continuous substance, they are composed of many joints, or are said to be articulated. In fig. 16 and 17, Plate XLI., we have copied from Du Hamel a few of the varieties, both of single and jointed hairs; but the forms they exhibit are so numerous and diversified, that we must refer to the writers on botany for minuter information. In some examples the point of the hair is terminated by a small rounded globulet, and sometimes by a fine filament, that seems to proceed out of the hair.

Some writers distinguish hairs into two general classes, —the glandular and lymphatic. In the former class, the hair or filament is sometimes the excretory duet of the small gland situated at its base; and in other instances, as in the chick-pea, the glandular or secreting organ is seated at the extremity of the hair. The lymphatic class embraces a much greater variety of hairs, which differ widely from each other in consistence, direction, and form. They spring only from the parts of plants which are exposed to the air, and are not found, therefore, on the parts concealed below the earth or in water: they are also rare on plants that grow in the shade, are wanting altogether in etiolated plants, and are most abundant, in general, on plants that grow in warm places, and are well exposed to solar light. With respect to their seat or origin, as compared with that of the pores, although, says Μ. de Candolle, these two kinds of organs appear sometimes mingled, yet each has a determined place; for the pores are seated on the cellular or parenchymatous parts, while the lymphatic hairs constantly spring from the vascular or ligneous parts. Yet the hairs, he adds, are almost always placed on the same surface as the pores; and hence the superior surface of the leaf, which is commonly destitute of pores, has in general but few or no hairs. The pores, as before stated, seem to be the true organs of transpiration; and the various kinds of hairs serve to protect the plant against the excess of solar light, against variations of temperature, against humidity, or sometimes against insects. (Organog. Vég. tome i. chap. 10.) Hairs, also, of various kinds are met with on the different parts of the flower. These often have the colour of the petals or other parts on which they grow, and are distinguishable from the true lymphatic hairs, though sometimes associated with them on the same organ.

With regard to the structure of these minute bodies, little that is satisfactory can be said. They seem to originate either directly from the cuticle or from the cortical texture beneath it; but not often from the ligneous texture, except in those instances where they are very long and rigid, as in the awns of wheat. Du Hamel observes that almost all of them are implanted on small bodies, similar to the bulbs which give origin to the hairs o'f animals. (Phys, des Arbres, tome i. p. 183.) They, commonly resemble simple filaments, but often appear like elongated cells threaded on one another, and, instead of terminating in a sharp point, end in a small papilla or utricle, which yields in many instances a viscid or oily matter, or sometimes a coloured liquor, which has led many to regard them as exercising a glandular function.

One species of these supposed glandular organs has been more particularly examined, and their fluid analyzed, by Μ. Deyeux, who gives the following account of it. Soon after the seeds of the chick-pea (cicer arietinum) are sown, its first leaves are seen to be covered with hairs, at the extremity of each of which is a transparent globule, about the size of a small pin-head, consisting of a fluid matter. It abounds most in mid-day, when the air is warm and dry, and is scarcely perceptible at night, or when the air is cold and moist; after rain, indeed, it does not again appear for two or three days. When these fluid globules were removed in a dry day by blotting-paper, they soon re-appeared; they were acid to the taste, reddened litmus paper, and caused an effervescence in carbonate of potash when brought in contact with it. He regarded them as composed of oxalic acid, the properties of which they precisely resembled. (Mém, de ľìnstit. Nat. tome i. p. 157.)

Art. IV.— Of Prickles and Thorns.

It is not easy to discriminate between some of the harder species of hairs, described in the former article, and those to which the appellation of prickles (aculei) has been assigned. They are defined by Du Hamel to be excrescences, often hard, and always terminated by a sharp point, which are developed with the other productions of plants, but are not inclosed in particular buds; so that they may for the most part be regarded as hard and solid hairs. They spring equally from the stem, the branches, the petioles of the leaves, and also from the leaves themselves in various plants; and' in the chesnut and some others they are seen to cover the fruit. They are frequently straight, but in the rose and many others are curved at the point, as in fig. 18, Plate XLI.; and, according to Malpighi, possess sometimes in this plant a little head, which yields a viscid fluid.

Regarding their structure, Grew remarked that they were connected only with the skin or the bark, and he therefore named them cortical, to distinguish them from thorns properly so called, such as those of the hawthorn, [7:3:70]which spring from the wood, and which he denominates ligneous. These latter, he adds, always ascend, while the cortical thorns commonly point downwards. (Anat. of Plants, p. 33.) In proof of their origin from the bark, Du Hamel remarks, that if, after maceration in boiling water, the bark of such plants be stripped off, all the prickles come away with it, and leave not the smallest impression on the wood, nor even on the more interior layers of the bark itself. When a section also is made of the branch and prickle, as in fig. 18, Plate XLI., the wood y and the pith z are both seen to have no connection with the prickle, but the inner layer x of the bark is interposed between the base of the prickle and the wood. The prickle does not, however, spring from the skin, for it is formed of many layers like the bark. As the parts become more solid, it is less freely supplied with juice, and therefore hardens and turns brown. (Phys, des Arbres, tome i. p. 188.)

In the nettle (urtica dioica), Malpighi states, that beside the common prickles on the leaves, there are among them others of a different description. They possess more of a ligneous character, are hollow internally, and contain a juice which, when it gains admission beneath the skin, excites itching and tumour. (Anat. Plantar, p. 137.) Dr Hooke had previously given a much more minute account of the sting of this plant. Almost every part of it, says he, is covered with prickles like sharp needles. Each prickle consists of two parts, very different in shape and quality from one another; one is shaped much like a round bodkin, is very hard and stiff, exceedingly transparent and clear, and hollow from top to bottom. When this bodkin is thrust into the skin, it does not at all bend; but a certain liquor is then seen to move up and down in it, rising towards the top, when the point is pressed down on the base. This base is formed by a little bag, is more pliable than the bodkin part, and within it is a cellular structure, which contains a thin transparent liquor (see fig. 19, Plate XLI.): it is this liquor that rises in the tube, and, being deposited beneath the skin after it is punctured, excites the irritation that succeeds. (Micrographia, p. 142.)

The true thorn (spina) derives its origin, as Grew remarked, from the ligneous part of the plant, and in some plants is produced from the degeneration of some other organ, as of the leaf, or even the branch itself: we shall therefore defer the further consideration of its structure, until we come to treat of that of the branch.

Section V.

Of the Glands of Vegetables.

Perhaps in the whole science of anatomy there is no word that has been employed with such latitude of signification, and is therefore exposed to so much ambiguity, as the term gland. In animal anatomy it was doubtless used at first to denominate certain organs, from the external resemblance which they bore to certain fruits or seeds; and in that sense it is still employed on several occasions. Afterwards it was understood to signify not so much the external form as the internal organization, and was considered to express a certain structure, by which alone the function of secretion could be exercised; but it is well observed by Dr Thomson, in his valuable work on Inflammation, that “the definition of a secreting glandular part must be taken from its function, and not from its structure; for nothing can be more various than the internal structure of those organs that are denominated glandular secreting organs: they consist sometimes of convoluted vessels, sometimes of follicles or small hollow bags, and sometimes of transparent membranes, in which neither convoluted vessels nor mucous follicles can be perceived,’.’ (Lectures on Inflammation, p. 318.) Besides these mor c simple structures, it is well known that most of the internal viscera are likewise denominated glands, though dif? fering in all their characters from those just mentioned.

The ambiguity which thus prevails in animal anatomy,i in relation to the use of the term gland, has been increased! tenfold in the applications that have been made αf it to t the organs of vegetables. It is justly observed by Μ. de Candolle, in reference to this subject, that the numerous approximations in structure between vegetables and animais have often promoted our researches into the former but have sometimes led physiologists astray, and int∏> duced into the language of botany many inexact expres∙ sions. In animal anatomy the term gland is understood to express some organ that exercises a secretory function; but in vegetable anatomy this term has often been applied to bodies that are not known to be real secretory organs. Thus the cells of the cellular tissue, which frequently contain resinous or oily matter, have been sometimes named cellular glands ; the little globulets or utricles at the extremities of the hairs on the edges of leaves, utricular glands ; the small organs formed by the pores on the leaf, cortical or miliary glands ; certain fleshy tubercles on the leaves, urceolar glands ; and the little scales that cover the fructification in ferns, scalifornι glands. The netía-rium of the flower commonly contains a sweet juice, and is therefore deemed a gland; but. Linnaeus, with his usual disregard both of the structure and function of organs, considers as a nectary, not only the body which may secrete, but any other that may serve as a receptacle of the secretion; and indeed is said to comprehend under this term all those bodies which have no resemblance to the other parts of the flower, in whatever variety of form they may appear, or whatever purpose they may serve. (Will-denow’s Principles of Botany, p. 87.) In some other instances the term gland has been used, not to express the secreting organ itself, nor even the receptacle of the secretion, but the solid excreted matter on the surface of certain leaves; and others consider hairs, and every other protuberance that projects from the surface, and contains a fluid different from the common sap, as entitled to the distinctive appellation of gland.

Amid such diversity of opinion concerning the structure, position, and function of these minute organs, and. such vagueness in the methods employed to characterize! them, it is extremely difficult to define their true nature, or declare the principle on which this definition should proceed. The mere existence of a fluid, distinct from the common sap in any organ, cannot be considered as bestowing on it the title of gland, otherwise the greater portion of some plants would come to be regarded as glandular: those varieties of structure which exercise no secretory function may also be excluded from the list of glands; and so likewise the hairs of plants, though containing peculiar fluids, may be excluded, since these peculiarities appear to arise frequently from circumstances foreign to the action of the organ itself; and even if they do not, some specific variation of the general name they bear is preferable to the employment of so ambiguous a word as gland. But where any organ is distinct from the com· mon textures of the vegetable, and by the peculiarity of its structure is fitted to produce those changes on the vegetable fluids which we name secretion, it may be deemed a secreting organ. This secretory function, however, may sometimes be exercised, as in animal bodies, by membranous surfaces, and sometimes by small isolated bodιe⅛ to which, perhaps, may properly belong the denomination of glands.

But even though this method of defining glands were [7:3:71]adopted, it still is a matter of no small difficulty to distingish their species by appropriate appellations. In animal anatomy no settled rule obtains; but the name of the gland is assigned from some accidental circumstance of situation, figure, use, &c. In vegetable anatomy the btanist, regarding glands only as aiding the discrimination of species, refers commonly to their situation, and speaks of foliaceous, stipular, or petiolar glands, according as they happen to be seated on the leaves, the stipules, or the petioles. The anatomist imposes names according to their forms, as they chance most to resemble a globule, an utricle, or some other figure; and the physiologist is chiefly directed by ideas which indicate their functions, disstinguishing them into mucous, oily, resinous, or nectariferous glands, according to the nature of the fluid they furnish. Of these different modes, that which proceeds on the apparent form, where it can be discovered, seems the most precise; but as this cannot always be accomplished, the situation of the organ, or the nature of the secreted fluid, must occasionally be had recourse to.

Of these bodies it is to be remarked that they differ in one respect from most of the corresponding organs in aninimals, almost all of them being seated on the external parts of the plant, like several of the more simple glandular bodies in animals. This arises from the greater simplicity of vegetable organization, particularly as it regards the absorbent system, the mode of growth, and the permanence of the organs produced; whence it happens that the living parts of aged perennial plants are situated only at or near the surface; and it is only in such parts that active secretory functions can have place. Hence it is on .cculent stems, on leaves, flowers, and fruits, during the active state of vegetation, that the glandular functions of vegetables must be exercised; and which parts, therefore, are the appropriate seat of glands.

In some leaves a secretory function extends over a great part of the surface, as in some species of Cistus, of sugar-maple, of larch, and others enumerated by Du Hammel, on which various collections of saccharine, gummy, and resinous matter arc found. (Phys, des Arbres, tome i. p. 183.) On the leaves of sage, Hooke mentions the occurrence of an infinite number of round balls resembling pearls, and which, says he, are nothing but a gummy exudation. (Micrographia, p. 142.) Μ. Guettard has described not fewer than seven species of glandular bodies on the leaves of different plants, to which he assigned names, chiefly from the appearance of their form; these are the miliary, the vesicular, the squamous, the globular, the lenticular, the utricular, and the urccolar glands. Of these reputed species, those called miliary are no longer held to be glands, but cuticular pores; and the squamous species is found to be identical with the thin scale that covers the fructification of ferns. Others add to this list the organ called nectary; but the very vague notions entertained of its nature and use altogether preclude the possibility of assigning to it any precise anatomical character.

Other writers have proposed to reduce all the bodies: called glands to two classes, the cellular and the vascular, according as they conceive them to be formed of cellular tissue simply, or of this tissue and vessels combined. But such an arrangement would lead us, in some instances, to confound the mere receptacles of secreted fluids with the organs that secrete them,—would bestow a secretory function on organs considered by these writers to be non-vascular, — and convert the entire cellular tissue of the plant into a simple glandular body. The glands of plants may indeed possess the form and size of cells; but they are not, like cells, close cavities; they resemble more the follicles and mucous glands of the animal system, and, from the nature of their function, must always be regarded as vascular.

Of organs so minute, and so very imperfectly known and characterized, nothing can be attempted in the way of anatomical demonstration. This, however, is the less to be regretted, as the glandular system in plants appears in general to be of much less consequence in the vegetable than it is in the animal economy.

PART II.

THE ANATOMY OF THE INDIVIDUAL MEMBERS AND ORGANS OF VEGETABLES.

In the preceding Part we have described, in general, the nature of the elementary organs and common textures that compose the entire plant; and we come now to the second division of our subject, viz. the description of individual members and organs. Following the method of Grew, we shall first exhibit the anatomy of the seed, and the changes of form and structure displayed in its evolution. We shall next treat of the mature plant, and exhibit views of the more remarkable varieties of structure observed in its several members, as the trunk, the branch, and the root. The organs that originate from these members, as buds, leaves, flowers, and fruits, will then be duly noticed; and we shall terminate our descriptions by a brief exhibition of the formation and structure of the vegetable ovum in its progress towards the state of the perfect seed.

The limits within which, in a work like the present, we are necessarily circumscribed, will render our view of these individual structures, when compared with the immense variety that obtains in nature, exceedingly imperfect; but our purpose will be accomplished if we succeed in exhibiting a correct and tolerably comprehensive outline of the great features of vegetable organization, as they are displayed in the individual parts and organs of the more perfect plants.

CHAP. I.

THE ANATOMY OF SEEDS.

Section I.

Of the Structure of Seeds in general. Art. Ĩ.—Of the External Characters and Component Parts of Seeds.

The seed or egg of vegetables (semen vel ovum) is formed at the base of the pistil of the flower, in an organ called the ovary (ovarium), hereafter to be described. This organ takes the name of pericarp when the seed is mature, and is then said to be formed of three parts, enveloping each other. The outer membrane has been called epicarp, and the inner one endocarp, between which the thicker portion, called sarcocarp, is placed. In fleshy fruits, as the peach and apple, the sarcocarp is very thick, and contains the matter which nourishes the seed. Interiorly, the pericarp is either simple or separated by one or more partitions into cells, which contain the seeds. The seeds accordingly communicate with or are attached to the pericarp, at the point of the seed named hilum, from which point the vesSels[7:3:72] pass through the inner membrane of the pericarp, and are continued to its fleshy portion, which supplies the materials for its growth. These vessels form the umbilical cord (funiculus umbilicalis') ; but sometimes, it is said, this cord, from its extreme tenuity, or implication with other organs, cannot be discovered. When the seed has attained to maturity, the umbilical cord dries up and breaks, and the pericarp opens in various ways in different plants, to permit the escape of t∣ħβ seed-

• The seeds of different plants exhibit the greatest diversity in number, size, and figure. Sometimes they are few, in other instances very numerous. In one plant of white poppy Grew reckoned 32,000 seeds, and on the spike of a species of typha he numbered 40,176 seeds; so that, upon the three spikes which one stalk of this plant bears, there are every year produced more than 120,000 seeds. (Anat. of Plants, p. 198.) In figure the varieties in seeds are so numerous as to baffle description; and, with respect to size, many are so minute as not to be visible to the naked eye, and others so large as to reach several pounds in weight.

The part at which the seed has separated from the ovary is indicated by a small mark or scar, called by Malpighi fenestra, by Linnaeus hilum, and by Gaertner umbilicus. In some seeds this scar is of considerable extent, and is the only mark that is visible; in other instances there seems to be present a foramen in addition to the scar. All seeds, says Grew, have their outer coats open, either by a particular aperture, or by the breaking off of the cord, or by the entrance of the cord into the substance of the seed, as in those which have a shelly or stony covering. In the bean this aperture is placed on the side, in the chesnut on the top, in the gourd at the bottom; and in each case the point of the radicle is opposed to the aperture, and first pushes forth through it. (Anat. of Plants, book i.) In many seeds, however, the radicle is not thus opposed to the umbilicus, but, according to Gaertner, is variously placed with regard to it; and in the seeds of the apple and pear, Malpighi considers no proper umbilical aperture to exist, but only an hiatus to be formed by the relaxation of the tunics, through which the radicle makes its way. (Anat. Plantar, p. 9.)

There can be no doubt that, in every case, a connection subsisted between the seed and the ovary during the formation of the seed; but this, in different examples, may have much varied. In all cases an umbilical cord must have existed, and appears, in many examples, from inspection of mature seeds, to have been the only visible medium of connection. In the bean, however, besides the umbilical cord there are marks of a connection also between the coats of the seed and those of the ovary that contained it, so that the scar or cicatrix, in that and similar cases, is distinct from the umbilical aperture. The scar is the mark left by the separation of the tunics continued from the ovary; the foramen is the aperture produced by the separation of the umbilical cord. It will be afterwards stated that the outer coat of the seed appears always to have originated from the inner coat of the ovary, so that it forms a sheath about the umbilical cord. If, therefore, as in the bean, these coats are thick, and the umbilical cord short, then traces of the separation, both of the coats and of the cord, will remain on the seed; if, on the other hand, the coats are thin, and the cord more elongated, this latter will be closely invested by the former, and the umbilical scar will present the appearance only of a simple aperture, as is common to many seeds.

A late writer, Μ. Turpin, regards the umbilical scar as consisting of three parts, viz. the proper cicatrix itself, in the centre of which he describes an aperture through' which the nutrient vessels passed to the embryo; and near' to it another smaller aperture, through which he believes the spermatic vessels to have passed. The former he calls the omphalode, the latter the micropyle, and declares that he observed it in more than 1200 seeds. (An. du Mus. d’Hist. Nat. tome vii. p. 199.)

From the situation of the umbilicus, the several parts or regions of the seed have been defined. They are six' in number. The part where the umbilicus itself is placed is termed the basis, and the point at the opposite extremity the vertex of the seed; the upper or back part is named the dorsum or back, and the part opposite to it the venter or belly; while the two lateral portions are called the sides (laterd). The point where the umbilical cord is inserted into the inner coat has been named the internal umbilicus. This point usually coincides with that of the external, but, from a change in the relative position of the parts during their formation, this coincidence is not always to be observed. (Gaertner de Fructibus et Seminibus Plantarum, vol. i.)

When examined in its mature state, the seed is found' to be composed of certain coats or tunics, which inclose' a kernel or nucleus, that also consists of several distinct parts. At an early period of growth, while the parts are still green and succulent, two coats are easily distinguished, as in the transverse section of the bean (fig. 30, Plate XXXVIII.), in which the inner coat a appears much thicker than the outer, and the radicle b is seen rising through it.

When these coats are stripped off, the parts which form the nucleus are brought into view. They consist, in the bean and most other seeds, of two distinct parts, the lobes or cotyledons, as they have been called, and the radicle and plume. These several parts can be seen only by separating the two lobes from each other, as is done in fig. 31, where the letters cc denote the cotyledons, d the radicle, and e the plume. Such seeds as have thus two coty ±edons are named dicoyledonous.

In many seeds, however, the part called cotyledon is single, and often bears but a small proportion to the entire bulk of the seed. Seeds which have thus but one cotyledon are named Monocotyledons ; and to this division the seeds of wheat, of barley, and of all the grasses belong.

Many of the lower tribes of plants are entirely destitute of a cotyledon, and are called Acotyledons. Some writers assert that some seeds have more than two cotyledons, and such seeds they have denominated Polycotyledons; but others, again, choose to consider these appearances not as distinct cotyledons, but only as deep fissures, or divisions into two primary lobes; and hence conclude, that all seeds having a cotyledon may be classed under the two divisions of mono and di-cotyledons.

Art. II.— Description and Structure of the Coats of Seeds.

Having given this general view of the several parts that compose the seed, we proceed now to a more particular description of their structure; and, as the tunics come first into view, we shall begin with them. These tunics, in some seeds, are two; in others, three in number. By Grew they were named coats or covers; by Malpighi> secundinae ; and by Gaertner, testa and membrana interna. We shall speak of them in the familiar terms of the outer, the inner, and the middle coats or tunics.

The outer coat or testa of Gaertner is described as a constant and essential part of the seed. It existed before the period of fecundation, and is sometimes the only apparent [7:3:73]∣, covering possessed by the mature seed. Some seeds have indeed been considered to possess no tunic whatever, and have therefore, says Gaertner, been named acocca ; but in such seeds there existed a coat before they arrived at maturity, and its apparent absence has been inferred from its extreme thinness, or its condensation with the sides of the surrounding ovary. (De Fructib. et Seminib. Plantar. vol. i. p∙ 132.) A distinguished botanist, however, Mr Brown, is said to have discovered two examples of seeds absolutely destitute of a covering, from their first appearance to their state of maturity. (Thomson’s Annals of Philosophy, vol. i. p. 310.)

∣ In different seeds this tunic possesses a very different structure, being in some thin and membranous; in others of a spongy or fleshy nature; and in others, again, it approaches to the consistence of leather or bone. But how various soever in this respect, it is always an entire tunic, and has no aperture but that of the umbilical foramen. Its colour is usually deeper than that of the other parts of the seed; and in this particular it presents every possible variety. It has rarely any connection with the nucleus, except in some monocotyledons. (Gaertner, vol. i. cap. 9.) With regard to its origin, Malpighi describes it, in its earliest state in the almond, as derived from the ovary itself, being composed of reticulated vessels, which spring from the surrounding organ. In other instances it is thicker, and is distinctly seen to be cellular as well as vascular; and in the bean and pea, little tubes are said by Malpighi to originate from the cells, and terminate by open mouths on the surface. (Anat. Plantar, p. 9.)

Both Malpighi and Grew discovered, in some instances, a very thin membrane to cover this outer coat, which, according to Gaertner, may be found in most seeds, if the parts be scrupulously examined. Its structure is sometimes membranous, often downy, and sometimes mucilaginous; it possesses occasionally considerable thickness, and at other times is a mere pellicle, and thence has been named pellicula. It covers the whole seed, and does not ever separate spontaneously from it. (Gaertner de Fructib. &c. vol. i. cap. 9.)

Besides this pellicle, another fine tunic named arillus is sometimes observed on the surface of the seed, as in the seed of euphorbia. It originates from the umbilical cord at the base, and extends more or less completely over the body of the seed. Its structure is very various, being sometimes soft and pulpy, at others thin and membranous, and in others forming a husky covering. It forms, in some instances, only a loose and partial covering; and in others it invests the seed so closely and completely, that it can scarcely be distinguished from the outer coat itself. Both in figure and colour it often varies greatly, but, like the pellicle before described, it is regarded rather as an accessory than a necessary integument.

To the exterior coat of the seed various appendages are sometimes attached, as down, wings, spines, hooks, all designed either as a defence to seeds or to facilitate their dispersion. They are distinguished and described by the botanist, but are in general of too fine a texture to be made the subject of anatomical demonstration.

she inner tunic (membrana interna of Gaertner) is a common but not constant part of the mature seed. It appears sometimes to be wanting, when in reality it is present. During the formation of the seed it is frequently so extenuated, or coalesces so completely with the outer coat, that it cannot be properly distinguished. In its earlier state it is represented by Grew as a very spongy and succulent body, and as thick and bulky as one or the lobes itself; but it dries and shrinks up as the seed approaches maturity, so that it is sometimes scarcely discernible. (Anat. of Plants, p. 47.) In the seeds of mpst plants it closely invests the nucleus, but is easily separable from the outer tunic. In those of the Gramineae, where the bulk of the seed consists almost entirely of unorganized matter, no separation of this tunic from the contained parts occurs; but its inner surface is formed into a cellular tissue, in the cells of which the nutrient matter is lodged. In other instances the inner surface is prolonged into processes, which penetrate into the nucleus, and intersect it in various directions.

This inner tunic does not, like the former, exist before fecundation, but is formed subsequently to it. It is composed of vessels and cellular tissue. The cells are commonly larger than those of the outer coat. It has no aperture, says Gaertner, not even an umbilical one; but resembles a shut sac, over whose external surface the umbilical vessels creep, and open, in an insensible manner, within its cavity. (De Fructib. &c. cap. 9.) The distribution of the vessels throughout the whole of this coat Grew compares to that in the leaf.

Beneath this inner tunic Grew describes another fine membrane, which immediately invests the lobes or cotyledons of the seed. In the bean it is exquisitely thin, and so firmly continuous with the lobes, that some dexterity is required to accomplish its separation. It is spread not only over the convex surface of the lobes, but also over the inner or flat surfaces, where they are contiguous, extending likewise over the radicle and plume, and so over the whole nucleus of the seed. It does not, like the other tunics, cease to grow in germination, but is augmented and grows with the organic parts. (Anat. of Plants, b. i. ch. 1.) This tunic may be regarded either as a covering to the nucleus, or as an actual portion of it, as Gaertner, who speaks only of two coats, seems to have considered it. It appears, however, more proper to regard it as the inmost coat, and thus to conclude, with Grew, that the covers in most seeds are three. (Ibid. b. iv. ch. 3.) The coat last described must in that case be considered as the middle tunic, being situated between the outer one and that which is continuous with the nucleus. In many seeds, however, only two coats are distinctly visible.

The foregoing tunics not only contain the nutrient matter, and afford a mechanical protection to the organic parts, < but seem fitted also, by their chemical constitution, to resist the operation of agents that might otherwise effect their decomposition, and that of the nucleus they inclose. From the experiments of Fourcroy and Vauquelin, on the tunics of certain seeds that grow in marshy situations, it appears that, besides the usual ingredients of vegetable substances, there exists in them a compound, formed by a combination of tannin with a peculiar matter of an animal nature, in union with a vegetable acid. This combination of tannin with the matter just mentioned renders these tunics insoluble in water, and enables them to resist putrefaction, although buried for long periods in the moist earth. The tunics of those seeds which do not possess this chemical constitution may, it is added, by their ligneous or horny texture, or by the oily matter with which they are penetrated, present similar obstacles to the action of decomposing agents. (An. du Mus. d’Hist. Nat. tome xv. p. 77.)

Art. III.— Description and Structure of the Nucleus of the Seed.

We proceed next to describe the parts contained with-; in the above-mentioned coats or tunics, and which con-i stitute the nucleus of the seed. These parts, as already 1 observed, consist of the radicle, the plume, and cotyledons, together, in most instances, with the nutrient mat [7:3:74] ter destined to support their future growth. The three former bodies are completely organized, but the nutrient matter is wholly inorganic, varies greatly in quantity and proportion in different seeds, and is very variously situated with respect to the organized parts.

These parts, which in the progress of their evolution give birth to the new vegetable, derive their visible origin from a medullary point, that succeeds to the act of fecundation. By some these organized parts have been called the corcuhιm, by others embryo, by others foetus, and by others plantula seminalis. The primary point or particle from which they originate may with propriety, says Gaertner, be termed corculum, since it is the source and scat of vegetable life, and from it the whole vascular system of the embryo proceeds. In some instances this corculum increases so little as to be scarcely visible even in the mature seed, or exhibits only a palish spot, which has been termed cicatricula ; in others it forms a roundish radicle, whose apex is free and rises above the nucleus, but whose base is firmly connected with it; in others, again, the corculum is still more disengaged, enlarging at each extremity, and producing at one end the radicle, and separating at the other into the two lobes called cotyledons, between which the first bud or plume of the future plant is situated. From this varying growth of the corculum, an embryo, more or less perfect, is produced. When the embryo presents only a mere germinating point, it is styled imperfect ; when it exhibits a simple radicle, it is deemed incomplete; when it possesses both radicle and cotyledon, it is considered perfect ; and when it consists of radicle, cotyledon, and plume, it is pronounced complete. (Gaertner de Fructib. &c. vol. i. cap. 13.)

In the mature seeds of the less perfect plants the embryo is altogether invisible until after germination, and even in many other instances its characters cannot be accurately traced. Its general figure is determined by that of the radicle and cotyledons, and is exceedingly various in different seeds. In size it ranges from a minute point to that of a body of considerable magnitude. In consistence it is almost always soft and herbaceous, but its radicle possesses sometimes a ligneous hardness. No seed contains more than one embryo, except in cases of superfoetation, of which Gaertner saw one instance in Pinus Cembra, a seed of which contained two embryos within one and the same cavity. (De Fructib. &c. vol. i. p. 168.) Malpighi also records a similar occurrence in a seed of Prunus Armeniaca ; and the seeds of some Gramineae, as will afterwards be shown, are capable of evolving an indefinite number of embryos. Every complete embryo is said to consist of three distinct parts beside cotyledons. These are the radicle, the stem, and the plume.

The radicle (radicula), called rostellum by Linnaeus, is the most constant part of the embryo, being found in some seeds in which no other trace of that body can be discovered. In the seeds, however, of the less perfect plants, and even in those of some monocotyledons, no radicle is visible antecedent to germination. In some rare instances among dicoyledons, as in nelumbo nucifera, no radicle exists; but in germination the stem first rises upward, and afterwards emits rootlets from its sides. (An. du Mus. d’ Hist. Nat. tome xiii.)

The size of the radicle is very various, and so also is its figure, being either conical, cylindrical, filiform, or tubercular, &c. It always, says Gaertner, occurs solitary, except in secale, triticum, and hordeum, to which alone, of all known seeds, three, four, or six radicles, properly formed, and distinct from each other, are furnished to each embryo. (De Fructib. &c. p.,169.) This plurality of radicles in the cerealia had before been remarked by( Malpighi. Μ. du Hamel describes the seed of misletoe'' (viscum album) as emitting numerous radicles like those of wheat. (Mém. de l’Acad. des Sciences, 1740.)

The stem (scapus) of the embryo is a continuation of] the radicle, and connects it with the plume. It is frequently wanting altogether, nor, when it is present, can we fix precisely on the point where the radicle ends and the stem begins. What is called stem descends frequently into the earth, and becomes a true root; so that every part of the embryo situated beneath the cotyledons might without impropriety be denominated radicle. The place of junction between the radicle and stem was called by Grew the coarcture, from its presenting often an evident degree of contraction; but Μ. Bonnet and others have given it the more appropriate name of the neck (collum) of the seed.

The plume (plumula) is the first bud of the new plant In seeds that possess but one cotyledon it is very generally wanting; and even in those which have two cotyledons it is not unfrequently absent, or is at least concealed within the stem. In most of the latter sort of seeds, however, the plume is met with. It is placed on the top of the stem or radicle, and lies between the cotyledons, by which it is variously compressed and folded on itself. In the greater number of seeds it is not entire, but at its free end is divided into several pieces, all closely couched together, like feathers in a bunch, and thence called the plume by Grew. In different seeds its several little leaves vary much in figure, size, and number. The structure both of the radicle and plume will be most advantageously displayed in connection with that of the cotyledons.

Of the organized parts of the seed, the organs called by ’ Grew lobes or dissimilar leaves, by Malpighi seminal leaves or cotyledons, remain to be described. The cotyledons derive their origin from the embryo itself, of which they constitute a part. The seeds, however, of some tribes of vegetables, as before remarked, are held not to possess these organs; and in many others the mass of nutrient matter has been confounded with them. When present they are either simple or divided. The simple cotyledon is formed by the mere extension of the corculum, and is in truth scarcely distinguishable from the stem itself. The double or conjugate cotyledons are produced by fissures, which usually divide that part of the embryo that is opposed to the radicle into two or more equal portions or lobes. These lobes have at first the appearance of mere tubercles, and in many seeds they retain this form unchanged; but in others they gradually expand into lamella or plates, which augment in size, and finally exhibit the proper form of cotyledons. This form is very various, as likewise is the size of these organs. Sometimes they are so small as to be scarcely visible, and sometimes so large as to form the chief portion of the seed. Their substance is either thin, or thick, or turgid. Their colour is commonly white, but sometimes yellowish, purple, or green, the colour into which they all pass during germination.

Concerning the structure of the cotyledons, it may be said that, in the more perfectly developed seeds, they are formed of cellular tissue, through which vessels are everywhere distributed; and, as we have already remarked, they are everywhere covered by a fine pellicle or coat, which prevents alike their adherence to the plume and to each other. This cellular structure of the cotyledon is well displayed by Grew (fig. 32, Plate XXXVIII.) in a slice of the cotyledon of the recent bean; and it is easily seen in a thin slice of almost any mature seed, if it be held [7:3:75]. against the light after it has been soaked in water. This 'cellular structure extends into the radicle and plume, but in much smaller proportion, constituting, according to Grew, about three fifths of the plume, five sevenths of the radicle, and nine tenths of the cotyledon.

Through all the organs that compose the nucleus vessels are distributed, by the medium of which a general communication is established among them. This vascular system is likewise exhibited by Grew in the dissection of a bean (fig. 34, Plate XXXVIII.), in which the vessels are seen to branch off on each side from the radicle, and spread themselves by innumerable ramifications through the cotyledons. From the radicle vessels also pass upwards to the plume. These vessels of the radicle are visible when a transverse section is made through it, as in fig. 33, α, Plate XXXVIII., in which they are seen to occupy the middle of that body. When the section is made higher up at the neck of the embryo, as in the same figure, b, then the central trunk, surrounded by several smaller fasciculi of vessels, passing to the different parts of the plume, is still more clearly exposed. In many seeds, however, the organized parts are so small that their general structure cannot be traced, except during the progress of their germination. We shall therefore defer the description of them till we come to treat of their evolution, and shall then also go more fully into the structure of the parts just mentioned.

Within the cells of the cotyledon, in many dicoyledonous seeds, the nutrient matter destined to support the future growth of the embryo is entirely contained. In other instances this matter is only in part received into those organs; and in the Gramineoe, and other monocotyledons, it is often placed almost entirely exterior to the cotyledon. This matter is produced from a clear liquor that is secreted in the tunic during the formation of the seed. To this liquor Grew gave the name of albumen, from its likeness, not only in appearance, but, as he conceived, in use also, to the white of egg in animals. By Malpighi this matter, considered in connection with the tissue that contains it, is often called the flesh (card) of the seed; and from its being sometimes situated around the embryo, it has been denominated perisperm by Μ. Jussieu. We follow Grew and Gaertner in the use of the term albumen, meaning to express thereby, not the primaryanimal compound to which chemists have of late assigned that term, and which is found but in few vegetables; but that compound substance which, whatever be its situation, quantity, or colour, constitutes the nutrient matter of the seed.

This albumen is a very constant part of the mature seed, but its proportion in some seeds is so extremely small, that the seeds in which it occurs have been termed exalbuminous ; and in a few instances it seems to be entirely wanting. Its quantity, situation, and figure, in different seeds, are subject to very great variation. In the seeds of the Gramineoe, where the embryo acquires only a very small size, the albumen constitutes almost the entire bulk of the seed, and is placed wholly exterior to the embryo. In the Leguminosoe, on the other hand, the embryo is more completely developed, and the whole of the albuminous matter is contained within the cotyledons. In beet (beta), and many others, the albumen is partly received into the cotyledons, and lies in part exterior to them; and where this occurs the embryo sometimes encircles the albumen, and is sometimes encircled by it. In Rheum the embryo is placed in the centre of the albumen, in Rumex and some others it is applied on the side of it, in Atriplex the long cylindrical embryo surrounds the albumen, in B'óerhaavia the embryo and its cotyledons cover entirely the granulated substance of the albumen. (Μ. Jussieu, An. du Mus. d ’ Hist. Nat. tome v. p. 224.) In the onion (allium cepa) the embryo makes several curves within the substance of the albumen, and in dodder (cuscuta) it is twisted around it in a spiral form; so that the relative positions of the embryo and albumen, as well as their quantity, proportion, and figure, are subject to endless variation.

But however much in these respects the albumen may vary, it is always contained within an organized structure. Sometimes this structure is that of the cotyledon, as already exhibited in the bean (fig. 32, Plate XXXVIII.), the cells of which contain this albuminous matter. Where the albumen is placed exterior to the embryo, as in the seeds of wheat, it is nevertheless contained in a cellular tissue. This is exhibited in fig. 27, Plate XXXVIII., copied from Lecuwenhoeck, in which cells of an hexagonal form are seen to be filled with the albuminous particles that constitute the white matter or flour of that seed. This mealy part of wheat he describes as consisting of minute globules, inclosed in a kind of membrane so exquisitely thin as scarcely to be observed, within which the globules are contained, as it were, in cells. The globules appeared to be of different sizes, not perfect spheres, but having an indentation on one part, which led him to suppose that they were not formed by simple accretion, but by some mode of growth, and that “the membranes which inclose them in cells must be provided with so many veins or vessels, that every particle of meal may have its separate vessel.” He even conceived the globules themselves to be inclosed individually in a thin skin or shell; but this opinion he never brought to ocular demonstration. (Select Works by Hoole, vol. i. p. 169.) Similar observations on the albuminous part of wheat have since been given by Mirbel; and Kieser and others have delineated the globular particles contained in the cotyledonous cells of the bean and other seeds; so that whether the albumen be situated in the cotyledons, or be placed exterior to them, it is in every case contained in a similar and distinctly organized structure.

In consistence the albumen is said to be either farinaceous, fleshy, or cartilaginous; and it may exist in ' various intermediate states. The farinaceous kind is 1 readily reduced to powder, and is dissolved by water into a viscous mass. The embryo is generally placed exterior to this species of albumen, as in the Gramineoe. The fleshy albumen is more frequent. It is softer than the former, and dissolves by water into a gelatinous mass. It is often entirely contained within the embryo and its cotyledons, and yields the thick oil that is expressed from many seeds. Lastly, the cartilaginous species has a horny consistence, is difficultly soluble in water, and not easily reduced to powder. The embryo is never placed exterior to it, and when it contains oil, this is usually very thin. (Gaertner de Fructib. &c. vol. i. cap. 10.)

In many seeds the albumen serves as a support and defence to the embryo, as well as for nutriment. If it be removed previous to germination, as was done by Mirbel (An. du Mus. d’Hist. Nat. tome xiii. p. 157) in the seed of the onion, and by Dr Yule (Wem. Trans, vol. i. p. 591) in different species of Gramineoe, the embryo, though planted in a rich soil, and carefully tended, grows but feebly, and for the most part dies.

Besides the albumen above described, Gaertner has re- Ί vived the use of the term vitellus, but employed it to designate a very different part from that to which it was originally applied by Grew. The latter made use of this term to designate the inorganic matter of the mature seed, which in the early stage of its production he called [7:3:76]albumen (Anat. of Plants, book ív. chap. 3); but it is employed by Gaertner to indicate a small membranous body, which in many seeds is placed between the embryo and albumen, and is closely connected with the former, but separates easily from the latter. The figure of this small body is described as being very various in different seeds. It is said not to rise out of the earth during germination; but, like the albumen, seems destined to afford nutriment to the embryo. In the Gramineae it represents a thin scale interposed between the albumen and embryo, to which, from its shield-like form, he gives the name of scutellum. (De Fructib. Plantar, vol. i. cap. 11.) There can be no doubt that this scutellum of Gaertner is the little “conglobate leaf” first observed in wheat by Malpighi, and which later writers have denominated the cotyledon of that seed. In the next section its form and situation will be clearly displayed.

Section II.

Of the Structure of Monocotyledonous Seeds, as displayed in their Evolution.

All seeds have by some botanists been distinguished into such as possessed one or more cotyledons, and such as were entirely destitute of them. In treating of seeds under the two divisions of mono and di-cotyledons, we would not be understood to deny the existence of seeds that possess more than two.

Some seeds are so extremely minute, that, until lately, their existence was not clearly ascertained; and it is only during their germination that their general form and character can be detected. In many others, the organized parts are so small as to be scarcely capable of demonstration, except by following the progressive changes of form they exhibit in their evolution. We propose, therefore, to select, from each of the two divisions of mono and di-cotyledonous seeds, an example or two of the successive appearances displayed in their evolution, which will, besides, form the best introduction to a knowledge of the structure of the mature plant.

In most of the monocotyledonous seeds the cotyledon does not appear above the soil during germination, but is retained within the coats of the seed, and consequently undergoes but little alteration in size. As an example of the evolution of a monocotyledonous seed, we shall select that of wheat (triticum hibernum), because its development has been studied with great care, and, in common with some others of the same natural family, it exhibits some striking peculiarities, which add greatly to its productive powers. The successive appearances exhibited in its evolution have been given with great accuracy by Malpighi, who has carried its anatomy farther, in some points, than most of his successors. In the earlier stages of growth, some very accurate representations of it have also been given by Μ. Poiteau; and Dr Yule has likewise obliged us with some valuable observations. From these different authorities, confirmed generally by our own observations, we shall endeavour to present a concise view of the structure and evolution of this very important seed.

If we take a grain of wheat, and examine its convex side, we observe, at its base, a small oblong body (fig. 35, Plate XXXVIII.) lying in a semicircular depression, which is well defined through the tunics that cover it. These tunics are two in number; an outer one, to which the chaffy filaments at the vertex of the seed are attached, and which readily separates when moistened; and an inner one, which everywhere adheres closely to the cellular tissue that contains the albumen. If these two tunics be raised and thrown back, as is done in fig. 36, the little oblong body h, and its semilunar appendage' i, placed behind it, are brought into view; and, together, they constitute the embryo.

Let next a vertical section of another seed be made in the direction of the furrow that runs along its flatter side, and let this section pass through the embryo, as is represented in fig. 37: we then observe the seed to be composed almost entirely of albumen k, with which the embryo I, consisting of minute convoluted leaves, is in close contact. The part of the embryo that is applied against the albumen is the cotyledon, which on that surface is convex, and on the opposite one concave.

In fig. 38, the entire embryo has been removed from its connection with the albumen, and a front view of it, considerably magnified, is there given, in which the letter m denotes the cotyledon, in the concavity of which the plume n is lodged, and o indicates the protuberances from which the radicles afterwards spring. If now this same embryo be reversed, as is done in fig. 39, then the convex back of the cotyledon only is seen, with the extremity of the principal radicle at the base. It is this side of the cotyledon that was applied against the albumen; and its polished surface, says Μ. Poiteau, proves that it nowhere adhered by any organic structure. Gaertner also remarks that the connection between these parts is not organic, but merely superficial,—an observation that is true as far as relates to the embryo itself and the albuminous matter, but not as applied to the tunics which envelope them; for, at the base of the seed, the inner membrane, which contains the albumen, appears to be continuous, as Lecuwenhoeck remarked, with that which covers the cotyledon, being reflected from the albumen over the cotyledon, much in the same way as the pleura and peritonaeum, that line the sides of the great cavities in animal bodies, are reflected over the viscera they contain. Such are the appearances presented by this seed antecedent to germination: let us next follow it through the several stages of that process.

After a seed of this species has been in contact for 24 or 30 hours with the humidity necessary to its germination, its embryo becomes swollen, and, when removed from the other parts, and moderately magnified, presents the appearance exhibited in fig. 40. In this figure the radicle is rendered more protuberant, and the fine tunic that invests it has undergone an alteration, being changed from a smooth, opaque, and solid texture, to one that is villose, transparent, and cellular. A vertical section of the same] embryo, as exhibited in the next figure (41), shows the elongation of the principal radicle p, which caused the protuberance below, and the sprouting of the two lateral radicles p p, which push forth more slowly on the sides. These three radicles soon force their way through the sac that envelopes them, which then forms sheaths around their origins. In the same figure, the letter q denotes the plume, consisting of several convoluted leaves, and resting on the cotyledon. In fig. 42 the appearance of the seed, in a stage a little more advanced, is exhibited. The plume r is now seen to have risen above the cotyledon s, and the three radicles, surrounded at their origins by their proper sheaths, have greatly increased in length, and in- i numerable capillary rootlets are emitted from their sides. (An. du Mus. d ’ Hist. Nat. tome xiii. p∙ 383.)

The daily appearances exhibited in the evolution of this seed, as previously given by Malpighi (Anat. Plantari p. 103), accord well with the above representations of Μ. Poiteau; and he has noticed some additional particulars of considerable importance. On the first day of germination, he represents the plume of the embryo as beginning to [7:3:77]open, and the protuberances, whιeh indicate the eruption of the three radieles, as beginning to form. The radicles at this period are completely enveloped in a membranous sac or involucrum; and the body of the embryo is closely connected with a “conglobate farinaceous leaf, by which nutriment is administered.” This conglobate leaf is the cotyledon before mentioned, and its connection with the radicle and plume is well shown by Malpighi. In fig. 43, t, he exhibits a front view of the radicle and plume, as they appear when separated from the cotyledon; and at the letter v of the same figure a back view of the same body is displayed, in which the letter x points to the mark or scar that denotes the place of separation. Malpighi believed these parts to be united with each other by a little node, hereafter to be described; but it is by the medium of vessels that this connection between the cotyledon and the other parts of the embryo is maintained; and by this route alone can the nutrient matter or albumen be conveyed through the cotyledon to the radicle and plume. To these vessels Μ. Bonnet gave the distinctive appellation of mammary : the union they form between the different parts of the embryo is so close, that at this part, says Gaertner, the cotyledon, and radicle, and plume, form one undivided body. (Gaertner de Fructib. Plantar. vol. i. p. 149.)

On the second day of germination the exterior tunie of the seed, according to Malpighi, gives way; the plume rises upward; the radieles do not as yet pierce their investing sac, but this sac is turgid with juice, and is covered exteriorly by a fine white down: the cotyledon also, at this period, is rendered moist.

During the third day the cotyledon is quite turgid with juice; the plume is much enlarged, and begins to look green; the three radicles have pierced the enveloping sac, and are everywhere thickly covered with hairs; and above the first two lateral radicles, two small protuberances, y, z, fig. 44, the origins of two more radicles, are now seen to emerge, while the sac that envelopes them is observed sensibly to waste.

When the third day has elapsed, the plume, inclosed in a fine transparent membrane, is still more elevated, and acquires a greenish colour: the protuberances of the two new radicles are more prominent, and the three former radicles have greatly augmented; the cotyledon is much softer, and, as if milky, yielding, when compressed, a white and sweetish liquor.

After the completion of the fourth day, the plume, continuing to ascend, pierces the membranous covering a' (fig. 45), and pushes into day a permanent leaf, green and convoluted, around which the membrane forms a sheath. Inferiorly, the first three radicles have greatly extended, and the two others, b' b', are much increased: the outer coat of the seed now begins to lessen, but still contains a sweetish liquor. Μ. Poiteau gives a section of the entire plantule about this period of its growth,, which agrees very exactly with the figure of Malpighi. In -this section (fig. 46) the plume d is seen to have pierced the membrane d’ that formerly inclosed it; the albumen d is diminished; the cotyledon J* ʹ retains its situation and form; and the five radicles ġ d are nearly of a length, and covered with hairs.

About the sixth day the plantule, still invested by its sheath, begins to open and expand; the seminal tunics shrink, and the surface of the outer coat is corrugated. ∏ these tunics are cut open, the cotyledon within is observed in some parts to be firmer than before, and has the appearance of a concave leaf; but in other parts it is more vascular and filled with juice, especially in that part near to the mammary vessels.

After the eleventh day these tunics still adhere to the plantule, but appear much wasted, and the juice they contain is mixed with bubbles of air; while the stem forming many knots, and the radicles emitting innumerable rootlets, continually augment in size. Where the vegetation has been very active, the whole original contents of the seminal tunics are by this time exhausted, and, when compressed, they yield only a watery fluid.

After the lapse of a month, when the parts already developed are still farther advanced, new buds break out from the primary seat of growth and rise upward, and new radicles push forth and descend. So readily are these radicles produced, that sometimes, if the primary ones be removed, others in crowds spring forth; at the same time new buds or shoots, protected in their proper sheaths, arise from the same part, and surrounding the primary plantule, are borne upward with it. Of these appearances accurate delineations are given, and they may be observed in every field of growing wheat.

The foregoing descriptions of Malpighi are in general very correct, and his figures, though somewhat rude, exhibit faithful delineations of the objects they are destined to represent. In one or two points, however, he has fallen into error, which, in the above statement of his opinions, to avoid confusion, we corrected as we went along. Thus, though he distinctly points out the “conglobate farinaceous leaf” as the organ by which nutriment is administered to the radicle and plume, he assigns to the sac that, in an early state, envelopes the radicles, the function of placentula, and even gives to the exterior tunic of the seed the title of seminal leaf. The true cotyledon, however, which never in this seed is produced into a seminal leaf, is the little conglobate body above mentioned; and the common tunies of the seed have no title to the appellation of seminal leaves. To this cotyledon Gaertner, from its shield-like form, gave, as before observed, the name of scutellum. He held it to be characteristic of the Gramineoe, and analogous to the organ to which, in some other seeds, he gave the name of vitellus. (De Fructib. Plantar, vol. i. p. 139.) But later writers, as Jussieu, Smith, Brown, and Poiteau, have all restored to it its proper office of cotyledon.

Μ. Poiteau has gone even farther, and asserted the existence of a second cotyledon in this seed, and in the oat, which he describes as situated directly opposite to the former. (An. du Mus. d’Hist. Nat. tome xiii. p. 388.) In this instance, however, he has mistaken the rudiment of the second bud for a second cotyledon, as Dr Yule ascertained by “tracing the growth of this supposed cotyledon from its first becoming visible to its final development as a plant.” (Werner. Transac. vol. i. p. 594.) Μ. Mirbel considers the sac that invests the plantule to be the cotyledon of this seed, and this cotyledon to form the first enslicaihing leaf. (An. du Mus. d’Hist. Nat. tome xiii. p. 148.) But, as already remarked, the cotyledon never in this seed rises out of the tunics; and, as Dr Yule observes, differs totally in situation, structure, and consistence, from the ensheathing leaf of the plantule.

A very remarkable peculiarity in these plants is their great reproductive power, as displayed in the indefinite number of new plants which we have seen to be evolved from one primary seed. Malpighi not only observed this peculiarity, but has described the structure from which it originates. He considered the radicle and plume of the embryo to be connected with the cotyledon, not by the mammary vessels, as we have stated, but by a little body which he called the umbilical node. In a section of the lower part of the stem of the plantule, made after the third day of germination, he delineates this mode as situ [7:3:78]ated at the junction of the radicle and plume, as represented by the letter Zt' (fig. 48, Plate XXXVIII.); and describes it as solid exteriorly, and softer and more medullary within. If a section of the same part be made on the fourth day, as in fig. 47, the stem i' of the plantule will be seen, says he, to spring from, this node, from which also the radicles equally take their origin.

This peculiar property was also observed by Lecuwen-hoeck, though he seems not clearly to have apprehended the nature of the organs from which it proceeded. In the embryo of wheat he describes three points, from which not only three distinct radicles spring, but they are also, he adds, “the beginnings of three several spires or stalks of wheat; so that from every grain of wheat (which is well worthy of observation) there will arise not merely a single stalk, but three distinct ones, which are formed in the seed itself.” Select Works by Hook, vol. i. p. 169: and in vol. ii. p. 289, are to be found similar observations on the seeds of oats, barley, and rye.

By Μ. Mirbel, the umbilical node of Malpighi is considered as a fleshy knot (un noeud charnu), by the medium of which the plume and radicle are united. The lateral radicles which issue from it he regards as distinct in their nature from the primary one, and as resembling those which spring from knots in the stem; he therefore names them articular roots, les racines articulaires. (An. du Mus. d’Hist. Nat. tome xiii. p. 149.) According to Dr Yule, however, this fleshy knot is to be considered as a tuber, analogous to the tuberous substance interposed between the bulbs and roots of the Liliaceae and other monocotyledonous tribes; and which is destined to produce an indefinite number of young plants, a greater or less number of which are subsequently evolved by the joint agency of the roots and leaves. The “articular roots” of Μ. Mirbel he regards as in reality young plants, the roots of the Gramineae being invariably fibrous. It is by means of these lateral shoots and their tubera that bushes, consisting of from sixty to several hundred stems, are sometimes seen to originate from one seed.

The above important peculiarities in the germination of the seeds of the Gramineae are very perspicuously displayed by Dr Yule in the three figures which we have copied from his Memoir. In fig. 49, Plate XXXVIII., Dr Yule represents the embryo of wheat as it appears when detached from the albumen, a short time after germination has commenced; the ascent of the plume covered with its membrane, and descent of the three primary radicles, which have pierced their containing sac, are clearly exhibited; and the letter Ä' points to the little cotyledon placed at the junction of the two parts just mentioned. In fig. 50 the germination of the same seed is shown in a more advanced stage; the plume l’ has now risen to a considerable height, and pierced the investing membrane; and at m! a second bud or plume (which Μ. Poiteau mistook for a second cotyledon) is seen to shoot from the tuber like the first. The letter n' denotes the seminal tunics. At a still more advanced period, four young plants, o'o'o'o', fig. 51, of the second month, with their sheaths in part withered, are seen to have sprung from the same part; but the two seminal tunics of the seed, exhausted of their contents, still remain attached, as indicated by the letter pt. ( Wernerian Trans, vol. i. p. 589.)

The description given above of the evolution of wheat is applicable, with little variation, to the seeds of all the cerealia. The seed of the oat emits from four to six radicles, all of which break through their enveloping sac at the same place, and thus appear to be contained in one sheath. Such too is the case with barley, the plume of which extends beneath the seminal tunics, and pushes out at the vertex of the seed.

This peculiar constitution of the seeds above mentioned is attended with important advantages in their culture, and explains the source of their great productive power. A single grain of barley was observed by Du Hamel to have produced 200 ears, each of which yielded 24 grains; so that one single seed planted in a good soil has produced 4800 grains. Curtis and others, by transplantation of the several plantules of wheat, obtained still higher returns from single seeds. For the same reason, these plants are better enabled than others to resist the injurious effects of accident or disease. If a seed, says Dr Yule, be buried under a stone or lump of indurated clay, the seminal plantules cannot shoot upward; but stems are then sent off in a horizontal direction, until they can effect their escape upward. Sometimes it happens that a small insect (Musea pumilionis) deposits its egg in wheat, and the grub is lodged in the very centre of the stem, just above the root, by which the stem is invariably destroyed, and the root so materially injured as to prevent its throwing out fresh shoots on each side, or stocking itself, as the farmers term it. Nevertheless, the plants thus attacked are not permanently injured; for, in the instance where these depredations occurred, the crop of wheat was good, and the ears large and fine through the whole field; so that these injured plants, by the production of lateral shoots, yielded an abundant crop. (Lin. Trans, vol. ii. p. 76.)

In the germination of other monocotyledonous seeds a similar succession of phenomena present themselves, with the exception of those which relate to the multiplication of so many individuals from a single seed. In all, the radicle first shoots forth, and the plume soon follows; the cotyledon is commonly of small size, and is retained within the tunics. As the embryo grows, the albumen is taken up and conveyed through the cotyledon to the young plantule; and before the albumen is exhausted the embryo is enabled to draw its nutriment from the soil in which it grows.

In many instances it appears that the primary radicle of seeds, after a short time, becomes dry and falls off, and is replaced by a great number of secondary rootlets. Μ. Poiteau regards this last circumstance as common and peculiar to monocotyledonous seeds. He has remarked it in many hundred palms, not one of which had a descending or tap-rbot. No plant in the numerous family of the Liliaceae is said to possess such a root. The radicle of the Cyperaceae does not, perhaps, perish so soon; but it does not continue long. This premature and constant destruction of the radicle he considers as the cause of the bulbs and truncations which occur, particularly in the Li∙ haceae ; for the lateral roots not being capable of receiving all the descending sap, it collects at the lower part of the stem, and occasions these different enlargements. (An. du Mus. d ’ Hist. Nat. tome xiii. p. 392.)

The effects which thus succeed to the spontaneous destruction of the radicle occur partly in other plants, in which the first radicle is naturally permanent, if it be artificially removed. Du Hamel found that if the extremity of permanent radicles were cut off, lateral rootlets were produced; that even mechanical obstruction to the descent of the radicle frequently gave rise to divisions in it, and the production of these lateral rootlets. He ascertained also, by experiment, that roots extend invariably, not by an elongation of parts already formed, but by new matter added to their extremities; and hence it is that roots, whether ligneous or herbaceous, do not elongate if even the smallest portion of their extremity be cut off. (Phys, des Arbres, tome i. p. 83.) The results of observa- [7:3:79] . tions on the growth of carrots in different soils, by Mr 'Knight, correspond with those of Μ. du Hamel.

Section HI.

Of the Structure of Dicoyledonous Seeds, as displayed in their Evolution.

From the greater number of seeds which have two cotyledons, the phenomena of their evolution may be expected to exhibit at least as great variety, as those of the division last described. In different species they differ in this respect as much from one another as they do from monocotyledonous seeds. Some seeds of this class raise their cotyledons above ground during germination; in others these organs are wholly retained within the tunics. Of each of these modes of evolution we propose to give an example, selecting, as before, those seeds which have been most accurately observed; or which, by the forms they exhibit, seem best calculated to illustrate the general laws by which their evolution is accomplished.

In the seeds of this division the embryo is commonly much more completely developed than in those of the class last described, so that the several organs of the plantule become distinctly visible. The radicle and plume are readily distinguished, and the cotyledons are frequently so large as to form nearly the entire mass of the seed. Within the cotyledons the albuminous matter provided for the nutrition of the embryo during its evolution is often entirely contained; and these organs, as before remarked, rise sometimes out of the earth, increase greatly in size, and after a certain period decay. In other instances no increase in size nor elevation above the surface occurs, but, like the greater number of monocotyledons, they remain beneath the soil, and yield gradually their nutrient matter for the support of the embryo. Even in plants of the same natural order, the Papilionaceae for example, some, as lupinus, says Dr Smith, raise their cotyledons into the air and light; while others, as lathyrus, retain them under ground, concealed within the tunics of the seed. As an example of the latter, we shall give from Malpighi an abridged account of the successive appearances exhibited by the common pea (pisum), which, in its evolution, approaches in some respects to that of the seeds last described.

The figure and size of this seed are familiar to every one. After being placed for a day in circumstances favourable to its germination, it is much increased in size; its outer coat is rendered softer, and becomes more white and thin; the umbilical aperture continues closed, but near to it an irregular opening or laceration is visible. If the outer coat be now stripped off, the nucleus comes into view. It is seen to consist of two distinct parts or lobes, which are the proper cotyledons of the seed. These cotyledons are closely invested by the inner tunic; externally they have a convex surface, but internally, where they are in contact, their surfaces are nearly plain. Between them, in a small depression formed in their substance, lies the plume: it is composed of a number of yellowish leaves folded on each other, and bent inward, and is united by a little curved stem to a small white conical body, the radicle. These appearances are exhibited in fig. 1, Plate XXXIX., in which one of the cotyledons has been removed, so that the inner surface of the other, together with the plume and radicle, is fully brought into view: the letter a denotes the cotyledon, b the plume, and c the radicle. This radicle at its neck, or point of junction with the stem, sends off on each side a little stalk or pedicle to each cotyledon. In the above figure one of these pedicles has been cut through, and the other, that remains attached to the cotyledon, is con-∣ cealed behind the plume. It is by these pedicles alone' that the two cotyledons are connected with each other.

When the second day of germination is completed, the: cotyledons are rendered more tumid, the tunics give way, and the radicle begins to protrude. Soon after, the cotyledons separate a little, and become somewhat concave internally. After the third day the radicle has pushed out through the tunics; it is white, except at its point, which is more deeply coloured, and it emits on all sides fine capillary rootlets; the cotyledons are now farther separated, and by degrees the stem of the embryo, with its curved plume, is disclosed.

About the fifth day the stem d, fig. 2, mounts upwards: it is white, and bears on its summit the plume e, still curved, and now becoming green: the stem now also begins to exhibit the marks of knots at particular parts: the radicle/is farther advanced, and small protuberances, the origins of future rootlets, appear on it: the cotyledons g retain their place, are turgid and solid, and still surrounded by the lacerated tunics.

At the close of the seventh day the plantule is much more advanced: the knots on the stem h h, fig. 3, are quite distinct, and its apex is furnished with broad green leaves, but which are not as yet unfolded. The substance of the cotyledons is still solid, and when compressed yields a bitterish juice: the radicle is much elongated, and has emitted numerous rootlets.

After the ninth day the plantule is completely formed: its stem i, fig. 4, is now erect, and the leaves of the plume k are expanded: the cotyledons I are reduced in size: and the radicle m, or root as it may now be termed, with its numerous rootlets, is greatly augmented. Every part of the plantule except the cotyledons continues daily to increase: at the end of a month these organs are still found to adhere; but are wrinkled, thin, and exhausted of their nutrient matter, with which the stem and other organs are filled.

The progress of evolution in those seeds that raise their cotyledons above the earth is exhibited by Malpighi in that of the gourd (Cucurbita). This seed is of an oblong figure, and has a flattened form. It possesses three distinct coats or tunics: the outer one is thick, tough, and coriaceous; the middle one thin, membranous, and of a greenish colour; and the inmost is that transparent colourless pellicle that closely invests, and is inseparably connected with, the cotyledons of the seed.

After this seed has been made to imbibe moisture, the outer and middle tunics readily separate, and expose the nucleus, which is seen to consist of two leaf-like cotyledons, which have no connection with each other, except by the medium of the little conical body, or radicle, at their base. The size and figure of these cotyledons, and the situation of the radicle that connects them, are represented in fig. 5, Plate XXXIX. Their external surface exhibits to the naked eye prominent lines, which indicate a vascular structure, the vessels of which proceed from the radicle at their base. They are commonly five in number, and from their main fascicular trunks ramifications are produced, which, in their distribution, form a finely reticulated appearance over the whole organ. On their inner side the cotyledons are quite plain, and closely applied against each other, but, as already remarked, are nowhere connected, except at the base. This surface is displayed in fig. 6, in which the great vascularity of the organ is rendered more apparent. It is between the two cotyledons that the plume, consisting of minute convoluted leaves, is lodged and cherished. In fig. 7 a part of the nucleus of this seed is represented a little enlarged, and [7:3:80]the two cotyledons have been removed at different places by transverse sections, to show more clearly the situation of the radicle and plume. The letter n denotes the place from which one of the cotyledons has been removed, so as to bring the plume o into view; and p points to the conical radicle below.

When this seed has been twenty-four hours m circumstances favourable to its germination, it is rendered tumid, and the umbilical aperture at its base is enlarged by the swelling of the parts within: the cotyledons become turgid, and the plume is augmented in size. After the second day the outer coat is much softened, the middle one appears as if torn and decaying, and all the parts within still further augmented in size. During the third day the colour of the outer tunic becomes darker, the cotyledons are more swollen, their vessels more conspicuous, and the radicle pushes out through the umbilical aperture·

When the fourth day has elapsed, the plantule is still retained within the tunics; and if these be now removed and examined, the middle one is found to be dry and thin: the cotyledons (q, fig. 8) are whitish, soft, and flexible, but the vessels on their surface are much more distinct: the radicle r is elongated and covered with down, as likewise is the stem s. At the top of the radicle a protuberance is seen, which is white and soft; and farther down appear several smaller tumours, indicating the places of rootlets about to break out.

About the sixth day the cotyledons t, fig. 9, emerge from beneath the tunics, representing the “dissimilar leaves” of Grew, and the “seminal leaves” of Malpighi, but which we shall in future denominate cotyledonous leaves. They are thick, soft, and a little separated from each other; their position is pendent; the colour yellowish; they are very vascular, and between them the plume is still concealed. The radicle u, at this period, is much elongated, and rootlets everywhere spring from its sides; the stem v is also lengthened and curved, and, in common with the radicle, is everywhere covered with a white curling down.

Towards the ninth day the cotyledonous leaves (x, fig. 10) assume an erect position. At their points they are still yellowish, but elsewhere green, and their cellular tissue is filled with a greenish-yellow juice; they begin to separate a little, but still entirely conceal the plume. The stem y, at this period, is greatly elongated, and its lower extremity has become green; the protuberance that existed at this part is greatly lessened; and below it the radicle z is continued, from which numerous rootlets, covered with capillary productions, break out.

During the following days the cotyledonous leaves continue to enlarge, the stem to elongate, and the plume to augment in size; but it is not yet unfolded. About the twenty-first day the development of the plantule appears to be completed. A representation of its foliage at this period is given in fig. 13; the cotyledonous leaves, a', have now reached a great size, are of a deep green colour, and very vascular: they rise by a short pedicle from the summit of the stem. On each leaf seven fasciculi of vessels are visible, which, beyond the middle, terminate in a nét-work, from which is produced the cellular structure that contributes to form the breadth of the leaf. In the axil, formed by the cotyledonous leaves, the plume b lay concealed; it is now disclosed by the removal of one of those leaves. At first the leaves of the plume are curled and convoluted, but afterwards they expand, and their figure is then seen to differ entirely from that of the cotyledonous leaves; they have notched margins, and their surface is covered with down. In this manner the young plant continues to increase, and acquires at length∣ 'its full magnitude, in the progress towards which the cotyledonous leaves waste gradually, and finally fall.

The seeds of the radish (Raphanus), of the lettuce: (Lactuca), and of the kidney-bean (Phaseolus), are repre-< sented by Malpighi as exhibiting, in their evolution, a si- s milar succession of appearances. In all these seeds, and in many others of this division, the radicle first pierces the seminal tunics; next the cotyledons come into view, and assume generally the form of leaves, between which the tender plume is for a time concealed, and at a later period is disclosed. In different seeds, however, the forms of these organs, and the periods of their successive evolution, are subject to the greatest variation, not only as relates to the species of seed, but to the soil, the climate, and season in which it is destined to grow.

Having thus surveyed the changes in external form: which the germinating seed exhibits, we shall conclude s our description by a display of the peculiarities of its internal structure. This we shall find to consist entirely of vessels and cellular tissue, variously proportioned and combined. In a longitudinal section of the radicle of the pea in an early stage, Malpighi represents it, as in fig. 11, to be composed entirely of cellular tissue exteriorly, in the centre of which the vascular system, separating at the top into three divisions to supply the plume and cotyledons, is placed. A similar section of the radicle (fig. 14) on the seventh day exhibits corresponding sections of the rootlets it puts forth; which are seen also to consist of cellular tissue, and of vessels that come off from the central fasciculus of the radicle.

The stem, when about a month old, is composed of a thick bark formed of cellular tissue, within which are several fasciculi of vessels. In some very thin slices of the stem of the pea, viewed through a microscope of considerable power, the arrangement and distribution of the vessels and cellular tissue appeared to us as represented in fig. 15. The centre of the stem was occupied by cellular tissue, round which was a zone of vessels, c,' forming four principal fasciculi. Exterior to this zone was a small ring of thickened cellular tissue, and beyond this the proper cellular substance of the bark. Near the circumference of the stem were four larger fasciculi, which may probably be considered as the “proper vessels,” while those near the centre may be deemed the sap-vessels.

In the plantule of the gourd a similar structure is observable. In fig. 19 is represented a longitudinal section of its stem and radicle on the ninth day, as given by Malpighi. In this stem the vessels are disposed in a circle that surrounds the pith; but as they descend towards the root, they approach each other, and give off ramifications to form the rootlets. In a transverse section of the stem, on the 21st day, Malpighi describes it as hollow in the centre, around which six fasciculi of vessels are disposed, and the intermediate portion is occupied by cellular tissue. Hedwig and Kieser, however, enumerate not fewer than ten fasciculi of vessels in the stem of this plant, some of which are placed next the pith, and others near to the bark, as in fig. 28.

To these representations of the structure of the pea and gourd by Malpighi, we shall add that of the bean in its early stages, as delineated by Grew, who has traced the distribution of the vessels in the radicle and cotyledons with great minuteness. In fig. 12, Plate XXXlX., is exhibited a vertical section of a young bean, which is made to pass through the cotyledons, the plume, and radicle. From the extremity of the radicle the vessels ascend in fasciculi to the neck of the plantule, where they are seen to diverge towards each cotyledon, and ramify through it, while a [7:3:81]central vascular portion is continued to the plume. In fig. 20 the vascular and cellular structures of the germinating 'bean are shown in conjunction, in a highly magnified representation, presented here in a reduced size from Grew. In this figure d' denotes the cotyledon, d the enveloping tunics,/' the cellular tissue, f the vascular system continued from the fasciculus 7√ in the radicle, and ramified through the substance of the cotyledon; the letter ï points to the plume, which also receives vessels from those of the radicle; and ⅛ ’ indicates a depression in the cotyledon, in which, antecedent to germination, the plume was partly lodged; a similar depression existed in the other corresponding cotyledon.

CHAP. II.

THE ANATOMY OF THE MEMBERS OF VEGETABLES.

Section I.

Of the Structure of the Stem or Trunk. Art. I.— Of the Stem in Herbs.

In the foregoing chapter we have traced the successive changes of form which the seed exhibits in its progress to constitute the perfect plant; we have next to display the structure of the plant itself in its more remarkable varieties and forms. The leading features of this structure have already been laid before the reader when discoursing on the common textures of plants: it remains now to exhibit individual examples of it, as they occur in the several members of the trunk, the branch, and the root.

Botanists employ different terms to distinguish the different kinds of stems or stalks that support the leaves and the organs of fructification. These are the stem (caulis), which is considered peculiar to herbaceous plants; the trunk (truncus), which is proper to herbs and trees; the straw (culmus), which is the appropriate stem of the grasses; and the stalk (scapus), which differs from the other varieties in bearing flowers only, and not leaves. For the peculiarities in external form and character which distinguish these several kinds of stems, as they occur in different species of plants, we must refer to the writers on botany; and shall proceed to exhibit a general outline of their internal structure.

Perhaps there is no plant in which the simplicity of vegetable organization is more clearly displayed than in the sugar-cane, which belongs to the family of Graminece. In its stem or culm the cells and vessels are comparatively large, retain much of their more perfect forms, and are quite distinct from each other. When treating of the cellular tissue we referred to the cells of this plant exhibited in the very thin transverse slice, fig. 16, Plate XXXIX., as illustrative of their hexagonal figure, and of their being bounded on every side apparently by a single membrane. In some parts, when the observer is viewing these cells through the microscope, some of them appear quite transparent, from the upper and lower bounding membranes being entirely removed, and the light, in consequence, being freely transmitted; but in others one or both of these membranes remain, and though they are exquisitely thin, yet, when viewed by a strongly reflected hght, a degree of refraction seems to be produced, which communicates to the surface of the membrane an irregu-ar appearance, such as it has been attempted to express in the darker cells of the same figure. The deception that arises from viewing two layers of these cells in conjunction, which imparts the appearance of double sides, as in fig. 21, was before noticed; and the longitudinal appearance of the same organs, as seen both in a single and double series of columns (fig. 22 and 23) was at the same time described.

It is through this cellular structure that the vessels which constitute the other component part of the culm of this plant are distributed. They occur in fasciculi, which, towards the centre, are placed at considerable distances from each other, and preserve a symmetrical arrangement; but nearer to the circumference they are more numerous, and their distribution is much less regular. In fig. 25, Plate XXXIX., a very thin transverse slice of this plant is exhibited, in which this regular disposition of the vessels at and near the centre st s r , and their crowded state near the circumference t’, are well shown; the cells, too, at the centre are larger, and have a more perfect form than those near the circumference of the figure. In fig. 27 a very thin longitudinal slice of this same plant is delineated; it is considerably magnified; the letter u' denotes the cellular tissue, and d v' two fasciculi of vessels which ascend through it.

In the palm, which, though belonging to the division of trees, we shall notice in this place, a similar disposition of the elementary organs is observed; but both the vessels and cells are smaller than in the sugar-cane, and the fasciculi of vessels are also much more numerous. This structure is exhibited in the transverse section of the trunk of the palm, fig. 26, in which the dark spots indicate the vascular fasciculi, and the whiter portion the cellular tissue. As before observed in the sugar-cane, the vessels are seen to be less numerous at the centre than at the circumference, where they are much crowded together, and very irregularly distributed, in consequence of the peculiar mode in which the growth,of these trees is accomplished.

In fig. 24 of the same plate we have also copied from Μ. Desfonta,ines a portion of the longitudinal section of the trunk of another species of palm (dracaena draco), which displays more clearly the irregular direction which the vessels take in the lax cellular tissue through which they are distributed. It was before remarked that these plants do not naturally produce branches, but that their vascular system is expended wholly in the production of leaves at their summit, and their trunk is perfectly cylindrical. If, however, the top of the plant has been cut off, or broken by accident, a division into branches is said to take place. (Mém. de l’lnstit. Nat. tome i. p. 486.)

As in these plants the ligneous and cortical textures are uniformly blended together through the entire stem, it must be presumed that the sap-vessels and proper vessels are everywhere associated. This fact is accordingly pointed out by Malpighi as occurring in several species of the Gramineae, who, as we before remarked, delineates a proper vessel as existing in each fasciculus of sap-vessels.

The next variety of structure we shall notice is that of certain herbaceous plants, in which the proportion of eel-, lular tissue in the stem much exceeds that of vessels; and, though belonging to the division of dicoyledons, they in several points resemble in structure those just described. In a section of the pea already given (fig. 15, Plate XXXIX.) the greater portion of the stem was seen to be made up of cellular tissue, through which several fasciculi of vessels were dispersed. The stem of the gourd has been examined with great minuteness by Μ. Kieser, who has represented in several sections, both transverse and longitudinal, its appearance in different parts at different periods of its growth. The total number of fasciculi of vessels that exist in the mature stem of this plant he [7:3:82]makes, with Hedwig, to be ten; but the number of vessels in each fasciculus he shows to vary at different periods, and in different parts of the same plant, according to their relative age. In fig. 28, Plate XXXIX., is a piece of the stem of the gourd, cut horizontally near the root, of its natural size and appearance, as represented by Kieser. The portion l’ of this figure is represented in fig. 29, magnified 130 times: it comprehends only one of the 10 fasciculi of vessels that exist in the stem. In this magnified view the letters m'm’ indicate the elongated cells of the bark; rid the larger cells, which form the parenchyma of the stem; d d the cells which surround the fasciculus of spiral vessels; p' /the fasciculus of spirals, containing 29 vessels, in part simple and in part punctuated spirals; ι j ι ’ the great cells situated towards the centre of the stem: at (į are four spiral vessels of different sizes, filled more or less with small punctuated vesicles, which spring from the walls of the vessel, and in two instances nearly close up their cavities.

The horizontal sections of the stem of the gourd exhibited above are taken from the same plant as the vertical sections of the vessels in figures 13, 14, 15, &c., Plate XXXVIII. The number of fasciculi that exist in the stem of this plant is 10; but the number of vessels in each fasciculus continuallyincreases as the plant approaches maturity, and varies even in different parts with the different stages of its growth. Thus, of six sections made of the stem, that taken near the summit, and therefore of latest growth, exhibits only six or seven vessels in each of the 10 fasciculi, corresponding in this respect with the number found near the bottom in the earliest stage of growth. In the next section, made through the internodial space below, the vessels in each fasciculus have increased both in number and size; in the third section, the vessels have still further increased in size, and in number amount to 19 in each bundle; still lower down, in the fourth internode of the same stem, both the number and size of the vessels go on augmenting. In the fifth section, taken at the largest part of the stem, a still further increase in size is found, and the number in each fasciculus amounts to 23 vessels. Again, the section made in the stem of the same plant very near the root, and taken at a more advanced age in autumn, exhibits vessels approaching in character to those of the ligneous part of trees, and now amounting to 29 in each fasciculus. It is this last section which we have copied above in fig. 28 and 29, where the vessels may be seen smallest next the pith, and gradually increasing in size as they recede from the centre towards the circumference. Lastly, in a section of the branch of the root of the same plant, in place of 10 fasciculi of vessels met with in the stem and in the principal root, there were only four, but each fasciculus contained 37 vessels: the cells of this part were smaller than in the stem, and the whole construction was comparatively more vascular and compact.

The general characters of these vessels, from the earliest period of their development to the mature state of the plant, and the successive changes in size, in form, and appearance which they undergo, have been already exhibited in the series of figures 13, 14, 15, 16, 18, 19, of Plate XXXVIII., representing vertical sections, corresponding to the horizontal ones just described. As the plant increases in age the vessels augment in number, and in character they approach to those of arborescent plants: their sides are thickened, their transparency is almost lost, and the cavities of some are filled by vesicles. The balance, also, in the different orders of vessels at mature age is nearly the reverse of that which obtains in youth. In the young stem, says Kieser, the. simple spirals are to the punctuated variety as 17 to 6,. while in the adult plant there are only 6 simple spirals to 23 of the punctual-ed variety in the stem; and in the root the simple spirals, which at first were equally visible, have altogether disap- v peared. A metamorphosis of the simple' into the more complex varieties of vessel must, therefore, during th e progress of growth, be deemed to take place.

In the stem of the gourd, as thus described, we observe an arrangement of parts in many respects agreeing with that of the sugar-cane. In both, the cells in certain parts are large, and their figure is well preserved; and the vessels are distributed in distinct fasciculi through the cellular tissue. In the gourd, however, the fasciculi are fewer in number, and do not suffer that displacement during the progress of vegetation which those of the cane experience. In neither plant are the vessels so numerous, or so situate, as to compress the cellular tissue into transverse partitions; nor is there in the transverse section of either any ’ appearance of concentric layers. In neither stem is there any proper pith, the central part of the sugar-cane being occupied by cells and vessels, and that of the gourd being alike destitute of both.

In the sections of various herbaceous stems represented by Grew and Malpighi, the number of vessels and their disposition exhibit the greatest variety, and approach more or less to the arrangement of parts that is found in shrubs and trees. Thus, in holly-hock (Alcea) Grew describes the vessels of the bark as yielding a thin mucilage, and within them are the sap-vessels, postured in short rays, which comprise twelve or sixteen vessels. In Scot- ∑onera, the “proper vessels” in the bark yield a milky fluid, and are postured in a radiated manner with the sap and spiral vessels that extend to the pith, all of which form parts of the same radiated lines. (Anat. of Plants, p. 103.) In endive, Malpighi describes the structure of the stem as approaching in character to that of trees. Near the circumference the vessels, says he, are disposed in lines directed towards the centre, and are separated by small ranges of cellular tissue, which is compressed into a solid and dense form; and more interiorly the vessels are larger, and extend a considerable way in rays through the stem. (Anat. Plantar, p. 25.) Where the vessels have this radiated position, the cellular tissue between them assumes the form of membranous partitions; but the greater portion of the stem of this plant is occupied by cellular tissue, not thus compressed into a membranous form. We thus clearly observe that, even in herbaceous stems, membranous partitions, extending more or less completely through the stem, are formed, whenever the vessels are disposed in radii, and are sufficiently numerous to compress, on either side, the cellular tissue that envelopes them.

Art. II.— Of the Trunk in Shrubs and Trees.

In shrubs and trees the several textures that compose the trunk are-commonly distinct from each other; and some parts that are but imperfectly distinguished in herbs become in them well defined. In the trunks of these plants, however, though a general uniformity of structure is observed, yet particular modifications of it exist in every species, and these are still further varied by the peculiar nature of the individual plant, and the mode and circumstances of its vegetation. Before descending to particular examples, it may be useful to exhibit a concise view of the parts that compose the trunk, and the terms employed to denote them.

Externally in every tree we have the skin or cuticle, ' the structure of which has been already described in a former section. In very young plants, and in young [7:3:83]branches, it is succulent, and its surface is entire; but in older trunks and branches it is frequently dried and 'broken, and in some instances is thrown off; so that the exterior covering of the plant appears to be formed by the cellular tissue of the cortical texture.

Beneath the skin is the bark, constructed, as we have seen, of cellular tissue, and of vessels collected into fasciculi, which at first are straight, and run parallel to each other; but, by the subsequent augmentation of the parts within, are separated at certain places, and touch only at a few points, so as to form a reticulated appearance. In the annual shoot only a single ring of vessels is observed, and these, with the tissue in which they are placed, form the cortical layer, as it has been called, of that period. Within this layer a new production of vessels and of cellular tissue takes place; and this being annually repeated, constitutes the series of layers of which the bark is ultimately composed. The new layer that is thus annually formed, and which appears to exercise an active vegetative function, was more particularly distinguished by the appellation of liber by the ancients, from its being the substance on which, before the invention of paper, they were accustomed to write. Very frequently, instead of a ring, the vessels of the bark are collected into distinct parcels or clusters, which, in the progress of vegetation, assume very different shapes.

Next to the bark is placed the wood, constructed, like the bark, of vessels and cellular tissue. Like it, too, it consists, in the young plant and in the annual shoot of the older one, of a single ring of vessels, which immediately surrounds the pith. In the following year a new ring of vessels is formed around the first, and in every succeeding year this process is repeated; so that the wood consists at length of a series of rings inclosing each other, the number of which denotes the age of the tree. The outer ring of newly formed vessels is more succulent than those of older growth, and is generally of a whiter colour; whence it has received the names of sap-wood or alburnum. By Du Hamel and most French writers it was named aubier, and by Hill it was called the blea. In every annual shoot the newly formed liber and alburnum are in contact, but in every succeeding year they are separated more and more from each other by the interposition of new matter between them; so that at length die first layer of bark and the first ring of wood occupy respectively positions the most distant; those, namely, of the centre and circumference of the tree.

The vessels that are thus annually formed, and constitute the alburnum, are disposed in radii which extend more or less completely from the circumference to the centre. In some trees the vessels are much more numerous than in others; and, in the progress pf vegetation, they frequently suffer great alterations in size and i external figure, as was before observed when treating of the vascular system. In the alburnum die size of the vessels is very uniform, and so it continues ever after in some trees; but in others it is very various, some being enlarged more than others by a greater influx of sap. With this increase of size the vessels seem to acquire the spiral character. They are also frequently closed up in the more aged parts of trees, by the production of vesicles within their cavities; and, in common with the smaller vessels and the cells, they are in some trees ultimately filled with gummy or resinous matter. In other instances, where the “proper juices” of the plant differ but little in quality from the common sap, the vessels, when no longer employed inactive vegetation, become dry, and appear in some plants hke empty capillary tubes; and in others, from the minuteness of their size, no aperture is visible. In this state they have been regarded as solid filaments, and denominated the ligneous fibres of the plant.

From the disposition of the vessels in radii more or less regular, the cellular tissue by which they are surrounded will be more or less compressed in the same direction, and form those transverse partitions between the several rays of vessels which we have already so frequently noticed. In addition to these fine partitions, described both by Grew and Malpighi, which are thus interposed between every ray or line of vessels, larger portions of cellular-tissue are observed at certain distances, which extend in the same direction through the wood, and in some instances are continued through the bark also. By the authors just mentioned these partitions were described as stretching from the bark towards the pith, and were named insertments by Grew, and transverse utricular ranges by Malpighi. Others have held them to proceed rather from the pith to the bark, and have called them medullary rays. We shall venture to name them in future transverse septa, which appellation denotes simply their direction, and the fact of their forming partitions between the vessels, without having any reference to their origin. For similar reasons, the finer partitions of cellular tissue, pre-, viously described, maybe denominated the lesser transverse septa.

Beside these transverse septa, there are sometimes portions of cellular tissue intercepting the vessels in a longitudinal direction. This was remarked both by Grew and Malpighi in the oak and several other trees: they appear sometimes to be very short, and sometimes to form nearly a continuous ring around the trunk. It is probable they are formed simply by the pressure of the vessels acting in a direction opposite to that by which the transverse septa are produced: they may, for distinction’s sake, be named the longitudinal septa. In many trees they are not to be observed, or, if they exist, are so thin as to form only a sort of fascia on the adjacent vessels. To the cellular tissue promiscuously intermingled with the vessels some writers give the name of parenchyma of the wood: others term the elongated cells of this part ligneous fibres.

The only other part of the trunk that remains to be ’ mentioned is the pith. It is situated at the centre, and is surrounded commonly by a ring of vessels, but sometimes in part by thickened cellular tissue. Its proportion in shrubs is usually much larger than in trees; and in the young shoots of trees it occupies more space than in the older branches. At first in the young plant it is succulent, but afterwards becomes dry; and in aged trees it frequently is entirely obliterated, or at least rendered solid.

In a description of the growth of the trunk in a young plant of chesnut, Malpighi has given clear views of the gradual development of these several parts, and exhibited the appearances successively displayed in the first years of growth. (Anat. Plantar, 'p. 35.) In the vine some of these parts are very distinct. Fig.L Plate XL., represents a transverse section of the annual shoot of this plant. The pith a, in the centre, is large, and composed of cells of different sizes. From the pith to the skin in the vine, and, according to Grew, in the elm and some other trees, extend the transverse septa b. In some places the cellular structure of these septa in the recent plant is still visible, as at c; but generally they have the appearance of thickened membranes. Between every two septa two or sometimes three lines or rays of sap-vessels are interposed, which from their size are readily visible, and their place and disposition are indicated by the dotted marks di the circular ring that bounds these sap [7:3:84]vessels seems to be formed of condensed cellular tissue, and exterior to it the “proper vessels” are placed in the cellular tissue of the bark.

In the next figure (fig. 2) is exhibited the transverse section of an apple-branch of one year s growth, which, like the former, is considerably magnified. The angulated figure and relative size of the pith at this period are shown. From the pith to the bark proceed numerous very fine septa, possessing somewhat of a curved direction; and between them innumerable black points, indicating the sap-vessels, are seen. These vessels are disposed in radii, and are bounded exteriorly by the thickened tissue of the bark: the bark is proportionally large, and in die midst of its cellular tissue clusters of “proper vessels,” e, are observed.,

In a similar section of the apple-branch of two years growth, fig. 3, more highly magnified, the pith/ is seen to have assumed a rounder form: from the pith towards the bark the transverse septa extend, between which the vascular radii are situate. The letter g denotes the place of junction of the two years’ growth; h the newly formed alburnous vessels, situate between the bark and the wood; and i two ranges of “proper vessels,” placed in the cellular tissue of the bark. Grew gives a highly magnified view of a section of this same plant about the third year of its growth, in the 25th table of his work, the appearances of which correspond with those just related, except that the transverse septa, as in all his figures, are made to represent straight lines, which, in the earlier periods of vegetable growth, does not appear to be quite correct.: Proceeding next to the plant of more mature age, we shall copy from Grew part of a section of the oak, which in fig. 4 is exhibited on a reduced scale. The letter k denotes the bark, I the alburnum, m the wood, and n the pith. In this section the larger transverse septa o, and the smaller ones p, are very distinct, between which the sap-vessels of different sizes, indicated by the dark points, are situate; the letter q points to the “proper vessels” of the bark, some of which are collected into round parcels, and others form a ring.

Beside the transverse septa that intersect the diameter of the tree, we observe, in this figure, other lines, r, placed at right angles to them, and which appear to be interposed between each annual ring of wood. Grew considered it probable that these lines were produced in the oak, and also in the fig-tree and walnut, by a peculiar sort of sap-vessels that existed once in the bark, as did the turpentine vessels distributed through the wood of the pine. (Anat. of Plants, p. 115.) Hill also calls them sapvessels seated on the outer edge of each annual circle, which in the young plant contain sometimes a limpid liquor, and at other times appear empty. He notices also the existence of “proper vessels,” containing a thick juice in the alburnum of this tree. (On the Construction of Timber, p. 9.)

An examination of this wood at an age still more advanced may contribute to throw some further light on its structure. In fig. 5 is given the representation of a transverse section of the central portion of a piece of the wood of very old oak, of its natural size and appearance. The pith s, at the centre, was completely obstructed, and its cellular character obliterated; the letters t11 denote the larger transverse septa, rendered completely solid everywhere, and in some places partially obliterated; and u the white lines that extend in the opposite direction, and mark the boundaries of each year’s growth. In the substance of these concentric lines numerous traces of minute apertures were visible; and from one line to another, small irregularly curved lines, as represented in the drawing, were everywhere observed to extend. No vacuity was anywhere perceived to exist, but the whole formed one compact and solid mass.

A small and very thin slice of this wood, comprising a part of one of the larger transverse septa, and small portions of two of the concentric lines above mentioned, was submitted to a pretty highly magnifying power; and a delineation of its appearance is given m fig. 6. In this figure the letters v v denote the two concentric lines; w one of the smaller irregular lines that frequently extend from V to V, but are often intercepted, and form irregular spots or patches, as traced in several parts of the drawing; x the smaller transverse septa, situated at unequal distances from each other; and y the larger septum, the cellular character of which is entirely obliterated. Several small irregular lines, running in the same· direction with the larger concentric ones, are also observed: these, at their junctions with the lesser transverse septa, produce a number of little squares, the areae of which are filled up with a dotted or punctuated membrane.

This microscopical representation seems to confirm the opinion of Grew, that the concentric lines v v are chiefly composed of vessels; for their apertures, now rendered visible, are of different sizes, and the canal of many of the larger ones is filled up with a vesicular substance, often observed in the vessels of aged plants. The substance that surrounds these vessels appears to be condensed cellular tissue, rendered perfectly solid by the resinous matter with which it is filled. The irregular thickened lines w that run transversely to the former are similarly composed of vessels and condensed cellular tissue; and the smaller patches, we have noticed, are made up of the same. Both the large and small transverse septa consist of condensed cellular tissue alone; and such, too, appears to be the matter of which the fine lines or longitudinal septa that intersect them are composed. Some observations of Du Hamel on the formation of the ligneous layers may aid our inquiries into the nature of these latter septa, and the minute structure of the wood.

In a transverse section of the trunk of the oak, the elm, or the fir, says this very intelligent writer, the ligneous layers are distinctly visible, and it is commonly believed that each of these layers is the product of one year’s growth. If, however, we cut obliquely a piece of oak, each layer is then seen, with the aid of a common lens, to be composed of a number of thinner layers, which mutually cover each other. By macerating pieces of wood in water, he was able to separate the annual layers into a great number of leaflets, thinner than the finest paper; and these primary layers, as they may be called, he afterwards ascertained by experiment to be formed successively during, the whole period of active vegetation; so that the layer which is the product of one year’s growth is itself formed of a number of layers exceedingly thin. (Phys, des Arb. tome i. p. 31, and tome ii. p. 19.)

Now it is probable that between each of these primary layers cellular tissue is interposed; and this tissue, by its compression, gives origin to the minute lines which in this tree have been noticed. In several trees, however, no similar lines are visible, because in all probability they do not, like the oak, afford secretions by which the tissue is thickened and rendered apparent. Even in a very thin slice of oak, when viewed in the microscope by a strongly reflected light, these lines almost entirely vanish, while those of the two orders of proper transverse septa remain.

M.Kieser examined the same wood in the transverse slice ∣ of a tree 100 years of age: and his representation of its structure is copied in fig. 23, Plate XL. In this figure, in [7:3:85]which the appearances are magnified 130 times, c¢ d indicate the cellular tissue, or, as he calls it, the parenchyma 'of the wood. At the top and bottom, the cells are obstructed and appear like obscure points (as is the case also in fig. 6 of the same plate); but in the middle of the figure the cavities of the cells are said to be still visible: the letters U U denote the apertures of enlarged spiral vessels, all of which were filled with vesicles, but a few only are so represented, as in d ; d’ indicates the smaller vessels dispersed through the cellular tissue; d a portion of one of the larger septa, and f' the smaller ones, which are often displaced by the augmentation in the size of the vessels.*

In this highly magnified representation by Kieser, there is no appearance of the concentric lines or septa which intersect the transverse ones, as noticed by Grew and Malpighi, and exhibited in fig. 5 and 6; but the two orders of transverse septa remain. We before remarked, that in a slice of wood so very thin as, when viewed by a strong light, to become translucent, these lines in a great measure vanished; and in the figure of Μ. Kieser this evanescence is made to extend even to the thickened portion that intercepts the annual layers. It is therefore probable that all these appearances of concentric or longitudinal septa are produced simply by the thickening and compression of the cellular tissue in the oak, and such other trees as have viscid juices, and whose vessels are subject to irregular enlargement; for in whatever position these thickenings occur, numerous vessels are always to be observed. Hence in very thin slices they entirely vanish, while die proper transverse septa, being formed by the condensation of continuous portions of membrane, remain visible. In these figures it may also be remarked, that the surface occupied by cellular tissue seems equal nearly to that occupied by vessels; but it is probable that the apertures of many minute vessels dispersed through the tissue have not been distinguished from the cells. In a highly magnified representation, however, of the wood of this tree by Grew (Anat. of Plants, tab. 3, fig. 7), the proportion of the vascular to what is deemed the cellular part is still less.

The vessels of the bark in the oak, as delineated by Grew, fig. 4, Plate XL., arc represented as disposed partly in parcels and partly in a ring; and in all the representations he has given of different trees, the greatest diversity in regard to the position of these vessels is found to exist. From the difference in the qualities of their juices, in the texture of the parts, and in the modes and circumstances of their growth, this variation may be expected to occur. In the oak, the pine, and many others, it seems certain also that a part of the cortical texture, or at least vessels containing the “proper juices” of the plant, are constantly mixed with those of the ligneous texture, and with them contribute to form each annual layer of wood. For examples of the diversity of structure exhibited in the trunk of a great many different trees, we may refer to the plates of Grew.

Section II.

Of the Structure of the Branch and its Appendages. Art. I.— Of the Branch.

From the trunk springs the branch, which in structure resembles very exactly that of the trunk itself; and hence the description of the one serves entirely for that of the other.

Branches, in common with leaves, originate from buds, of which we have subsequently to treat, introductory to a description of leaves. At present we shall describe only the mode of connection between the trunk and the branch.’

The branch, says Grew, springs not from the surface, but so deep as to take with it not only the bark and the wood, but the pith also, making its way at the parts where the vessels are separated by the transverse septa, and carrying with it the skin, which is extended with it. (Anat. of Plants, p. 28.) The separation of the vessels of the trunk to form the branch is not like the separation of a few filaments from a skein of thread; but they spring, says Du Hamel, from a centre, and bear with them a part of each portion of the tree. Hence, if a tree that is divided into two branches be cut a foot above the bifurcation, the surface of the sections resembles that of two trunks cut horizontally: if the section be made lower down, at the place where the branches spring, the same appearances of two series of concentric circles are seen at the axis of the trunk, but they are surrounded by other layers common to those which belong to each of the branches; and still lower on the trunk the concentric circles belonging to each branch diminish, and are finally lost in those which form the trunk from which they sprang. (Phys, des Arbres, tome i. p. 93.) These primary branches divide into others, and these again into still smaller branches, which form angles with each other, more or less great, according to the species of tree, and other causes.

The position of the branches on different trees is very various, but in the same species it is generally uniform. From the measurements of Du Hamel, it appears that in many trees the solid matter of the branches that go out from the trunk exceeds that of the trunk itself in the proportion nearly of five to four. (Phys, des Arbres, tome i. p.96.)

Some have supposed that the branches proceeded always from, and were nourished by, the pith; but this opinion has been combated by Du Hamel and Hill. The latter gives a longitudinal section of the trunk of a species of American dog-wood (copied in fig. 7, Plate XL.), representing the origin of two buds g ’ g ’ , which are seen in the state of pushing out on either side through the vessels of the wood. In this trunk the pith is of a brown colour, the inmost ring of vessels green, and the outer ones white. Through the white rings the inner green vessels are seen to shoot, and the brown pith is left entire behind; though the new branch at length obtains a pith for itself, which, however, has no connection with that of the trunk. In a similar section of an older trunk (fig. 8), the branches are seen to spring also from the inmost circle of wood; the pith Ji is not at all disturbed, but each branch is furnished with its own pith i ’ i'. In a section of the vine (fig. 9), a similar origin of the branches, from the inmost circle of wood, is observed; and not only is the pith h f of the branch distinct from that of the trunk, but the pith of the trunk itself is intercepted by the shooting of a branch l’ across it. ( On the Construction of Timber, p. 35.) This latter fact of the interception of the pith by the shooting of a branch across it was previously noticed by Grew in the vine and some other plants. (Anat. of Plants, tab. 19.)

. When a bud thus protrudes to form a branch, the perpendicular vessels of the trunk are compelled to separate; and they afterwards meet above, and pursue their former direction. This is shown in fig. 11, Plate XL., copied from Du Hamel, in which the bud ni, in the act of sprouting, is seen to push to either side the vessels of the trunk, which again meet above it.

The vessels both of the wood and bark, according to the same author, have their direction determined chiefly [7:3:86]by the course of the sap. If the sap preserve a perpendicular direction, as in trees that have no branches, the vessels arc perpendicular also; but if it move to one side, these vessels then change their direction. This is strikingly evinced in a tree that has been cut over above a branch; for then all the sap being obliged to pass to the young branch, the vessels all at once take the same direction; so that if a tree has been cut oyer in winter, and at the end of the succeeding spring its branch below be examined, the new vessels of the branch will be seen to cross those of the trunk, as exhibited in fig. 12, n', Plate XL. (Phys, des Arbres, tome ii. p. 53.)

In figures 7, 8, and 9, copied from Hill, the young branch, in every instance, is seen to originate from the inmost ring of vessels that surrounds the pith; and he was of opinion that all buds and branches sprang from this part alone. To this vascular circle he gave the name of corona, and held it to be the most important part of the vegetable body,—-that it was like no other part of the plant, but contained within itself the essence of them all. (On the Construction of Timber, p. 21.)

It is however notorious, that trees in which not only this first circle, but almost every other circle of vessels, has perished, produce leaves and shoots from the trunk where the bark is entire, as this author himself admits, p. 44 of his work. There does not appear, either in the anatomical character, or in the functions of this circle, any thing that distinguishes it from the others, except priority of formation, and its being in consequence the seat from which the first buds and branches spring.

A much more correct idea of the origin of buds and branches was entertained by Du Hamel, who illustrates it by the diagram fig. 10, Plate XL. Let us suppose .this figure to represent a tree of four years’ growth, as indicated by the four ligneous cones which at its base envelope each other. In the first year a bud o' springs from the inmost ring, which by the fourth year is seen to consist of four ligneous layers. In the second year a second bud fi springs from the ring of that year, and consists only of three layers; the next year a bud cį is developed on the first bud o', and possesses only two layers; and the bud τ j , developed the fourth year on the outmost ring of wood, is seen to consist only of one layer. (Phys, des Arbres, tome ii. p. 53.) In this way every ligneous layer may be considered equally capable of giving origin to buds and branches.

Art. II.— Of Thorns.

Connected both in origin and in structure with branches are those appendages, frequently observed upon them, called thorns. They are very conspicuous on the hawthorn, and are constituted, says Grew, of the same parts as the bud itself, and in a like proportion. They spring from the outer portion of the ligneous texture, and may be considered as abortive buds. (Anat. of Plants, p. 33.) Malpighi describes them as being frequently produced in the axils of leaves, and assuming for a time the form of a branch, but at length degenerating into a thorn. He considers them to arise from defective nutrition, and adds, that they disappear frequently under higher culture. (Anat. Plantar, p. 138.)

According to Willdenow, most species of our fruit-trees naturally possess thorns, which disappear entirely and become branches under higher culture. Even in the black-thorn the prickles diminish in number under improved culture, but do not entirely disappear.

Sometimes, however, instead of being produced from abortive buds, thorns owe their origin to the degeneration of other organs; the petioles of some pinnate leaves which are persistent become thorns, and the same thing is observed of the peduncles of some flowers. Certain stipulae also, as those of Mimosa, are said sometimes to change into thorns (Principles of Botany, p. 270); and in the date, according to Μ. de Candolle, a lobe of the leaf has been converted into a thorn. (Théorie Element, de la Botanique, p. 344.) In other instances, as in the holly, the leaf produces thorns round its entire margin, which are formed by the vessels that bound it. (See fig. 11, Plate XLI.)

When the bark is removed from a branch that possesses thorns, the thorns still remain, by which they are distinguished from prickles that originate from the bark. In fig. 20, Plate XLI., is a representation of a branch deprived of its bark, in which thorns of different forms are seen to spring from the ligneous texture; and in fig. 21 of the same plate is a longitudinal section of another branch covered with its bark, in which the ligneous and cortical textures of the thorn are displayed. For the descriptions of the varieties of thorns, and their external appearance, we must refer to the writers on botany.

Art. III.— Of Claspers and Tendrils.

From the similarity in structure which these bodies exhibit to the branches from which they spring, we have placed them as appendages to those members. As is well known, they are met with only in certain plants, and their obvious use is to attach the different parts of the plant to one another, for mutual support, or to the objects in their neighbourhood. In a general manner they are denominated fulcra : the most remarkable species are those which, like the claws of ivy, called claviculi by Malpighi, are not rolled into a spiral form; and those of the vine, named tendrils (cirri'), which possess a spiral conformation.

According to Malpighi, the claws or claspers of ivy possess a roundish form, and are covered with hairs, which yield a viscous humour, by which they are agglutinated to stones or to walls and trees. Their forms are exhibited in fig. 22, ë f', Plate XLI., copied from Malpighi; and in the lower part, g ’ , of the same figure, their origin and structure arc displayed. They are seen to spring from the ligneous texture of the stem, and to possess a similar structure.

The tendril is described by the same author as springing from knots between the origins of leaves. It possesses a round stem, which is sometimes covered with hairs; and this stem frequently divides into several branches. At first it is very tender, but gradually acquires solidity, and assumes the spiral character; its colour is green, and it is composed, like the trunk, of all common textures. (Anat. Plantar, p. 139.) By Willdenow it is regarded as an abortive leaf, being simply a prolongation of the petiole, without the leafy expansion. (Principles of Botany, p. 272.) Its form, as it occurs in the vine, is exhibited in fig. 23, Plate XLI.; but in this particular it is subject to the greatest variation.

The leaf itself, as well as the petiole, is said to be sometimes prolonged into a tortuous appendage resembling the tendril; and similar transformations are sometimes exhibited by the peduncle and petals of the flower. In öne species of vine mentioned by Malpighi, the extremity of the tendril, which is at first pointed, becomes gradually incurvated and reflected, and at length is formed into a roundish body, which is furnished with small papillae, that yield a viscous fluid, by means of which it attaches itself to walls or wood, so strongly as not to be easily separated.
[7:3:87]

In dodder (cuscuta), small tubercles are formed on the 'stem, which, according to Μ. de Candolle, are so organized as to fix themselves on any other plant, and derive nourishment from it. ( Théorie Element, de la Botanique, p. 344.) For descriptions of the several varieties of these organs enumerated by botanists, and for the terms employed to express them, we must refer to their works.

Section III.

Of the Structure of Roots.

Considered in relation to their general form, roots, says Μ. de Candolle, exhibit two very distinct appearances. Some roots have a conical trunk, simple or ramified, but springing always from a single base, and their radicle only elongates or ramifies; others spring always in a bundle, more or less marked, from a common base, which is so confounded with the neck of the plant that it is sometimes taken for the base of the stem, and sometimes for the trunk of the root The first form of root is met with in the greater number of exogenous plants; the second occurs chiefly in the endogenous, and also in those exogenes which have fascicular roots.

Of the conical roots, some ramify greatly, and have numerous fibrous or capillary rootlets; others produce few or no branches, and are furnished with few fibrous roots. In the former class, the principal differences arise from the degree of ramification; in the latter, two varieties of form are very distinct—that of the carrot, named fusiform, from its tapering to a point, like a spindle—and that named rapi-form by De Candolle, from the swollen form it assumes at its origin, and sudden shrinking to a point below, as in the turnip.

The fascicular roots present also many varieties of form: Is/, Those of the Gramineae, in which each rootlet, though distinct at its birth, ramifies so much that the divisions resemble the fibrous productions of the preceding class: 2dly, Those of the hyacinth, in which many cylindrical rootlets spring from a common base, and descend nearly parallel, or with a slight divergence: 2dly, Those of the orchis, in which, besides cylindrical rootlets, there are others swollen into tubercles: ⅛thly, Those of Ranunculus, where all the fibres that spring from the base are more or less swollen, so as to form a bundle of oblong tubercles. Between all these distinct forms there are several intermediate varieties. Under the name of tuberous roots arc included all those which have swellings in any part of their length, and that, too, whether they sprang originally from a single or common base. It may, however, be observed, that many tubercles which spring apparently from roots, are in reality productions from inferior branches of the stem, concealed beneath the soil, as Μ. Dunal first proved to be the fact in the potato. (Organog. Végét. tome i. p. 252.)

The root is held to consist of two parts; the main body with its branches, and the fibrous productions called capillary rootlets. These parts are commonly hid beneath the soil, but in some plants they are attached to rocks or stones, in others they are fixed in other vegetables. Some plants, again, have no fixed attachment, but float loosely on water; and others are considered to possess no root at all.

With respect to structure, the body of the root of trees, says Malpighi (Anat. Plantar, p. 145), maybe regarded as an elongation of the trunk beneath the soil, being constructed of the same textures, disposed nearly in the same manner. In various herbaceous plants the root has been examined wifh much minuteness by Grew. (Anat. of Plants, book ii.) The cuticle, says he, is of different colours in different plants: sometimes its surface is smooth, at others rough; and its texture is either thin and transparent, or thick and opaque. At an early period the cuticle appears to be a continuation of that of the radicle, but afterwards seems to be derived from the bark. It is composed chiefly of cellular tissue, through which vessels are distributed in the direction of the length of the root. The existence of these vessels, though scarcely visible by the microscope, may be inferred from the fact that it is more difficult to tear the skin in a longitudinal than transverse direction. If, also, the skin be cut transversely, and then allowed to dry, all the parts where there are no vessels shrink below the surface of the cut end; but where there are vessels there is no shrinking. Or if, after making a transverse section of the root, the skin be pressed very gently with the edge of the nail, sap will sometimes arise from the skin, as from any other part where vessels are placed; which does not happen with fluids contained in the cellular tissue, unless the cells be broken. (Ibid. p. 62.)

Beneath the cuticle lies the bark, which is found to differ very much, both in colour and thickness, in different plants; scarcely in some instances exceeding the skin in thickness, whilst in others it is thicker than the woody part of the root. It is composed of vascular fasciculi and cellular tissue, the cells of which are smaller than those in the trunk: these cells are generally of a spherical form, or nearly so, often transparent, but not visibly pervious into one another. In some roots the cellular tissue is disposed in rays extending from the inner to the outward edge of the bark; in others it extends only in part through the bark. The cells themselves are sometimes uniform, and sometimes they differ both in size and figure. They are often filled with lymph, or other matter. (Ibid. p. 64.)

In the cellular tissue vessels running lengthwise through the bark like small threads are observed; and in a transverse section the sap is seen to exude from them. They often form a net-work, and seem as if inosculated; but are only contiguous, like the braces in a drum. They are never single, but form clusters of 20 or more together: their form is cylindrical, and they are of the same size through their whole course. Some vessels frequently separate from a cluster and are transferred from place to place; but they never ramify so as to be successively propagated one from another. Their position in the bark is very various, forming sometimes a ring at its inner edge, and dispersed sometimes in clusters through it, or disposed in the form of lines or rays. The fluid they contain varies in colour and consistence in different plants, being sometimes thin and clear, sometimes thick and mucilaginous, often milky or resinous. Sometimes the fluids of the bark vary in the same root, which, when cut transversely, will yield both a limpid and milky sap. (Ibid. p. 67.)

The woody part of the root, like that of the trunk, is made up of cellular tissue and vessels. This tissue varies greatly in proportion in different roots, but is less abundant in the root than in the stem. It is disposed sometimes in rings, but oftener in lines and rays, like the transverse septa in the stem. Through this tissue two kinds of vessels are distributed, one containing a lymph, and the other being empty; whence they have been denominated by Grew the succiferous and air-vessels. In some roots these vessels are disposed in clusters or fasciculi; in others they form rings; and in others they have a radiated form. Both kinds are much intermixed, and everywhere surrounded by cellular tissue. The number of empty vessels is sometimes small and sometimes great; [7:3:88] and the same may be said of their size, even in the same root. Malpighi, says Grew, has described them as being constructed of a thin transparent zone, of a silvery colour, having a spiral form; to which Grew adds, that the spiral zone is not a single piece of a flat form, but consists of two or more round and true fibres, not inosculated side to side, but knit together by other smaller fibres,—a construction similar to that which Kieser has since demonstrated. Their spiration, however, is better seen in the stem, and better m young parts than in old ones. Though these spiral vessels seemed empty, or containing only air, yet, says Grew, “whether these vessels may not, in some vegetables, and at some times, contain liquor, is doubtful.” (Ibid. p. 74.)

In illustration of the structure above stated, Grew ex- • hibits, in twelve successive tables of his work, the external forms and internal structure of various roots. As a specimen of this structure, we have copied his representations of the root of wormwood. In fig. 13, Plate XL., is a delineation of a transverse section of this root, of its natural size; and in fig. 14 is a magnified representation of a portion of the same section, here presented on a reduced scale, in which A denotes the cuticle, B the bark, and C the wood. The smaller letter a indicates the ligneous rays formed by the vessels, and b the cellular tissue or transverse septa continuous with that of the bark. The dark spots in the bark are described by Grew as balsam vessels, intermingled with lymphaeducts; and those in wood as the spiral or empty vessels. It will be seen that the vessels of the woody part are smallest at the centre, and gradually augment in size as they approach the circumference, as Kieser has since observed; and that, with the exception of a few vessels said to convey lymph, all those of the wood are considered as air-vessels, and therefore as possessing a spiral conformation. In his chapter on roots, Malpighi also frequently describes tracheae as occurring in the dissections made by him of the roots of various plants.

In fig. 21, Plate XL., is exhibited a horizontal section of the root of the gourd, of the natural size, as given by Kieser, representing four fasciculi of vessels, intercepted by cellular tissue. One of these fasciculi, consisting of 37 vessels, is represented in fig. 22, magnified 130 times. Of these vessels, the largest, as usual, are situated exteriorly, and they pretty regularly diminish in size as they approach the centre. Two of these vessels are nearly filled with vesicles, and the whole of them are generally of smaller, size than those in the stem of the same plant. A vertical section of one of the largest vessels near the circumference, and of two of the smallest at the centre of this root, have been already exhibited in figure 19, Plate XXXVIII. They all belonged to the variety of punctuated spirals; so that all the vessels in the root of this plant, as well as in the stem, may be considered as possessing the spiral conformation.

To the testimony of Grew, Malpighi, and Kieser, as to the existence of spiral vessels in roots, we may add that of Du Hamel, who says that if a young root, as that of the horserchesnut, be duly macerated in boiling water, and its bark afterwards removed, the ligneous portion will be found so tender as to be divisible into fine filaments; which filaments, when viewed with a microscope, will be found to be spiral vessels or tracheae, which, he adds, might lead one to think that all the ligneous fibres are really spiral fibres, while they are yet tender. (Phys, des Arb. tome .ii. p. 13, 14.) Mirbel and some other writers, however, deny altogether the existence of spiral vessels in the root; and Μ. de Candolle declares that one of the chief differences in structure between the stem and the root consists in the total absence of tracheae ⅛ the latter. ( Organog. Végét. tome i. p. 242.) Yielding credence, however, to the positive testimony of Grew, Malpighi, Du Hamel, and Kieser, rather than to the negative assertions of their opponents, we are disposed to conclude, that all the vessels which convey the ascending sap in the root possess, like those in the stem, a spiral structure.

It cannot fail to be observed also, in the sections of roots just exhibited, that the proportion of vessels to cells is much larger in the root than in the stem of herbs, and that consequently the cells are smaller and more compressed. The vessels occupy also the very centre of the root, to the exclusion of all cellular tissue, in the form of pith, in exogenous plants; but in the endogenous, the internal structure of the roots, as compared with that of the stem, is said to present no sensible difference. In exogenous plants, also, the diametral insertions, or septa, and the appearance of the bark, are as distinct in the root as in the stem; while in the endogenous the vessels and cells are intermingled through the entire root, as well a$ through the stem. In trees, the structure of the root is more distinct than in herbs, and still more nearly resembles that of the trunk.

In treating of the evolution of seeds, we exhibited several examples of the manner in which the fibrous or capillary rootlets spring from the trunk, or larger branches of the root. These rootlets are more simple in structure than buds, being made up chiefly of the ligneous portion of the plant. They are the organs which absorb nutriment from the earth; and hence, as Du Hamel observed, the earth is exhausted chiefly where these capillary rootlets are distributed, and not in the neighbourhood of the larger roots. On examining the roots of trees after severe winters, he often found the capillary rootlets dead; which led him to suppose that trees lose in the earth their capillary rootlets nearly as in the air they lose their leaves. He found, that even after slight frosts many of these rootlets were dead; and that, when the temperature became milder, new rootlets were developed, which abundantly replaced those destroyed. (Phys, des Arb. tomeì. p. 89.)

All roots raised from seeds have a tap-root, that grows in length in proportion as the earth is easily penetrable. The seeds of the oak, sown in a deep and rich sand, have produced plants, says Du Hamel, the stems of which were only six inches in height, while the root extended nearly to four feet. If, however, the root meet below the surface with any obstacle that opposes its elongation, it then continues short, and subdivides into many lateral branches. This result may be also obtained by simply cutting the extremity of the root; for roots, as Du Hamel first ascertained by experiment, do not elongate but by their extremity. (Phys, des Arb. tome i. p. 83.) Hence, when injured by insects, or in any other way, so as to check its elongation, the root, by throwing out lateral branches, will often derive more nutriment by these new productions than it would have obtained by a single trunk.

Roots arc provided through their whole extent with a quantity of germs, or a capacity of producing them, when placed in favourable circumstances; and this capacity ⅛ possessed also by the branch. If the branch of many trees be cut off and planted in the earth, it emits rootlets from its sides beneath the soil. If even the bark of a branch be removed so as to intercept the descending sap, and the part be then surrounded with moist earth, the upper portion of the divided bark will emit rootlets into the earth. The simple immersion of cuttings of the willow in water is sufficient to cause the production of ι ootlets, [7:3:89]which spring from the cutting, says Malpighi, just at the 'surface of the water, not below it. They appear at first like small tumours, and are composed of ligneous fibres and tracheae, which gradually force their way through the bark. (Anat. Plantar, p. 146.)

In like manner, as a branch may thus be made to produce rootlets, so may a root be made to produce branches. If a root be cut off, or an incision be made in its bark, so as to intercept the descending sap, a swelling, or bourrelet, will form around the upper extremity, from which new rootlets will spring; or if this cut end be left exposed to the atmosphere, then, says Du Hamel (Phys, des Arb. tome i. p∙ lθ2), in place of rootlets, new shoots or branch-lets will be developed. It is thus ∙ that many plants, as mint, which spread their roots horizontally, produce new stems from them when they happen to come sufficiently near to the surface. From the facts above stated, Du Hamel maintains that the tumour or enlargement (bourrelet) formed either on the cut stem or the root of a plant is'essentially the same; that in both it contains germs proper for producing either branches or roots; and that the one or the other is developed, according as the bourrelet is exposed to the air or is placed in the earth. (Phys, des Arbres, tome i. p. 102.)

Others have maintained not only a perfect identity of structure between the root and branch, but also between the bud that gives origin to the branch, and the rootlet that springs from the root; and further, that they are convertible into each other. In proof of this it is alleged, that if a willow plant be bent down so that its head shall be fixed in the earth, it will soon push forth rootlets; and that if, after these are developed, the trunk with its former roots be raised so as to be exposed to the air, there will shortly be formed a new head, furnished with leaves and branches. These facts, however, prove only a substitution, and not a conversion, of these organs into one another; for the young buds of the head die when placed under the soil, and the new roots spring from points not previously occupied by buds; while, in like manner, all the young rootlets developed beneath the soil perish in the air, and new buds arc produced from the older trunks of the root. In some plants the stem seems to perish, and the root in the following year produces a new stem; and from this it has been inferred that the root alone preserves the vitality of the individual: but the stem in these plants does not, says Μ. de Candolle, entirely perish; and in the phenomena exhibited by cuttings the contrary takes place, for the stem devclopes new roots. We ought therefore, says he, to regard the stem and the root as of equal importance. Their union constitutes the body of the plant. (Organog. Végét. tome i. p. 249.)

The knots or articulations in the stems of plants, by retarding the descent of the juices, favour the production of new parts; and according as the stem may be exposed to dryness or humidity, either branches or roots may be formed. Hence stems that have many knots most easily propagate by slips and layers; and fleshy'plants, as the Cactus, which possess a thick and moist cellular tissue, have a disposition to propagate roots even in the open air. It is the same with roots possessing tubercles, which are a sort of magazines of nutrient matter, and possess, therefore, a singular aptitude for emitting new productions. (Organog. Végét. tome i. p. 248.)

We have already stated that roots grow in length, not by an extension of parts, but only by the addition of new rpatter to theiŗ extremity. This power of growth Du Hamel found to reside within two or three lines of the extremity, both in ligneous and herbaceous roots. (Phys. des Arb. tome i. p. 84.) It is chiefly by their extremities, also, that they absorb fluids; for Senebier found, that if the extremity of a root, as that of a carrot or radish, was immersed in water, the water was absorbed; whilst if the whole root, with the exception of the extremity, was immersed, little absorption took place ;—results which have been confirmed by the experiments of Carradori. These facts have led Μ. de Candolle to consider the extremities of roots, and of all fibrous rootlets, as furnished with a peculiar hygroscopic power. This power he conceives to depend, as we Before remarked, on the existence of a minute cellular structure at this part, to which structure he has given the name of spongioles (spongiolae). These spongioles, according to him, are formed of a very compact cellular tissue, composed of roundish cells. He distinguishes several varieties of these organs, as the radical, the pistillary, and the seminal, according as they are seated at the extremities of the radical fibres, of the pistil, or in the coats of seeds. So great, he adds, is their absorbing power, that they absorb the colouring molecules of liquids, though these molecules will not enter the ordinary pores, which are infinitely greater in size. ( Organog. Végét. tome i. chap. 7, p. 89.) In his 10th plate he exhibits the appearance of these spongiolcs in the radicles of pandanus odoratissimus, where they are better seen than in any other plant examined by him.

The nature and qualities of the soil exert a great influence on the form and size of roots. If the soil be free and pervious, the root often descends to a great depth in the form of a tap-root; but if it encounter obstacles which oppose its descent, it then continues short and divides into lateral branches. Roots also extend into the portions of soil that are richest, while the barren parts are nearly destitute of roots. (Phys, des Arbres, tome i. p. 82.)

CHAP. III.

THE ANATOMY FΕ THE ORGANS OF VEGETABLES.

Section I.

Of the Structure of Buds.

In the preceding chapter we have described the larger and more permanent members of the plant. We have now to delineate the form and structure of organs, whose existence is of shorter duration, and which present a more diversified character. They may all be said either directly or indirectly to originate from buds, with which therefore we shall commence our descriptions.

A bud, according ,to Gaertner, is an organic body, sprouting from the surface of a plant. In the beginning it is distinct from the proper and permanent members of the plant, but after some time becomes a part of it; or, if separated, is capable, by the increase of its own proper substance, of growing into a new plant, perfectly similar to its parent. (De Fructib. Plantar, p. 3.)

Buds may in general be easily distinguished from seeds; but in some of the lower tribes of vegetables the conformity in external appearance and internal structure is said to be so great as to render this a matter of much difficulty.

Gaertner enumerates four species of buds, two of which are leafless, and two are provided with leaves or scales. The first, propago, is ’the most, simple species of bud; it is leafless, and possesses different forms: sometimes it is entirely naked, and sometimes is inclosed in a bark-like case: it at length separates spontaneously from its parent, and is dispersed like a s’ced.
[7:3:90]

The second species is termed gongylus : it is described as a perfectly simple, leafless, somewhat globular and solid bud, concealed within the bark of the parent plant, and never separating spontaneously till the bark decays by age. It has a great affinity to the tuber of roots, but differs from it in not being a proper member of the plant, and in not possessing the principle of multiplication which resides in the tuber.

The third species of bud is a compound germen, and named bulbus. Its figure is somewhat globular: it is nearly leafless, and formed of a very short keel· (carina) and thick succulent scales, and at length separates spontaneously from its parent. Of this species there are two varieties, the one solid, constructed of a solid fleshy body, and having generally the rudiments of the new plant fixed outwardly upon it; the other coated, being formed of several concentric scales, in the centre of which the young plant is cherished.

The last species of bud, and that which is strictly so named, Gaertner denominates gemma. This is a bud composed of a subulate keel, and distinct herbaceous leaflets: it resembles a branch in miniature, and never separates spontaneously from its parent. It is named an eye (oculus) when it puts forth flowers alone, or flowers and leaves together; and simply a gem (gemma) when it is unfolded into leaves alone.

Of these several species, the last, gemma, is generally considered as alone entitled to the appellation of a true bud by those who consider scales and leaves as essential to its constitution; but this definition, says Gaertner, would exclude even bulbs, and it is better to derive our idea of a bud from a general agreement in properties, and particularly from similarity of origin in formation and evolution, than from the ever-varying condition of external form. The two former species of buds occur in the lower tribes of vegetables; the two latter are observed on the stems and branches, and roots of various plants. On the present occasion we shall notice the latter only.

That variety of bud which is called a bulb, and which springs from, or is variously attached to, the roots of many plants, has already been stated to exhibit considerable difference in structuré. It is sometimes constructed of several thick scales or leaves enveloping each other, and is sometimes formed of a more continuous and solid substance. To the first description belongs the bulbous substance of the lily and tulip. Grew has given a section of the latter, made in the month of September. It displays the tunicated structure of the bulb, at the base and in the centre of which the young flower, destined to appear in the following spring, is observed. (See his Anatomy of Plants, table 63.) Of the tunicated variety, the common onion affords also a good example. The coats that compose its bulb are to be regarded as fleshy leaves, and the true root, according to Du Hamel, is the fleshy plate that supports the bulb, from which the rootlets spring. The common potato affords an example of the solid bulb, on whose surface numerous buds, all capable of producing entire plants, are seen. Botanists enumerate a great many other varieties of bulbs, for which we must refer to their writings; and to the works of Malpighi and Du Hamel for the anatomy of many of them.

The true bud of the stem and branch was distinguished by the ancients into two kinds, according as it produced either a leaf or a flower, and to it they assigned different names. į t( Germen aùtem est id quod ex ipsis arborum surculis 'primo vere exit, ex quo deinde folium producitur," says Pliny; and this,he distinguishes from the flower-bud, “nam gemma propriè fioris est.” By Grew the term germen, and by Malpighi the word gemtna, is employéd’ to denote each variety of bud. By Linnaeus the term germen is used to denote, not the bud of a branch, but the rudiment of a seed; and not the rudiment of the seed only, but the organ also that contains it. The wordgem. ma he uses to denote a bud. Gaertner adopts the word germen in a generic sense, to express every species of bud; and, with Malpighi and Linnaeus, employs the term gemma as indicative of that species now under consideration. This, therefore, as the most generally received appellation, we shall in future employ.

During summer, says Du Hamel, buds are gradually formed in the axils of the leaves, viz. in the angle which the petiole forms with the branch. They are at first exceedingly minute; are seen in winter chiefly on the young branches, sometimes on the larger ones, and more rarely on the trunk. They exhibit different forms, according to the kind of tree that bears them, and are attached to it by ā very short pedicle. Their position on the branch was considered·* by Μ. Bonnet to be reducible to five classes; sometimes they are situated on opposite sides of the branch, but placed alternately, and sometimes they are placed exactly opposite to each other. In other instances they form a kind of ring round the branch. Sometimes they have a spiral disposition, and at others constitute a sort of double spiral around the branch. In those cases where the buds stand opposite on the branch, the extremity of the branch is frequently terminated by three buds; but where the buds are only alternate, the young branch is commonly terminated by a single bud. In the pine the true buds are placed, not in the axils of the leaves, but at .the extremity of the branch alone. Some buds stand out a considerable distance, from the branch; others are placed in close contact with it; and these varieties occur sometimes on the same branch in regard to the buds that issue from its sides and extremity. The shape of buds is also very various—some being long and pointed, others short and round; some again are hairy, others smooth; some very small, and others large. (Phys, des Arbres, tome i. p. 99.)

Beside these varieties in position and form, which serve to distinguish the buds of different genera and species,' there are also many sorts of buds to be observed on the same tree, whose characters are discoverable by their form. Those which are pointed usually produce leaves and branches, and from those which are large and rounder commonly proceed flowers. The former are named leaf or wood buds (gemmae foliiferae), the latter flower or fruit buds (gemmae floriferae) : others which possess both leaves and flowers have been called mixed buds (gemmae mixtae). In some trees, as those of the apple and pear, two varieties of wood-buds occur, one of which is small, produces only a small bunch of leaves, and in the end becomes a fruit-bud; the other is larger, and gives origin to a branch.

As the rudiments of the flower appear in the bulb of roots the season before they are destined to bloom, so< those of the leaf and the flower are distinguishable at the same period in the bud of the branch. They are to be perceived, says Du Hamel, in autumn, and continue to grow even during winter, appearing to be clandestinely formed in that season, when the movements of the sap seem to be suspended; and are thus prepared to shoot forth on the return of spring. It is, however, only in perennial plants that these phenomena are observed. Annual plants do not produce buds, and even those whose roots survive the fall of the stem produce buds only on their roots. (Phys, des Arbres, tome i. p. 103.)

Climate appears to exert the greatest influence on the* formation and evolution of buds. In cold regions, asweļ [7:3:91]have just observed, the budis formed many months before 'it is destined to shoot into a leaf or branch; but in warmer regions scarcely any interval occurs between the periods of formation and evolution. The buds, in such climates, are said to unfold themselves immediately from the bark into branches, without having remained in the. form of buds for any length of time. Sometimes, in the milder seasons of our own climate, the evolution of buds rapidly succeeds to their formation, and the vegetative process with us emulates the productive powers of more favoured climes. In some examples, however, the specific characters of particular plants overcome these general tendencies of climate; and thus hot climates are said to possess some bud-bearing plants, and in colder climates there are a few shrubs which are said never to bud. (Willdenow’s Principles of Botany, p. 273.) It seems, however, more correct to consider all plants as bearing buds, from which the branches, the leaves, and the flowers, successively proceed; and to say that in warm climates in general no suspension of the vegetative process occurs, as in cold ones, and no marked interval is observed, therefore, between the formation and evolution of buds. The few exceptions that occur respectively in warm and cold climates must be considered in reference to the specific characters of the individual plants. The process by which buds are actually formed has been called Gemmation or Gemmifi-cation.

Having made these few general remarks on the nature and formation of buds, we have next to exhibit a few examples of their structure and evolution, confining ourselves at present to a description of those which produce either branches or leaves. The mature bud consists of two parts; one that forms the new branch or leaf, and in the language of botanists may be termed persistent ; the other serving only a temporary purpose, and falling when that purpose is accomplished. To the former may properly be applied the term germ or gem, and the latter, from its office, may be called hybernaculum. In its leading characters the bud bears a near analogy to the more perfect seed, for the germ very exactly resembles the plume; and the hybernaculum, as we shall see, in structure, office, and duration, approaches near to certain cotyledons.

The leaves or scales which constitute the hybernaculum, and which in future we shall denominate the hybernacular leaves, vary much in number, size, and figure, in different buds. Even in the same bud the inner ones are thinner, and much more tender and succulent, than the outer, and are besmeared with, a viscid humour which intimately unites them; while the outer ones are commonly hard, hairy, and of a sealytexture. Like the cotyledonous leaves of seeds, those of the hybernacle sometimes grow for a certain time with the germ, and fall successively at periods more or less distant: they are also not less distinct in figure from those of the germ, than the leaves of the cotyledon are from those of the plume. This arises from the peculiarity in the distribution of their vessels, which do not spring from one common central trunk, as in ordinary leaves, but are derived from several distinct fasciculi at the base, like those of the cotyledonous leaf, as seen in that of the gourd, d’, fig. 13, Plate XXXIX. According to. the manner in which they are disposed or folded up in the bud, botanists have assigned them different names, for an account of which we must refer to their writings. In the opinion of Du Hamel, they all derive their origin from the inner layer of the bark, of which they seen^to be only a prolongation. (Phys, des Arb.

The germ of the bud, which is contained within and protected by these enveloping leaves, is composed of one or more leaves generally folded and curled, but in some instances open and expanded. At first; they are. very small, and their form is indistinct, so that the pedicle alone is distinctly visible; from which branch off tire vessels that form the middle rib, and are afterwards expanded to construct the lobes of the leaf. From the figuré which the germ possesses before its expansion being like that of a keel, its vascular portion at this period has been named carina, and its softer part medulla or pith.

With respect to the particular portion of the branch from which the germ internally derives its origin, opinions have much varied. Some have held that it proceeded from the pith alone; others from the first circle of vessels that immediately surrounds the pith; others from the tender wood alone; and others from the pith, the wood, and the bark conjointly. Grew held this latter opinion. (Anat. of Plants, p. 28.) Malpighi describes the germ as a tender, ligneous substance, formed of vessels and cellular tissue, and surrounded by its proper cortical texture. (Anat. Plantar, p. 45.) According to Du Hamel, it originates from the ligneous texture and the pith (Phys, des Arbres, tome i. p. 103); and Hill considered it to spring from the first circle of vessels alone, but not to carry with it any portion of the pith.

As every germ is composed of the cortical and ligneous textures, it may be said to originate in part from both, as all these writers seem to admit. Sometimes also the pith of the germ is continuous with that of the ι branch; but in other instances no such connection subsists, and there is nothing in the character of the pith of the trunk that renders it at all essential to the constitution of the germ. That in many instances, also, the germ springs from the first circle of vessels, is most certain; but it is not less certain that buds spring from trees long after this first circle of vessels has lost its vegetative power, or has been entirely destroyed.

In the oak Malpighi describes the entire bud as consisting of many scales enveloping each other, and forming an oval body. When evolution commences, these open and expand, and in part fall; but two generally remain and protect the springing germ for a long time. In fig. 15, Plate XL., is represented one of the hybernacular leaves d of the oak-bud, at the base of which the germ e is placed. The leaf d is described as possessing an oblong form, and is very evidently vascular; the germ is exceedingly small, and is represented as possessing at this period only one fasciculus of vessels. By degrees three fasciculi become apparent, which are continued through the germ, and form three pointed extremities, as in fig. 16. These parts augment, and the vascular fasciculi separate, so as to produce successively the appearances in fig. 17 and 18. At length the small Curled leaf/ fig. 19, rising between the two hybernacular leaves g g, is seen to resemble the plume of the seed, and to possess a form altogether different from the enveloping leaves, which in appearance resemble more the cotyledonous leaves of certain seeds. In fig. 20 is given a vertical section of the germ, considerably magnified, in which h denotes the pith that occupies the centre: it is inclosed by vascular fasciculi, i, that send off through the bark ramifications to the several little processes that compose the serrated border of the leaf. (Anat. Plantar, tab. 10 and 14.)

But the method of nature in the evolution of buds, continues Malpighi, is not always the same; for the hybema-cular leaves do not always waste and fall as those of the germ increase. On the contrary, in many trees, these leaves, especially those about the base of the bud, losing [7:3:92]their primary figure, assume new forms, and are converted into the permanent leaves with which the branch is adorned. Examples of this sort occur in laurel, in the apple-tree, the almond-tree, and many others. In other instances, as in the rose, the permanent leaves seem to be generated out of those of the hybernacle, from the apex of which they emerge; and gradually the latter is changed into a sort of petiole, to the sides of which two slender appendages, the remains of the former hybernacular leaf, adhere. Similar transformations are said to occur in many otìier plants.

The buds, such as they have been described, pullulate variously from the sides of stems and branches, and always above the insertion of the fallen leaves; but they spring also from their extremities, and produce the annual elongation of the branch or stem. The manner in which this takes place, and the appearance which the parts exhibit, have been well illustrated by the observations and dissections of Μ. du Hamel. In the evolution of the seed, the plume, as we have seen, rises above the earth, and produces the stem, which puts forth leaves. When these leaves fall in autumn, the stem continues, and is terminated by one or more buds. The roots, as already remarked, do not increase in length but at their extremity, and therefore never elongate after the smallest portion of their extremity has been cut off. It is not the same with branches; for the newly formed part of the young shoot actually elongates, especially at its extremity, where it is most succulent and tender,—less in the parts lower down, where it is harder,—and in its more ligneous parts not at all; as Du Hamel, by very simple and decisive experiments, ascertained. (Phys, des Arbres, tome ii. p. 14.)

Tire appearance, structure, and evolution of a bud, at the extremity of a branch of the horse-chesnut tree, are given by the same ingenious author, whose candour, ability, and success in the prosecution of these curious researches render him worthy to rank by the side of Malpighi and Grew. In fig 1, Plate XLI., is represented part of the annual shoot of this tree, terminated by its appropriate bud, formed in autumn, and which is the commencement of the next year’s shoot. In fig. 2 a vertical section of the same bud is exhibited, in which the number and disposition of the hybernacular leaves that envelope and protect the germ in the centre are displayed. In the stem of the shoot the letter a denotes the pith, which is surrounded by the wood b b, and this again is covered by the bark cc. In fig. 3 is represented a section of part of the bud detached from the woody part of the shoot, and a little magnified, to show that the leaves of the hy-bernaculum take their origin from the inner portion of the bark.

Proceeding next to the interior part of the bud, Du Hamel represents it as composed of numerous small leaves (fig. 4), which are more and more minute as we proceed inward, and are covered with fine hairs. In fig. 5 is a branch-bud of the peach, as seen in February, after all the enveloping scales have been removed. It is composed of greenish filaments, ranged nearly as they appear in the figure. When some of these filaments were detached and viewed with the microscope, they appeared toothed at the edges, as in fig. 6, and were covered with hairs. All these filaments were afterwards detached in order to disclose, a small body lodged within them, and which appeared to consist of two small leaflets, folded and toothed at their edges, but not covered with hairs. It is represented in fig. 7. -It occupied the centre of the shoot, and seemed to be connected with the pith.

In the next figure, 8, is represented the bud of the horse-chesnut in the state of evolution. The letters d d indicate the scales of the hybernaculum thi∙ust aside by the shooting of the germ e, accompanied by two permanent 1 leaves ff. The letter g denotes the place of a second bud. A vertical section of the same bud is shown in fig. 9, from which all the sealyenvelopes have been removed It exhibits an entire shoot of one year’s growth attached to part of one of two years. From h to i denotes the growth of two years, and from i to k that of one year, with the germ k in the centre, and the lateral leaves as in the preceding figure. The letter I marks the pith, in m the wood, and η n the bark of the young branch. From I to n the pith is white; from n to o greenish; and towards i it is of a brownish-red colour. From i to k, which marks the extent of the annual shoot, the pith is green and succulent, and at p p it is seen to be prolonged into the lateral branches. The wood of two years’ growth, from m to i, is white, and forms a continuous tube round the pith, except where the branches go off. It is covered by another layer so thin as to be scarcely visible, but which will in the end become wood; and this layer is covered by the bark. The ligneous layer of the annual shoot appears to be a prolongation of the new layer of the older one, and, like it, possesses at first an herbaceous character. ∙ The cortical layer also seems to be a prolongation of that of the older shoot. As to the pith, though in both shoots it is continuous, it is to be observed that that of the older branch is white and dry, and that of the young shoot green and succulent. (Phys, des Arbres, tome i. p. 117.)

It is thus by the formation of a bud in autumn at the extremity of a branch, and the shooting and growth of that bud in the succeeding spring and summer, that the trunk of the tree and its branches are annually elongated. During the first season the shoot retains in great part its herbaceous characters, but in the second it becomes perfectly ligneous. In the axils of the permanent leaves of the young shoot, the rudiments of new buds become apparent, even in the' first season.

It was before remarked that trees receive an additional circular layer every year, and that from these new layers buds successively spring, so that the earliest branches may contain as many ligneous layers as the trunk, and those of later formation a smaller number, according to the year in which they shot forth, and the circle of wood from which they sprang. Combining this growth in breadth with that in length, it will appear, says Du Hamel, that at the base and centre of a tree 100 years old there is wood of 100 years of age; whilst at the exterior part of the same tree, and at the extremities of its branches, there is wood of one year’s age only. As the latitudinal growth was before illustrated by a diagram, a similar mode may be adopted to explain the longitudinal increase.

Let fig. 10, Plate XLI., represent in q r the ligneous portion of a tree that has proceeded from a seed in spring, and is observed in autumn. The following spring a second shoot proceeds from the bud r, which reaches as far as s'; but at the same time there is a second ligneous layer formed on the first shoot q r, by which its thickness is proportionally augmented; and at the end of the second year the tree has the form and extent of the unshaded portion of the figure q s. The next spring the bud 5 opens and sends out another shoot to t ; and ligneous layers are added as before to the two preceding shoots; and thus the tree is extended from q to t. The fourth year the same processes are repeated, and the tree extends from q to w ; and each annual shoot, from the base to the summit of the figure, is seen to be composed successively of four, three, two, and lastly of one layer in thickness. This figure, therefore, illustrates the mode in which trees increase at the same time in height and breadth. The lig-neons[7:3:93] layers may be compared to a series of cones which 'envelope each other, and which annually augment the diameter of the tree by the two thicknesses of the layers. It shows also that trees grow much more in height than in breadth, and that this growth is effected by the successive formation and evolution of buds at the extremity of the stem, precisely as the first shoot issues from the seed. (Phys, des Arbres, tome ii. p. 50.)

It is well known that if a bud be taken from one plant and inserted beneath the bark of a kindred species, it will grow, and retain in its new situation its original qualities and habits; or some buds, if directly planted in the earth and protected from too rapid exhalation, will produce a tree similar to the parent. Hence some writers, as Dr Darwin, have considered each bud as an individual plant, and that a tree, properly speaking, is a family or swarm of buds, which annually produce as many new plants. The bulbs produced beneath the soil by the tulip and many other plants resemble, in this respect, the buds produced in the atmosphere. Vegetables, therefore, are propagated by two methods—the oviparous, as by seeds, and the viviparous, as by buds and bulbs; and the individual plants, whether springing from seeds or from buds and bulbs, are all annual productions. The reproduction of plants, therefore, appears to be of two kinds, solitary and sexual; the former being accomplished by buds and bulbs, the latter by seeds, or through the agency of sexual intercourse.

Section II.

Of the Structure of Leaves.

These are organs of great importance in the vegetable economy: they are not however universal; for the Cactus, some species of Sćhoenus, and a few other plants, are considered to be destitute of leaves. Like other parts of the plant, they may be regarded with reference either to their external form or their internal structure. The former view belongs more especially to the botanist, who has very happily applied his descriptive language to portray the almost infinite diversity of figure, size, and character, which the leaves of different species of vegetables exhibit. On this branch of the subject we shall but lightly touch, recalling simply to notice the leading distinctions of the botanist, and such only of them as may appear to be more immediately connected with the structure of these organs.

The leaves (folia) are distinguished and denominated according as they are simple or compound. Simple leaves are such as have only a single leaf on the stalk or petiole that supports them, and where all the parts of the leaf are continuous with one another. Compound leaves are those which are madc up of more than one piece, or where the leaf is formed of parts or leaflets articulated together. In regard to their place, situation, and insertion, leaves are said to be determinate. By the place of a leaf is meant the part of the plant to which it is attached. By situation is understood the disposition of the leaves on the stem or branch, which corresponds to that of the buds. By insertion is expressed the mode of connection between the leaf and the stem or branch; and the direction of leaves is considered to bear reference to the position in which they stand to the stem.

_ The foregoing observations apply to leaves considered in connection with other members: when we regard them sin g 1 y, we remark several parts which are common to almost all leaves, and to which particular names have, been assigned. The part at which the leaf springs from the branch or 1 stem, ^whether, directly, or by means, pf a petiole, is called the base, and the point opposed to this, the apex, of the leaf. The expanded portion is termed limbus, which has its two sides, faces, or surfaces, as they are indifferently called. The prominent lines that appear on these surfaces were named riblets (costuloe) by Malpighi, and have very improperly been called nerves by most writers; for the term nerve denotes an organ of a totally different nature, of which not even the existence has yet been demonstrated in any part of the vegetable system. To the line that circumscribes and forms the boundary of the leaf the term margin or border is commonly applied.

The figure of leaves exhibits the greatest diversity, to express which various terms are employed. Their margin also is either entire or variously fissured, notched, or toothed. Their surfaces are naked and smooth, or clothed with hairs and studded with excrescences; sometimes they are plain and flat, at others furrowed or plaited; and in all these particulars the opposite surfaces present also the greatest diversity. In size, leaves exhibit the most remarkable differences; and, with respect to substance, some are exceedingly thin, and have a membranous texture, others very thick, and of a fleshy nature. As to colour, they exhibit every shade of green, from the most gay and lively to the deepest and most obscure. In some the tint of colour approaches to blue, in others to red; and previous to their fall they all undergo those changes of colour which produce the diversified beauties of an autumnal scene. The period, however, at which this occurs is very different. Some leaves fall early, before the summer has passed, and are termed caducous ; others retain their place till autumn, and then fall, and are named deciduous; others continue beyond the summer, and are styled persistent; and others, which have a still longer duration, are denominated perennial.

In the preceding section on the structure of buds we exhibited the form and appearance of leaves in the earlier periods of their existence. They were shown to possess a very minute size, and to be curiously folded up and concealed within their various scales or coverings, which effectually protected them from the rigour of the winter season. The germ of the bud, from which the leaves originate, was stated to be composed of the same elementary parts as the stem and branch from which it sprang. From the ligneous ring in the annual shoot it derived its vessels, which, after having traversed obliquely the cortical layers, were prolonged into the pedicle by which it remained attached to the branch. This pedicle was continued to form the keel of the germ in its folded state; and, in its more expanded forms, to constitute the vascular riblets of the leaf. By subsequent growth it gradually extends and elongates, becomes more ligneous, and is formed at length into the slender stalk or petiole by which the leaf remains attached to the branch. (Phys, des Arbres, tome i. p. 123.)

The vessels which are given off from the branch to form the petiole are not collected into one bundle, but constitute several fasciculi, which are disposed in different ways about the centre of the stalk. In some stalks there are three, in others five or six, and in others seven or more fasciculi, all placed, says Grew, either in an angular or circular posture, and at a greater or less distance from the centre. This centre is generally occupied by a pith, but sometimes it is hollow or tubular. In table 49 of the Anatomy of Plants, several transverse sections of the petioles of the leaves of different plants are given, exhibiting the different modes in which these fasciculi are dispersed through the cellular tissue that forms the greater portion of. the jstalk. In every instance the .pith and bark [7:3:94]of these petioles are represented as forming one continuous substance. In some plants the fasciculi of vessels that come off from the stem are not collected into a cylindrical figure, but, after being variously implicated with each other, expand at once into a leaf. Such leaves rise by a broad base, and, from being destitute of a petiole, are termed sessile, forming often a sort of sheath about the stem, of which several varieties are enumerated by botanists.

The petiole, as it springs from the branch, may be compared to a small stem, which at the basis of the leaf expands into numerous branches. Frequently it is continued through the centre of the leaf, forming its middle rib, and giving off in its course numerous branches. At other times, on reaching the base of the leaf, jt separates at once into three or more equal portions, which form as many distinct leaves or parts of a leaf, supported by. their respective petioles. The structure of the petiole is the same as that of the branch, being composed of the ligneous and cortical textures, in which sap and “proper vessels” are discoverable; and the whole is invested by the cuticle.

Where the petiole terminates, the proper or expanded portion of the leaf (limbus') commences, the figure of which is determined by the number and distribution of the vessels. These vessels divide and ramify in various modes, till at their termination they form a finely reticulated structure. In many recent leaves this structure is very visible; but when the softer parts are removed by spontaneous decomposition, the vascular system of the leaf remains nearly entire, and its extent and form are then more completely exposed. In fig. 11, Plate XLI., this distribution of the vessels is exhibited in a leaf of holly. From the central fasciculus branches are everywhere given off, which further subdivide, and form smaller ramifications, that terminate at length in a minutely reticulated structure. Around the margin of the leaf the vessels are continued, and at certain parts are prolonged into the thorns that bound the circumference of this leaf.

In this and similar instances the vessels appear to be ramified out of greater into less; but, as already observed, this does not appear to be really the fact, the vessels being all of the same size everywhere in the leaf, and all continued through it, like so many distinct tubes. This structure Grew represents in a highly magnified view of the leaf of borage, in table 50 of his work; part of which is represented, on a reduced scale, in fig. 13, Plate XLI., designed to show that the ramified vessels o are all clusters of tubes of the same size, which, though they separate continually, and come into contact, are never produced or ramified one out of another. Neither do they ever inosculate or anastomose with each other, until, according to Grew, they come to their final distribution.

The vessels thus distributed through the leaf belong chiefly to the order of spiral vessels, as was shown by Grew in the leaf of the vine and many others; and, from the observations of Malpighi, Darwin, and Knight, already related, it appears that “proper vessels” are everywhere associated with them. It would seem, from the observations of Darwin, that these two orders of vessels communicate in the leaf by continuation of canal, as the arteries and veins do in animal bodies. In the stalk he considered them to be disposed in two concentric rings, the inner one of which carried out the sap to the leaves, and by the outer one it was returned to the bark. (Phytologia, p. 43 and 58.) In branches of the apple and horse-chesnut, Mr Knight also observed coloured fluids to rise through the vascular fasciculi in the petiole of the leaf. These fasciculi were surrounded by others free from colour, which conveyed a different fluid, and, on being traced down the stalk, were found to enter the inner bark, and' to have no communication with those of the wood. The returning vessels he describes as being situated parallel to and surrounding those which carry up the sap. (Phil. Trans. 1801, p. 336.) . . ļ

Beside the vascular system which, by its ramifica-∣ tions, forms the skeleton of die leaf, there is another ∣ structure that requires to be noticed. We have seen that' the vessels, in their ultimate distribution, form a net-work more or less fine and minute in different leaves; so that a great number of intervascular spaces are produced. These spaces or areae, says Malpighi, are occupied by cellular tissue, which springs from the vessels themselves, and seems to depend from them; and by its means the thickness of the leaf is formed. In the recent part of the borage leaf, represented in fig. 13, Plate XLI., this cellular structure, occupying the reticulated spaces formed by the vessels, is observed; but it is most clearly seen in thick leaves, when the cuticle has been removed, and the vessels in part dissected away. The cells or utricles which form this parenchyma, as it has been called, are of different figures, and are mutually contiguous; they are composed of a membrane formed into the shape of a little vesicle or bladder, from the middle of each of which a little vascular production issues; they are all connected with each other, and with the vascular system of the leaf. In fig. 12, Plate XLI., this cellular structure is represented in the leaf of Cactus by Malpighi, in which the oblong cells arc described as proceeding from the central vessel, and mutually communicating with each other. (Anat. Plantar, p. 52.) A similar idea of the formation of the cells seems to have been entertained by De Saussure. In different leaves the cells possess very different sizes and forms. In the experiments of Μ. de la Baisse, they are said to have become tinged by coloured fluids conveyed from the vessels. Whether they communicate with each other is not known; but analogy, derived from other similar structures, would lead us to suppose that they communicate only with vessels, as seems to be the case in the cotyledonous leaf of the seed.

Μ. de Candolle describes the leaf as consisting of a' superior and inferior face, between which is an interme- ’ diate part, of very various thickness in different leaves, toļ which he gives the name of mesophylle, answering to the parenchyma of preceding writers. The leaf is formed, he says, by ramifications of the vessels, and by cellular tissue, which fills up the intervals of these ramifications. It probably comprehends two systems; one which, receiving the ascending sap, brings it into contact with the air, and permits the exhalation of its superfluous parts; the other a system which receives the elaborated sap, and reconducts it to the stem, whare it serves for nutrition. Physiological phenomena, he adds, prove the existence of these two functions in leaves; but anatomy has not yet distinguished the organs by which they are accomplished. (Organog. Végét. tome i. p. 271, 273.) It has been before observed that Μ. de Candolle follows Kieser in regarding the alleged spaces between the cells, called intercellular canals, as the organs in which the vegetable fluids are conveyed; but the facts and arguments already stated establish, we think, with sufficient certainty, that, in the wood, the bark, and leaves, the sap is conveyed by appropriate vessels. The consistence of the leaf, continues Μ. de Candolle, depends much on the relative proportions of the cells and vessels that compose it. When the vessels are numerous, the cellular tissue is small in quantity, and the leaf is fibrous, firm, and thin; when, again, the vessels are few and distant, then [7:3:95] the cells are more abundant, and the leaf is softer and 'more fleshy.

s The two surfaces of this organ often differ much in appearance and structure. On the upper surface the vessels are commonly but little prominent; it is therefore smoother, has but few hairs or pores, and is sometimes destitute of them. The lower surface, on the other hand, presents the vessels more prominent, and is furnished with hairs and pores much more abundantly. These differences are more particularly observed in the leaves of trees: in those of herbs the two surfaces more nearly resemble each other. In aquatic plants, though the upper surface is smooth and green, and the under one pale and rough, yet the former only, as being exposed to the atmosphere, is furnished with pores. It is to be remarked, however, that whether the two surfaces exhibit an aspect either very different or very similar, each seems destined to enjoy a special function. Hence, if the upper surface be turned downward, and the lower one upward, the leaf tends to resume its natural position; and if it be retained by force in a position not natural to it, it dies. (Organog. Végét. tome i. p. 274.)

In the cellular tissue of herbaceous sterns, and of all leaves, are contained small globules, which by the action of light are rendered green, and bestow on the plant its verdure. In the elongated cells of Conferva reticulata they are represented by Kieser as small green globules. (See Plate IL of his Memoir.) They are of a resinous nature, and have been named chlorophylle by chemists; but as a matter similar in composition is said to reside in the flower, Μ. de Candolle deems the foregoing name improper, and proposes to call this matter chromule. (Ibid. tome i. p. 19.)

In the substance of the leaf Malpighi describes small bags or follicles as existing between the cells and vessels, and which, in most leaves, yield a peculiar humour. They spring from the vessels, have a roundish or oblong form, and the lip that bounds their orifice is often furnished with hairs. This orifice points towards the upper surface of the leaf, or that which regards the heavens. In different leaves the follicles have different forms; and they afford a secretion which, adhering to the contiguous parts of the leaf, renders them shining. They are numerous in the leaf of the fig, lemon, orange, &c.; and are rendered more visible as the cellular tissue of the leaf wastes and dries. (Anat. Plantar, p. 52.)

There are other small leaves called stipules and bracteae, which are attached to leaves and flowers, and are of some importance to the botanist. In the general features of their structure they resemble those already described.

The whole structure of vessels and cells that compose the leaf is covered by a cuticle variously furnished with hairs and pores, and other appendages already described.

Section III.

Art. I— Of the Structure of Flowers.

The flower is that assemblage of organs which, by the mutual action they exert on each other, give origin to h'uits and seeds. All the parts hitherto described contribute to the growth and perfection of the individual plant, and have been named the Nutritive or Conservative ⅛ystem; but the organs composing the flower are destined to continue the race, and from thence have been denominated the Reproductive Organs. In a limited sense the bud, produced by the conservative system, may be deemed a reproductive organ, since new individuals may successively be propagated from it; but after a certain period, plants thus produced seem to degenerate, and become incapable of producing fruit. Reproduction without fecundation presents no organic apparatus which is proper to it; but sexual reproduction, or that accomplished by fecundation, exhibits numerous and varied organs, which merit all our attention, not only because they exercise the principal functions of vegetation, but because, from the constancy and symmetry of their forms in the same species, though varied infinitely in different species, they afford the base on which Classification rests.

In relation to the organs of reproduction, vegetables, as before remarked, have been divided into two great classes: 1st, such as have flowers visible to the naked eye, and in which the sexual organs are distinct; 2dly, such as have flo'wers (if there be any) which can be rendered visible only by the microscope, and in which the sexual organs are not distinct. The former class, named phaenogamous or phanerogamous, comprises all exogenous, and the greater part of endogenous plants; the latter, or cryptogamous division, comprehends some endogenous vascular plants, and all those which possess a simple cellular structure. At present we shall treat only of the first or flowering class.

The flower is essentially constituted by the presence of one of the two sexual organs, or of two re-united by a common support, with or without the exterior envelope destined to protect them. In some instances the flower bears only a male or female organ, and is then named unisexual; but more commonly these organs are present in the same flower, and it is then called hermaphrodite.

This unisexual flower, again, is simply male or female, according as it possesses either stamens only, or pistil only. In some instances, though the same plant bears both male and female organs, it is not hermaphrodite, since these organs occur in different flowers; in others, again, the male and female flowers exist only in different plants. Lastly, we sometimes find both male, female, and hermaphrodite flowers mingled together, either on the same or on different footstalks. The distinctions founded on this separation, re-union, and mixture of the sexual organs, have served as the basis of certain classes in theLinnaean system.

•Sometimes the male or female organs alone, protected in a small scale, constitute the flower; but in general they are surrounded and protected by others, named the corolla and the calix.' All these parts are commonly borne on a stalk called the peduncle, which, expanding at its extremity, forms the receptacle, or torus as it has been called, upon which all the parts above mentioned are supported. In fig. 1, Plate XLII., the sexual parts which constitute a perfect flower are exhibited. The letter a denotes the corolla, formed, in this flower, of six leaves called petals; b the stamens or male organs, which surround the pistil or female organ c ; and d indicates the peduncle of the flower. In the next figure (fig. 2) the corolla has been removed, and the parts within are then more clearly seen. They now consist only of the sexual organs supported on the peduncle, and at their base are embraced by the calix e. The pistil, in the centre, is composed of three parts, viz. the ovarium f, the style g, and the stigma h ; while each stamen consists only of two parts, the filament i, and the anther h, which latter contains the prolific matter named pollen. Such an assemblage of organs constitutes' a perfect or complete flower; but it often happens, that in flowers where the sexual organs are present, one or other of the accessory organs is wanting; thus, in the tulip only one of the protecting envelopes is present, and botanists differ as to the name it should bear. Linnaeus considers . it as a corolla, because it is coloured like ordinary petals; but Μ. Jussieu regards it as a calix, from its analogy in structure arid use to that organ.
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The flower is sometimes attached immediately to the branch by its base, when it is termed sessile; but in general it is borne on a footstalk or peduncle. This peduncle of the flower, like the petiole of the leaf, is sometimes single and sometimes ramified: in the latter case its divisions take the name of pedicels.

Around the base of the flower we often meet with small leaves which, in size, colour, form, and consistence, differ from ordinary leaves. They are called bracteae, and from the axils they form with the stem or branch the flower-buds often spring. From thus being the birth-place of the flower, which draws from them a great part of the nutrient matter, the bracteae are smaller and more membranous than the ordinary leaves of the plant, and partake often of the colour of the flower. They must, however,be regarded as leaves, modified by the circumstances in which they grow, as is demonstrated by the fact, that examples frequently occur in which bracteae are changed into true leaves. (De Candolle, Or-ganog. Végét. tome i. p. 438-9.) When these bracteae approximate so as to form a sort of envelope around the flower, they take the name of involucrum, — when this envelope becomes a sheath, it is named spatha, — in the Gramineae it is termed gluma, &c. ; of all which forms there are numerous modifications, to which botanists attach different names. The bracteae approximate more or less in character to the calix, in proportion as they are coloured or exhibit the form of whorls; and the transition of the organs of vegetation into those of florescence is so gradual, says Μ. de Candolle, that the more we study it, the better do we comprehend that unity of composition which constitutes the base of philosophic organography. ( Organog. Végét. tome i. p. 447.) »

The flower, as well as the leaf, originates from a bud, and in many respects the buds of flowers resemble those of leaves. They are covered and protected in the same manner, but they are generally larger, and have a more rounded form. They consist of two parts, the gem or eye (oculus), as it is sometimes called, and the hybernaculum, or protecting envelope. In some trees they spring from the extremity of particular branches; in others from the branches in common with the leaf-buds; and in others from the axils of the leaves. In fig. 3, Plate XLII., is a portion of the branch of the peach-tree, bearing two flower-buds 11, between which the smaller and more pointed leaf-bud m is placed.

Grew discovered and has exhibited many examples of the complete formation of the flower many months before it is destined to bloom. (Anat. of Plants, tables 63, 64.) Of this fact Du Hamel has also given several examples. In fig. 4 is a longitudinal section of the flower-bud of the peach-tree, made in the month of February, to show the complete formation of the stamens and pistils within the surrounding envelope of the bud. In fig. 5 is another representation of a similar bud, from which the enveloping scales have been removed: it displays the calix, which at this period completely envelopes the other parts of the flower. When the leaflets of the calix are separated, as is done in fig. 6, then the stamens and pistils are fully disclosed: the petals also of this flower were visible, but at this period were very small. Even the anthers were found to contain a fine dust, but no rudiment of the future embryo could be detected in the ovary. In the flowerbud of the pear, examined in February, the parts of fructification were also visible, but indistinct; a month later, all the parts were more advanced, and the stamens, petals, and pistils were distinct, and towards the end of March, even the rudiments of future seeds were visible in the base of the pistil. (Phys, des Arbres, tome i. p. 200.) Thus it appears that through the winter season the several parts of the flower are clandestinely formed, though still extremely minute. As they enlarge, their position within the bud is very various in different species: they gradually expand and form the perfect flower, whose several parts we have next briefly to describe.

The peduncle of the flower, by which all the other parts are connected with the stem or branch, is frequently single, but often divides into several parts or pedicels, as they are named, each of which supports one or more flowers. The modes in which the flowers are disposed on the pedicels have received different names from botanists, and to the general circumstance they assign the term inflorescence. The peduncle in structure resembles the stalk of the leaf, being formed, like it, of the several common textures. In size, in length, and other external characters, it exhibits all the varieties already noticed in the petiole of the leaf.

At the extremity of the peduncle is placed the flowercup or calix, which has very different forms, and has received in consequence different names. Even in its most common forms it exhibits great variety. It is sometimes composed of a single piece, and has a tubular form; in others, of many pieces called sepals (sepala), which vary in number, position, and size, and for an account of which we must refer to the writers on botany. In some instances this organ falls when the fruit has set; in others it continues till the fruit is mature; and in others the fruit is formed within it, to which it becomes a permanent covering, and exercises the office of pericarp. The colour of the calix is commonly green, but sometimes partly red, or white, or yellow. In some flowers it is entirely wanting. From the dissections of Grew and Malpighi it appears, like the leaves, to be constructed of a cuticle, a pretty thick cellular tissue, and of vessels, all of which exist in the peduncle, and are derived from the common textures of the plant.

Above the calix, the corolla, the chief ornament of flowers, is borne. It is inclosed by the calix, but surrounds the interior parts of the flower, and is of every colour but green. It consists of one piece, or of several: these pieces are called petals, and according to the number of these pieces the corolla is variously denominated. The monopetalous corolla, or that composed of one piece, is also distinguished into several parts, expressive of form, position, or quality; and in the polypetalous variety, which consists of several pieces, each petal has its claw (unguis) situated at the base, and by which it is attached to the calix or receptacle, and its expanded part (lamina), which is of very various figure, size, and colour. At the base of the petal a tuft of hairs is often observed, and its surface is frequently covered with a fine down, or sometimes with jointed hairs bearing 'globulets, as occurs on leaves. With regard to structure, Malpighi describes it as composed of the same textures as the common leaf, as is distinctly visible in the thicker varieties of it. Grew showed it to possess spiral vessels,—a proof, as Du Hamel observes, that it is partly derived from the ligneous texture, since such vessels are not found in the bark. The odour these organs possess leads to the belief that they contain also a peculiar juice. (Phys, des Arbres, tome i. p. 215.)

The organ next to be noticed is the nectary (nectariumf In a strict sense this term designates any excretory gland which is situated on any one of the floral organs: and the juice it yields bears the name of nectar. The excretory glands observed on flowers deserve a common designation, because, whatever be their position on any part of the flower, whatever the nature of the juices of the plant, or whatever the size, form, or consistence of the gland itself, the secretion it yields is always saccha[7:3:97] rine, and very similar in all known plants. In regular 'flowers the nectaries may occur on all the organs, symmetrically disposed. Their most common seat is the receptacle or torus. In some flowers, however, they are seated on the ovary, in others on the corolla, calix, stamens, &c. They have the form sometimes of distinct tubercles; but in different plants they exhibit various other forms. In irregular flowers the nectaries are not thus symmetrically placed, but are found on one part and not on the corresponding part of the same flower. Sometimes they supply the place of abortive stamens or pistils. The juice they secrete is greedily sought by insects. ( Organog. Végét. tome i. p∙ 535.)

Art. II.— Of the Sexual Organs of Flowers.

Having exhibited this general view of the flower, we must now describe somewhat more particularly the forms, characters, and structure of its most important parts—the sexual organs.

The male organ or stamen consists, as before remarked, of two parts,—the anther, with the pollen it contains, and the filament on which the anther is borne. The filament, though a common, is not an essential part, and is sometimes wanting, when the stamen is said to be sessile: the perfection of the stamen resides, therefore, in the anther, and especially in the pollen it contains.

In regard to number, length, position, direction, &c. the stamens exhibit great diversity in different plants. The number of stamens in each flower varies from one to twenty or more; and on this number, progressively increasing in flowers truly hermaphrodite, the first ten classes in the Linnaean system are founded. In length, the stamens are equal or unequal; and this disproportion is sometimes symmetrical and sometimes not,—peculiarities which give rise to the formation of other classes in the Linnaean arrangement, Vx position, the stamens are some? times opposed to the divisions of the petals, and sometimes they alternate with them. Sometimes, from being shorter, the stamens are wholly included within the corolla: in other instances they protrude beyond it. Their direction is erect, pendent, or horizontal; and their summit is variously inclined to or reflected from the centre of the flower, a In some flowers the stamens are united by their filaments; in others by their anthers; and in others they are joined to and almost confounded with the pistil, giving origin, in their various módes and degrees of union, to the formation of other classes in the Linnaean system.

The filament which bears the anther is most commonly straight and filiform: sometimes, however, it is flattened or wedge-shaped, or awl-shaped. It is sometimes as small as a hair, sometimes large and flat like a petal, and its summit is either pointed or obtuse. To the summit the anther is commonly attached; but in some flowers this part is prolonged beyond the attachment of the anther. In structure, the filament resembles the corolla; and it often happens that these organs, are transformed into one another. Thus the rose, in its wild state, has only five petals and many stamens; but by cultivation the stamens gradually degenerate into petals, and the flower becomes sterile. Malpighi describes the filaments as originating from the ligneous texture, being formed of spiral vessels and elongated cells; and as they are sometimes produced from elongated floral leaves (petals), they must necessarily, le θ a ^ s > b c formed of the same parts. (Anat. Plantar.

The anther is that essential l part of the stamen that contains the fecundating matter. It is borne on the summit of the filament, and is generally formed of two small membranous sacs, attached immediately to each other, or ’ united by an intermediate connecting body. Anthers,’ therefore, are generally bilocular; but in some plants there is only one sac or loculament, and in others as many as four. The sacs are either round, oval, or elongated: each sac commonly exhibits on one of its sides a longitudinal furrow: this side is properly called the face, and the part opposed to it the back, of the anther. The anther is attached to the summit of the filament, sometimes by its face, sometimes by its back, &c. In form, the anther is subject to great variety; and the two sacs that compose .a bilocular anther are joined together in very different modes. In discharging the pollen the sacs open in different ways in different genera of plants, most commonly by the longitudinal furrow that runs along the face of the anther; but in some plants the opening, or dehiscence as it is called, is through two small holes or pores situated at the summit of the two sacs; and sometimes it is by a sort of little valves in some one or more parts of the anther. As the filaments of the stamens sometimes cohere, so likewise do the anthers, in a manner to form a sort of tube. In other plants the stamens, instead of being free or simply united together, coalesce so as to make one body with the pistil. The colour of the anther is often yellow, orange, violet, white, &c., but never green or truly blue.

The pollen, contained in the anthers, consists of numerous regularly figured small particles, which possess a very different figure, size, and colour in different plants. Many of their forms and sizes are delineated by Grew in table 58 of his Anatomy of Plants. In fig. 9, A, Plate XLII., copied from Grew, one of the filaments, bearing its anther, as it appears when detached from the pistil of the mallow, fig. 8, is exhibited. The particles of pollen are considerably magnified, and still more highly in fig. 9, B. In fig. 10 of the same plate, severalrepresentations of the forms of the pollen, as given by Du Hamel, are also exhibited. The number of these particles in each anther extends, it is said, from a few hundreds to many thousands. In the cell of the anther they are said to float in a viscous liquid, and to be nowhere attached to its sides, though it is probable they once were; and Μ. Turpin has even designated a salient point in each cell as the part which produced the pollen. (De Candolle’s Organog. Végét. tome i. p. 465.)

In some flowers the pollen consists of transparent grains; in others they are of à white, purple, blue or brown, and more frequently of a yellow colour. Their surface is either smooth or rough, or covered sometimes with a viscous matter. When examined under the microscope at the period of maturity, they may be seen to burst, and yield a fluid, in which, according to Du Hamel, small granules are seen to float. This fluid is described as being thin, or viscid, or oily. If a grain of pollen be thrown on the surface of water, it may be seen to swell insensibly, and finally burst. At this moment a minute quantity of fluid matter escapes, which spreads on the surface of the water, and forms a sort of slight cloudiness. It is to this liquid matter that the fecundating property of the pollen has been attributed. (Elemens de Botanique, par Achille Richard, p. 199.) This fluid is said by Hedwig to be discharged at once on the bursting of its containing capsule; but Koelreuter considers it to be slowly transmitted through pores in the side, or hairs on the surface of the capsule.

Instead of being formed of distinct granules, the pollem is sometimes met with in a solid mass. In many genera of the families Apocyneae and 0rehidece,fla the pollen contained in one sac is united into one body, having the form of the containing sac; in other instances it forms smaller [7:3:98]masses, and these are united by a sort of elastic net-work; 'while in others the masses are granular. The pollen is very inflammable, and in many plants its odour is said, to bear a striking resemblance to the corresponding secretion in animals. (Elemens de Botanique, par Achille Richard, p. 200.)

It has lately been said by some,.that the particles contained in the grains of pollen exhibit spontaneous motions, like the spermatic animalcules of animals; others ascribe these motions to causes distinct from life; and others, though they allow similar motions to be exhibited by the molecules of unorganized matter, maintain, nevertheless, that in the granules of pollen they are independent of physical causes, and resemble the less rapid motions of some of the simplest animalcules of infusions.

The parts of the flower hitherto described are constructed, says Malpighi, with reference to the female organs, in which the seed, the last result sought by nature, is curiously formed and matured. This organ, called the pistil (pistillum), consists of three parts. These are represented in the pistil of the almond, fig. 11, Plate XLII., where the letter p indicates the stigma, q the style, and r the ovary. To this latter organ Malpighi and Grew gave the name of uterus, Linnaeus that of germen, and Gaertner that of ovarium. In different plants the pistil exhibits great variety in form, size, number, and mode of attachment; upon which many of the orders in the Linnaean system are founded.

~ The ovarium occupies almost always the inferior part of the pistil. It is the organ in which the seed is produced. When cut open it exhibits one or more cavities or cells, in which are contained the rudiments of the seeds or ovula; and it is in it that the change of the ovula into perfect seeds is accomplished. Its form is various, but most commonly ovoidal. It is seated commonly on the receptacle, together with the stamens; but frequently it is placed below the flower. Its cavity consists sometimes only of one cell or loculament, in which one or more ovula are found. More frequently there are two or more cells containing ovula: these are sometimes disposed in regular series, sometimes they are scattered and without order, and sometimes they are united to one another in a globular form. When these ovula are fecundated they become seeds; but in some plants a certain number are constantly abortive.

The style is a prolongation from the summit of the ovarium, and supports the stigma. It is commonly so situate in the flower as to be surrounded by the stamens; but sometimes it is entirely wanting, and the stigma is then said to be sessile. The ovary in different plants may be surmounted by one or more styles; while in other plants there is but one style to many ovaries. Most frequently the style occupies the summit of the ovary, but sometimes it springs from its side, and very rarely from its base. The forms of the style are numerous :—the most common is the filiform; but frequently it is thick, angulated, or club-shaped. It is commonly a hollow tube which communicates with the ovary: there is usually but one style to one ovary, but sometimes more. In some instances the pistils correspond in number with the loculaments into which the pvary is divided; in other instances every seed that is formed has its distinct pistil, while in other examples only one pistil is allotted to a great number of seeds. In some flowers the style is so connected with the ovary that it falls after fecundation; in others it continues after that event, and forms a part of the future fruit.

The stigma, which forms the summit of the pistil, sometimes terminates the style by an open mouth. Sometimes it appears like a small bud; in other instances it is variously divided or forked. Sometimes it is smooth; and' is sometimes covered with hairs. The number of stigmata is determined by that, of the styles, or divisions of the styles, and therefore varies from one to six or more in different plants. Sometimes the stigma is attached to the summit of the ovary, without the intervention of a style; but most frequently it is seated on the summit of the style, or occasionally attached to its side, or to that of the ovary.

In consistence, the stigma is thick and fleshy, or thin and membranous, or formed of small glandular bodies. In form, it varies exceedingly; and its surface is smooth, pubescent, or plumons, &c. In structure, it is ordinarily glandular; and at the period of fecundation its surface is rendered moist by a peculiar viscous matter. When the pollen falls on this matter, its grains burst, and the granules, called by some favilloe, are supposed to be absorbed by the spongioles seated in this part, and conveyed to the ovula by the vessels which form the pistillary cord, and thus accomplish their fecundation. Μ. Bulliard is said to have traced coloured liquors, previously absorbed by the stigma, along the vessels in the interior of the style, to the ovula contained in the cells of the ovary. (De Candolle, tome i. p. 480.) In many plants, however, either from the structure of the pistil, the nature of the pollen, or the circumstances in which fecundation is effected, the pollen seems never to come into contact with the ovula.

Section IV.

Of the Structure of Fruits and Formation of Seeds.

In the preceding section we described the structure of the flower antecedent to fructification: we have now to exhibit as concisely as possible the changes of form it undergoes after that event, and particularly as it regards the production of the seed.

After fecundation has been effected, the calix, corolla, stamens, and even style of the pistil, commonly fade and fall; the ovary alone remains, and undergoes very different changes of form in different plants. In the latter periods of its enlargement it is usually called pericarp (peri-carpìum), a term which is understood by botanists to apply also in certain cases to the calix, the corolla, or any other apparatus of organs that serves as a support and defence to the seed.

In its early state the ovary is described by Gaertner as possessing at first a simple cellular structure, which at a i later period takes the form of distinct cavities or loculaments. Within these cavities minute globules are after-’ wards seen, which are the rudiments of future seeds. According to Mr R. Brown, the ovulum, in the unimpregnated ovarium, is attached only to a part of the membrane that lines its cell or internal cavity; but soon after fecundation in some cases, and still oftener during the growth of the seed, this membrane coheres so closely with the proper coat of the seed as to be no longer distinguishable or separable from it. In the ovarium of plants of the family Composite he observed two slender filiform cords, which, originating from the base of the ovulum or its short footstalk, ran up, and were more or less connected with the parietes of the ovarium, until they united at the top of its cavity immediately under the style, between which and the ovulum a connection was then formed. In some species of this family, as in tussilago odorata, these çords were easily separable from the ovarium, and could be removed entire along with the ovulum. These cords he regards rather as vessels conveying' nutrient matter, than as organs by which impregnation was accomplished. A similar appearance of vascular cords con [7:3:99]necting the ovulum with the style was observed in the family Brunonia; and in certain liliaceous plants the bulblike seeds, which separate from the plant before the em-biyo becomes visible, are supplied with distinct spiral vessels, which, entering at the umbilicus, ramify regularly through the fleshy mass, and appear to have a relation to the central cavity, where the embryo is afterwards formed. (Linnaean Trans, vol. xii.) Thus, then, the ovulum in the plant, like the egg in the animal, is, to a certain extent, produced without fecundation; but if that function be not performed, it soon degenerates and wastes. The progressive changes that occur in the ovary itself, and in its contained ovuia, subsequent to impregnation, have been observed by Malpighi in several plants, whose observations, as they relate to the almond, we shall briefly detail.

In fig. 11, Plate XLII., is represented the pistil of the almond, as it appears soon after fecundation, in which the ovary r is seen to be somewhat enlarged, and has an oval form. In the next figure (fig. 12) a longitudinal section of the same pistil is exhibited, exposing the cavity of the ovary s, within which a small vesicle, t, is placed. As the ovary (fig. 13) enlarges, it becomes rounder, and its style is contorted and diminished in size. If in this stage it be laid open, as in fig. 14, its vessels, which in the peduncle w are disposed cylindrically, are observed to be dilated at the place of the calix x, and to give off branches to the ovary itself, and to the shell y that now begins to be formed within it. In the centre, the ovulum z is now seen to be much increased in size. All the parts continue to augment; the ovary (fig. 15) becomes rounder, and the appearance of the style is obliterated. On exposing its cavity in this stage, as in fig. 16, the ovulum a! in the centre is observed to be much increased, and the outer layer b' of the shelly covering that invests it now begins to harden. Such are the changes of form and structure exhibited by the ovary: we have next to trace more minutely those of the ovulum that is produced within it.

The earlier appearances of this body have been exhibited in fig. 12 and 14. When removed from its seat, a few days after fecundation has been accomplished, and viewed by a moderately magnifying power, it exhibits the form and appearance represented in fig. 17. Externally it is covered by a vascular tunic, c , , derived from the inner coat of the ovarium, a part of which coat adheres to it at d’. If in this stage the ovulum be laid open by a vertical section, as in fig. 18, it is seen to be composed of two tunics or sacs, one within the other; the inner one is filled with a cellular tissue, that contains a transparent juice. To this inner tunic the term chorion may be properly applied.

At a period a little later, when the ovulum is examined, a tubular body (e', fig. 19) is observed to extend through the chorion or tunic last mentioned. Shortly after, this tube expands at its apex, and is found to contain a small vesicle. In fig. 20, which represents a section of the entire ovulum, the outer tunic, the chorion, and the tube f ’ expanded at its apex, are exhibited. To it the appellation of amnios may be given; for it is the organ in which the embryo, or corculum, as at this early period it has been called, is first seen to emerge. Through several successive days the expanded portion of this tube enlarges and forms a sort of sac, which is filled with cellular tissue, and the summit of which, says Malpighi, the embryo is seen to occupy. In fig. 21 this amnios is separated from the other tunics, and at its summit the embryo g ’ is observed. If removed from its seat, the embryo presents the appearance h' (fig. 22), and when expanded, as m fig. 22, i', is seen to consist of a body and two little wings.

Having thus viewed the several parts of which the ovulum is composed in their separate state, let us next observe them in connection, and trace the series of appearances they exhibit, and the effects they produce on each other. In figure 23 is given a vertical section of the entire ovulum in a more advanced state. The outer coat k' still envelopes the others; the embryo l’ occupies the summit of the amnios m', whose lower part, still tubular, is continued through the chorion n'. In the next figure (fig. 24), from which the outer coat has been removed, the embryo o' and the amnios p' are represented as enlarged; but the chorion q r is partly exhausted of its juice, and has fallen down in a collapsed state. At this period the embryo, when separated from the amnios, has the form r', fig. 25, and in its expanded state is represented by the letter √ of the same figure. The bulk of the embryo t’, fig. 26, continually augments, and encroaches on the capacity of the two tunics v', x ’ , whose forms are constantly changing; and from being successively emptied of their juices, with which the embryo becomes filled, they are gradually pressed downward. At last the embryo y ’ , fig. 27, is so much augmented as to fill the cavity of the outer tunic; and by this time the amnios and chorion, exhausted of their fluids, exhibit the shrunk and corrugated forms in which they appear at the bottom of the figure. According to this representation, the outer tunic k', fig. 23, derived from the ovary itself, and the fine membrane that immediately invests the embryo, form the only permanent coverings of the mature ovum or seed; for during the progress of formation the chorion and amnios (which are successively produced subsequent to fecundation) are again obliterated by the growth of the contained embryo. Malpighi describes the process of formation in many other seeds to be nearly similar; for his descriptions of which we must refer the reader to his work (Anat. Plantar, p. 71).

In the above descriptions of Malpighi, the several parts seem to be clearly exhibited, except in one important particular, namely, the situation and course of the umbilical cord. In almost every instance he designates the tube, which we have represented as the first form of the amnios, as the umbilical vessel (vasculum umbilicale), which the subsequent appearances it exhibits show to be erroneous. In the descriptions of Grew this deficiency in the representations of Malpighi is supplied. He has particularly observed the formation of the seed in the aprico, which in many respects resembles that of the almond; and we shall subjoin an abridged account of his observations.

In this fruit the pericarp that envelopes the seed is seen, in its mature state, to be composed of the pulpy part a, fig. 28, within which is the osseous envelope b, and at the centre the kernel or true seed c. At an early period both the pulp and stone are observed to consist of cellular tissue; and through the stone the vessels passing from the peduncle are continued. At the base of the figure the letter d denotes one fasciculus of vessels continued through the stone, and turning inward, where it reaches the apex of the seed. These vessels form the umbilical cord or seed-branch of Grew, while the fasciculus e that runs on the opposite side is continued to the flower. In fig. 29 a vertical section of the ovulum, as well as pericarp, is exhibited as it appears at a very early period; in which f denotes the pulp, g the stone through which the umbilical vessels pass and enter the outer tunic h of the ovulum, around which they make a ring. Within this tunic is another, i, filled with cellular tissue; and through its axis a small tube extends, at the apex of which the embryo k is first seen to emerge.
[7:3:100]

In fig. 30 these several parts of the ovulum are exhibited on a larger scale; the letter I denotes the outer tunic that immediately lines the stone; m the inner one, corresponding to the chorion of Malpighi; and n the tube answering to the amnios of the same author. Through the outer tunic Grew represents the umbilical vessels to pass and be continued to the middle tunic, the cavity of which is occupied by large cells that contain a pure lyitiph. At first this tunic is entire, but soon there appears in than a hair, and is dilated at each extremity into an oval cavity that contains a pure lymph. A few days after a soft node is seen to emerge in the upper cavity of this tube. This node (o, fig. 31) is described to possess a conical figure, and to be another tunic filled with very minute cells. It is at first entire, but when about the size of a carraway-seed it becomes a little hollowed near its apex, at which part the vessels enter and terminate in another very small node (fig. 32), which is the first appearance of the embryo of the seed. This embryo, when about one fifth part as big as a cheese-mite, begins to be distinguished by a little fissure, which marks the division of the lobes, as in fig. 33. When the lobes have increased, and are more fully formed, the node contracts at its base (fig. 34), indicating the place of the umbilical cord, which subsequently becomes the radicle of the seed. (Anat. of Plants, p. 209.)

This description corresponds nearly with that of Malpighi, as far as regards the situation and general form of the tunics, and the place in which the embryo is seen first to emerge. It also displays the course of the vessels to form the umbilical cord; but the growth of the embryo in this seed does not seem to produce the obliteration of some of the tunics in the manner delineated by Malpighi.

An example of a different kind is observed in the pear. Its structure has been described by Grew, and more minutely by Du Hamel. In its mature state it consists of a pulpy matter, in the centre of which are five loculaments that contain each two seeds. These appearances arc exhibited in the transverse section, fig. 35; and in the longitudinal section, fig. 36, the seeds are further shown to be attached by a small umbilical cord. The pulp of the fruit is made up of a very fine cellular tissue, filled with the proper substance of the fruit, and is everywhere furnished with vessels. Through this pulpy matter a number of solid particles are met with, which are more particularly accumulated at the top and about the core. They are formed of an assemblage of small particles, of a stony consistence, with which a little knot of vessels (fig. 37) is everywhere connected. In fig. 38 is a thin transverse slice, showing the relative position of these stony particles, as indicated by the knots of vessels with which they are associated. The stony matter is not observed at an early period, but seems to be deposited from the juices in a more mature state.

By long maceration in water the pulpy matter is dissolved, and the vascular system is obtained separate. In the peduncle of the fruit fifteen principal fasciculi of vessels arc contained. Ten of these are distributed to the seeds and flower, and the five others are dispersed through the pulp. This vascular structure is represented in fig. 39 after the removal of the pulpy part; the larger vessels embrace the core, and, after variously ramifying, terminate in the little vascular processes before described as connected with the stony matter of the pulp. In fig. 40 is represented one of the loculaments of the capsule, with the seed in it, receiving vessels from fasciculi continued from the peduncle; and in fig. 41 an entire seed is represented, and also a section of the same, in which the umbilical vessels that enter at the base are. shown, as in other instances, to be continued beneath the tunic to the apex of the seed. (Phys, des Arbres, tome i. p. 242.)

The last variety we shall notice in the formation of the seed is that of wheat (triticum), as given by Malpighi. In fig. 42 is represented the pistil of the flower of this plant, consisting of the ovarium q, the two styles r, and the feathered parts that form the stigmata. Previous to fecundation, the ovarium is found to contain a little vesicle, fig. 43, which is the rudiment of the future ovulum. After fecundation the styles soon fall, and the ovarium acquires a more pointed figure, as in fig. 44. If now it be opened, the little vesicle has changed its appearance, and contains within it another smaller vesicle, u, fig. 45. The ovarium continues to alter its shape, and assumes a more oblong form, ∙fig. 46, and the appearance of styles is now quite obliterated. Gradually the little vesicle is formed into a small plantule, convex anteriorly, fig. 47, but more hollowed within, fig. 48, and which is situated at the base of the ovarium. The two portions thus described in figures 47 and 48 arc the minute germ and cotyledon of this seed, which are represented in their appropriate place in fig. 49, in which the letter x denotes the germ, resting in the concavity of the cotyledon; y the albumen that forms the chief bulk of the seed; and z the ovarium, which in this seed continues permanent, and forms its outer tunic. (Anat. Plantar, p. 73.)

In his description of the progressive changes exhibit- 1 ed by the impregnated ovarium, Gaertner follows Malpighi and Grew, and refers to the figures given by those authors as illustrative of them. The liquor, however, in which the embryo is first seen, he confounds with the sac that contains it, calling it the amnios; but the term amnios designates the membrane or sac only, and not the fluid contained within it. Sometimes, he says, there is no proper sac for this fluid, but it is contained in the chorion, or in an appropriate cellular tissue. (De Fructibus, &c. tom.i. p. 60.) The changes in the ovary during the formation of the seed have likewise been observed by Mr Keith, whose account of them agrees with that of Malpighi and Grew. (System of Physiological Botany, vol. ii. chap. 8.) Very recently, also, the same subject has occupied the attention of several foreign writers, and particularly of Μ. Mirbel, whose observations we shall briefly detail.

According to Μ. Mirbel, little has been done to advance this part of physiology since the days of Grew and Malpighi, till the late observations of Μ. Schmitz and Mr R. Brown. The latter remarked that we ought not to judge of the structure of the ovulum from that of the developed seed. Acting on this idea, Μ. Mirbel applied himself to examine the ovarium, from the moment the ovulum begins to appear, and followed its progressive changes of development in plants of the same species. The ovulum at first he describes as a small pulpy excrescence of a conical shape, attached to the containing cell, and which does not appear to have any proper envelope or aperture of any kind. In plants whose ovaria produce many seeds, the ovula exhibit different degrees of development, according as they are nearer to or more distant from the vascular cords that supply nutriment. Μ. Schmitz, it is said, first discovered in the apex of each ovulum a small orifice or hole, which penetrates the two membranes or sacs, of which, at this early period, the ovulum is composed. To these sacs, which are placed one within the other, Μ. Mir-bel has given the names of primine and secondine. The orifices at their apex, which at first form but one aperture 1 externally, soon become more distinct from the enlargement of the parts within, and gradually a third membrane is seen to protrude through them. No sooner has it become visible in the form of a small pulp, than it begins, [7:3:101] in some species, to dilate interiorly, so as to form a sac ' with very thin sides, which afterwards becomes blended with the secondine: in other species it protrudes to a considerable extent, and in a conical form, through the primine and secondine, which appear to invest it as the cup of the acorn embraces the kernel. In this state it is free at its apex, but is attached at its base to the secondine. To this sac the name of tercine has been given. These three tunics, it is said, appear together as soon as the development of the ovulum begins; the primine is never absent, and the same is probably the case with the tercine, nor has he ever missed the secondine but in a very few instances.

In addition to the three tunics or sacs just described, two others, named the quartine and quintine, successively appear. The former, says Μ. Mirbel, is not rare, but has escaped observation by being confounded with the tercine, from which, however, it differs both in its origin and mode of growth. On its first appearance it forms a cellular lining over the inner surface of the cavity of the ovulum. By degrees it separates, and continues attached only to the summit of that cavity. In this state it forms a close sac, and so continues in the ovula of some plants; while in those of others its cavity becomes filled with cellular tissue, and forms a pulpy mass.

The fifth envelope, or quintine, is always developed either within the tercine or quartine when they remain filled with cellular tissue. It is first seen in the centre of the sac like a fine tube, and is attached at one end to its summit, and at the other to its base. It begins to enlarge at the summit, and the embryo is almost simultaneously visible in it. As the enlargement proceeds downward, the quintine presses at all points on the sac that surrounds it, so as often to push down and occupy the place of the tercine or quartine. From the summit of the ovulum a very fine thread (Ze suspenseur) descends in the quintine, bearing at its extremity a globule, which is the nascent embryo. This quintine, adds Μ. Mirbel, answers to the vesicle called amnios by Malpighi.

t The foregoing observations were made on the ovula of several plants, and in their more important points will be seen to correspond nearly with the descriptions of Malpighi and Grew. These latter authors have not so minutely examined the ovulum and its envelopes antecedent to fecundation; but after that event the chorion of Malpighi may be considered as answering to the tercine or quartine of Mirbel; for in the centre of this chorion the tube e , (fig. 19, Plate XLII.) is seen to extend from the summit to the base, as in the tercine of Mirbel. The subsequent expansion of this tube at its summit (fig. 21), its gradual enlargement downwards, and the appearance of the embryo at its apex as described to occur in the quintine of Mirbel, are all exhibited in the amnios of Malpighi; as is also, in fig. 23, the pressure which this amnios makes on the surrounding sac. In the representations of these appearances given by Grew in figures 28 to 34 inclusive, Plate XLII., a similar series of changes is exhibited; and in what relates to the vascular connection of the ovum with the parent plant, he has anticipated most of the observations of those who have followed him. Thus, in fig. 28, 29, Plate XLII., he exhibits the course the vessels take after quitting the peduncle, and shows that they penetrate the several tunics to reach the point where the embryo, in the form of a small globule (fig. 32), is first seen to emerge; and in fig. 33 and 34 of the same plate he delineates the thread by which it is suspended.

According to Μ. Mirbel, the vessels which form the funiculus or cord enter the ovulum at the point afterwards called hilum in the mature seed, and terminate at a point in the inner tunic or membrane, to which the term chalaza, or internal umbilicus, has been applied. This point or chalaza in the inner tunic corresponds, he says, in the ovula of some plants with that of the hilum in the outer; but sometimes the vessels after their entrance are prolonged in the manner described by Grew, so that the place of the chalaza is opposite to that of the hilum. Wherever the chalaza may be situated, that part, Mirbel adds, is to be considered as the base of the seed. (Recherches sur la Structure de l’ Ovule Végétal, par Μ. de Mirbel Annales des Sciences Naturelles, Juillet 1829.)

We have thus described in a general way the elementary organs that form the several textures of which the vegetable system, in phaenogamqus plants, is composed, and given examples of the various modes and proportions in which they combine for that purpose: we have next examined the structure of the mature seed, traced the changes of form observed in its evolution, and described the construction of the several members and organs that are successively produced: and, lastly, we have displayed the structure of the flower, and the formation and development of the ovum, from which a new seed originates, fitted to undergo and exhibit the same series of changes. The means and conditions required for the accomplishment of these changes, the circumstances in which they act, and the modes of their operation, belong to the department of Physiology, and will form the subjects of future consideration.
(w.)