# On the movements and habits of climbing plants/Part 3

By tendrils I mean filamentary organs, sensitive to contact and used exclusively for climbing. By this definition, spines or hooks and rootlets, all of which are used for climbing, are excluded. True tendrils are formed by the modification of leaves with their petioles, of flower-peduncles, perhaps also of branches and stipules. Mohl, who includes with true tendrils various organs having a similar external appearance, classes them according to their homological nature, as being modified leaves, flower-peduncles, &c. This would be an excellent scheme; but I observe that botanists, who are capable of judging, are by no means unanimous on the nature of certain tendrils. Consequently I will describe tendril-bearing plants by natural families, following Lindley, and this will in most, or in all, cases keep those of the same ​homological nature together; but I shall treat of each family, one after the other, according to convenience. The species to be described belong to ten families, and will be given in the following order:—Bignoniaceæ, Polemoniaceæ, Leguminosæ, Compositæ, Smilaceæ, Fumariaceæ, Cucurbitaceæ, Vitaceæ, Sapindaceæ, Passifloraceæ.
Bignoniaceæ.—This family contains many tendril-bearers, some twiners, and some root-climbers. The tendrils are always modified leaves. Nine species of Bignonia, selected by hazard, are here described, in order to show what diversity of structure and action there may be in species of the same genus, and to show how remarkable the action of the tendrils may be in some cases. The species, taken together, afford connecting links between twiners, leaf-climbers, tendril-bearers, and root-climbers.
Bignonia (an unnamed species from Kew, closely allied to B. unguis, but with smaller and rather broader Bignonia, unnamed species from Kew.leaves).—A young shoot from a cut-down plant made three revolutions against the sun, at an average rate of 2 h. 6 m. The stem is thin and flexible and twined, ascending, from left to right, round a slender vertical stick as perfectly and as regularly as any true twining-plant. When thus ascending, it makes no use of its tendrils or its petioles; but when it twined round a rather thick stick, and its petioles were brought into contact with it, these curved round the stick, showing that they have some degree of irritability. The petioles also exhibit a slight ​degree of spontaneous movement; for in one case they certainly described minute, irregular, vertical ellipses. The tendrils apparently curve themselves spontaneously to the same side with the petioles; but the movement was so slight that it may be passed over. From various causes, it was difficult to observe the movements of the petioles and tendrils in this and the two following species. The tendrils are so closely similar in all respects to those of the following species, that one description will suffice.
Bignonia unguis.—The young shoots revolve, but less regularly and less quickly than those of the last species. The stem twined imperfectly round a vertical stick, sometimes reversing its direction, exactly in the same manner as has been described in so many leaf-climbers; and this plant is in itself a leaf-climber, though possessing tendrils. Each leaf consists of a petiole bearing a pair of leaflets, and terminating in a tendril, which is exactly like that above figured, but a little larger. The whole tendril in a young plant was only about half an inch in length, and is very unlike most tendrils in shape. It curiously resembles the leg and foot of a small bird with the hind toe cut off. The straight leg or tarsus is longer than the three toes, which latter are of equal length, and, diverging, lie in the same plane; the toes terminate in sharp and hard claws, much curved downwards, exactly like the claws on a bird's foot. The whole tendril apparently represents three leaflets. The main petiole (but not the two sub-petioles of the lateral leaflets) is sensitive to contact with any object: even a small loop of thread after two days caused one to bend upwards. The whole tendrils, namely the tarsus and three toes, especially their under surfaces, are likewise sensitive to contact. Hence, when a shoot grows through branched twigs, its revolving movement soon brings the tendril into contact with some twig, and then all three toes bend (or sometimes one alone), and, after several hours, seize fast hold of the twig, exactly like a bird when perched. The tarsus, also, when it comes into contact with a twig, slowly bends, until the foot is carried quite round, and the toes pass on each side of the tarsus, or seize hold of it. If the main petiole bearing the leaflets comes into contact with a twig, it likewise bends round, until the tendril touches its own petiole or that of the opposite leaf, which is then seized. The petioles, and probably even the tendrils in a slight degree, move spontaneously; hence when a shoot attempted to twine round an upright stick, both petioles after a time came into contact with it, and the contact ​caused still further bending; so that ultimately both petioles clasped the stick in opposite directions, and the foot-like tendrils, seizing on each other or on their petioles, fastened the stem to the support with surprising security. Hence this species, differently from the last, uses its tendrils, by the intervention of the spontaneously moving and sensitive petioles, when the stem twines round a thin vertical stick. Both species use their tendrils in the same manner when passing through a thicket. This plant seems to me the most efficient climber which I have examined; and it probably could ascend a polished stem incessantly tossed by heavy storms. To show how important vigorous health is for the action of all the parts, I may mention that when I first examined a plant which was growing pretty well, though not vigorously, I concluded that the tendrils acted only like the hooks on a bramble, and that this was the most feeble and inefficient of all climbers!
Bignonia Tweedyana.—This species is closely allied to, and behaves in all respects like the last; perhaps it twines round a vertical stick rather better. On the same plant, one branch twined in one direction and another in an opposite direction. The internodes in one case made two circles, each in 2 h. 33 m. I was enabled in this species to observe, better than in the two preceding, the spontaneous movements of the petioles: one described three small vertical ellipses in the course of eleven hours, another moved laterally in an irregular spire. Some little time after the stem has twined round an upright stick, and is securely fastened to it by the clasping petioles and tendrils, it emits at the base of its leaves aërial roots, which curve partly round and adhere to the stick; so that this one species of Bignonia combines four different methods of climbing, generally characteristic of distinct plants, namely, twining, leaf-climbing, tendril-climbing, and root-climbing.
In the foregoing three species, when the foot-like tendril has caught any object, it continues to grow and to thicken, and ultimately it becomes wonderfully strong, in the same manner as we have seen with the petioles of leaf-climbers. If the tendril catches nothing, it first slowly bends downwards, and then its power of clasping is lost. Very soon afterwards it disarticulates itself from the petiole, like a leaf in autumn from the stem, and drops off. I have seen this process of disarticulation in no other tendrils, but when uncaught they soon wither away.
Bignonia venusta.—The tendrils are here considerably modified ​in comparison with those of the previous species. The lower part, or tarsus, is four times as long as the three toes; these are of equal length; they do not lie in the same plane, but diverge equally on all sides; their tips are bluntly hooked, so that the whole tendril makes an excellent grapnel. The tarsus is sensitive on all sides; but the three toes are sensitive only on their outer surfaces, which correspond with the under surfaces of the toes in the tendrils of the previous species. The sensitiveness is not much developed; for a slight rubbing with a twig did not cause the tarsus or toes to become slightly curved until an hour had elapsed; subsequently they straightened themselves. Both tarsus and toes can seize well hold of sticks. When the stem is secured, the tendrils are seen spontaneously to sweep large ellipses: the two opposite tendrils move independently of each other. I have no doubt, from the analogy of the two following allied species, that the petioles move spontaneously; but they are not irritable like those of B. unguis and B. Tweedyana. The young internodes also sweep fine large circles, one being completed in 2 h. 15 m., and a second in 2 h. 55 m. By these combined movements of the internodes, petioles, and grapnel-like tendrils, the latter are soon brought into contact with surrounding objects. When a shoot stands near an upright stick, it twines regularly and spirally round it; as it ascends, it seizes the stick with only one of its tendrils, and, if the stick be thin, the right- and left-hand tendrils are alternately used. This alternation follows from the stem necessarily taking one twist round its own axis for each completed spire.
The tendrils a short time after catching any object contract spirally. Those which have caught nothing slowly bend downwards, but do not contract spirally. With many plants the tendrils after a time contract spirally, whether or not they have caught any object. But this whole subject of the spiral contraction of tendrils will be discussed after the several tendril-bearing plants have been described.
Bignonia littoralis.—The young internodes revolve in fine large ellipses. An internode bearing immature tendrils made two revolutions, each in 3 h. 50 m.; but when grown older, with the tendrils mature, two ellipses were performed, each at the rate of 2 h. 44 m. But this species, unlike the preceding, is incapable of spirally twining round any object: this did not appear due to any want of flexibility in the internodes, or to the action of the tendrils, and certainly not to any want of the revolving power; nor can I ​account for the circumstance. Nevertheless the plant readily ascends a thin upright stick by its two opposite tendrils, both seizing the stick some way above, and afterwards spirally contracting. If the tendrils seize nothing, they do not contract spirally. Bignonia venusta ascended a vertical stick by spirally twining and by seizing it alternately with its two tendrils like a sailor pulling himself up a rope hand over hand; our present species pulls itself straight up, like a sailor seizing with both hands together the rope above his head.
The tendrils are almost identical in structure with those of the last species. They continue growing for some time, even after clasping an object, and when fully grown, though borne by a young plant, were 9 inches in length. The three divergent toes are shorter relatively to the tarsus than in the former species; they are blunt at their tips and but slightly hooked; they are not quite equal in length, one being rather longer than the others. The outer surfaces of the three toes are highly sensitive; for when lightly rubbed with a twig, they became perceptibly curved in 4 m. and greatly curved in 7 m.; in 7 h. they became straight again and ready to react. The tarsus, for a space of one inch close to the toes, is sensitive, but in a rather less degree than the toes; for after a slight rubbing this part required about twice as long a time to bend. Even the middle part of the tarsus, if acted on soon after the tendril has arrived at maturity, is sensitive to prolonged contact. After the tendrils have grown old, the sensitiveness is confined to the toes, when they will only curl very slowly round a stick. The maturity of the tendril is shown by the divergence of the three toes, at which period their outer surfaces first become irritable. The irritability of the tendril has little power of spreading from one part to another: thus, when a stick was caught by the part immediately beneath the three toes, these often remained sticking out, and never clasped the stick.
The tendrils revolve spontaneously. The movement begins before the tendril is converted into a grapnel by the divergence of the toes, and before any part has become sensitive; so that the revolving movement is at this early period quite useless. The movement is at this time slow, two ellipses being completed conjointly in 24 h. 18 m. When the tendril was mature, an ellipse was performed in 6 h.; so that even at this period the movement is much slower than that of the internodes. Large ellipses were swept, both in vertical and horizontal planes, by the tendrils. ​Not only the tendrils, but the petioles bearing them, revolve; these petioles, however, are not in the least sensitive. Thus the young internodes, the petioles, and the tendrils, all at the same time, go on revolving together, but at different rates. Moreover, the movements of the opposite petioles and tendrils are quite independent of each other. Hence, when the whole shoot is allowed freely to revolve, nothing can be more intricate than the course and rate followed by the extremity of each tendril. A wide hemisphere above the shoot is irregularly searched for some object to be grasped.
One other curious point remains to be mentioned. Some few days after the toes have closely clasped a stick, their blunt extremities become, though not invariably, developed into irregular disk-like balls, which have the singular power of adhering firmly to the wood. As similar cellular outgrowths will be fully described under B. capreolata, I will here say nothing more about them.
Bignonia æquinoctialis, var. Chamberlaynii.—The internodes, the elongated non-sensitive petioles, and the tendrils all have the power of revolving. The stem does not twine, but ascends a vertical stick in the same manner as the last species. The tendrils resemble those of the last species, but are shorter; the three toes are more unequal in length, two of them being about one-third shorter, and rather thinner than the third; but they vary in these respects. They terminate in small hard points; and what is important, they do not develope cellular adhesive disks. The reduced size of two of the toes, and their lessened sensitiveness, seem to indicate a tendency to their abortion; and the first-formed tendrils on one of my plants were sometimes quite simple. We are thus naturally led to the three following species with simple undivided tendrils.
Bignonia speciosa.—The young shoots revolve irregularly, making narrow ellipses, or spires or circles, at rates varying from 3 h. 30 m. to 4 h. 40 m.; but the plant shows no tendency to twine. Whilst very young and not requiring any support it does not produce tendrils. The tendrils of a rather young plant were five inches in length; they revolve spontaneously, as do the short and not sensitive petioles. The tendrils, when rubbed, slowly bend to the rubbed side, and subsequently straighten themselves; but they are not highly sensitive. There is something strange in their action: I repeatedly placed upright, thick and thin, rough and smooth sticks and posts, and string suspended vertically, near ​them; but these objects were not well seized. The tendrils, after clasping an upright stick, repeatedly loosed it again; often they would not seize it at all, or their extremities did not coil closely round it. I have observed hundreds of tendrils in Cucurbitaceous, Passifloraceous, and Leguminous plants, and never saw one behave in this manner. When, however, my plant had grown to a height of eight or nine feet, the tendrils acted much better; and one or both regularly seized an adjoining, thin, upright stick, not high up as with the three previous species, but in a nearly horizontal plane; thus the non-twining stem was enabled to ascend the stick.
The simple undivided tendril ends in an almost straight, sharp, uncoloured point. The whole terminal part exhibits one odd habit, which in an animal would be called an instinct; for it continually searches for any little dark hole into which to insert itself. I had two young plants; and, after having observed this habit, I placed near them posts, which either had been bored by beetles, or which had become fissured in drying. The tendrils, by their own movement and by that of the internodes, slowly travelled over the surface of the wood, and when the apex came to a hole or fissure it inserted itself; for this purpose the terminal part, half or quarter of an inch in length, often bent itself at right angles to the basal part. I have watched this process between twenty and thirty times. The same tendril would frequently withdraw from one hole and insert its point into a second one. I have seen a tendril keep its point in one instance for 20 h. and in another instance for 36 h. in a minute hole, and then withdraw it.
Whilst the point of a tendril is thus temporarily inserted, the opposite tendril goes on revolving. The whole length of a tendril often fits itself closely to the surface of the wood with which it is in contact; and I have seen a tendril bend at right angles and place itself in a wide and deep fissure, with the apex again abruptly bent and inserted into a minute lateral hole. After a tendril has clasped a stick, it contracts spirally; if it catches nothing, it does not contract. When it has adapted itself to the inequalities of a thick post, though it has clasped nothing, or when it has inserted its apex into some little fissure, the stimulus suffices to induce spiral contraction; and this contraction always draws the tendril away from the post. So that in every case the above described nicely adapted movements were absolutely useless, excepting once when the tip became jammed in a narrow ​fissure. I fully expected, from the analogy of B. capreolata and B. littoralis, that the tip would have developed itself into an adhesive disk; but I could never detect even a trace of this process. Improbable as the view may be, I am led to suspect that this habit in the tendril of inserting its tip into dark holes and crevices has been inherited by the plant after having lost the power of forming adhesive disks.
Bignonia picta.—This species closely resembles the last in the structure and movements of its tendrils. I casually examined a fine growing plant of the allied B. Lindleyi, and this apparently behaves in all respects in the same manner.
Bignonia capreolata.—We now come to a species having tendrils of a different type: but first for the internodes. A young shoot made three large revolutions, following the sun, at an average rate of 2 h. 23 m. The stem is thin and flexible, and I have seen one make four regular spiral turns round a thin upright stick, ascending, of course, from right to left, and therefore in a reversed direction compared with the first-described species; but afterwards, from the interference of the tendrils, it ascended either straight up the stick or in an irregular spire. These tendrils are highly remarkable. In a young plant they were about 2½ inches in length, and much branched, the five chief branches apparently representing two pairs of leaflets and a terminal one; each branch is bifid or more commonly trifid toward its extremity, with all the points blunt but distinctly hooked. A tendril when lightly rubbed bends to that side, and subsequently becomes straight again; but a loop of thread weighing ¼th of a grain produced no effect. The terminal branches of a tendril twice became in 10 m. slightly curved when touching a stick; and in 30 m. the tips curled quite round the stick: the basal part is less sensitive. The tendrils revolve in an apparently capricious manner, sometimes not at all, or very slightly, but at other times they describe large regular ellipses. I could detect no spontaneous movement in the petioles.
At the same time that the tendrils are revolving more or less regularly, another remarkable movement first begins; the tendrils slowly begin to bend from the light towards the darkest side of the house. I repeatedly changed the position of my plants, and the successively formed tendrils always ended by pointing, some little time after the revolving movement had quite ceased, to the darkest side. But when I placed a thick post near a tendril, and between it and the light, the tendril pointed in that direction. In two instances a pair of leaves stood so that one tendril was directed ​towards the light and the other to the darkest side of the house; the latter did not move, but the opposite one bent itself first upwards and then right over its fellow, so that the two became parallel, one above the other, both pointing to the dark: I then turned the plant half round; and the tendril which had turned over recovered its original position, and the opposite one, which had not moved before, now turned right over to the dark side. Lastly, on another plant, three pairs of tendrils were produced by three shoots at the same time, and all happened to be differently directed: I placed the pot in a box open only on one side, and obliquely facing the light; in two days all six tendrils pointed with unerring truth to the darkest corner of the box, though to do this each had to bend in a different manner. Six tattered flags could not have pointed more truly from the wind than did these branched tendrils from the stream of light which entered the box. I left these tendrils undisturbed for above 24 h., and then turned the pot half round; but they had now lost the power of movement, so that they could not any longer avoid the light.
When a tendril has not succeeded, either through its own revolving movement or that of the shoot, or by turning towards any object which intercepts the light, in clasping a support, it bends vertically downwards and then towards its own stem, which it seizes together with the supporting stick, if there be one. A little aid is thus given in keeping the stem secure. If the tendril seizes nothing, it does not contract spirally, but soon withers away and drops off. If it does seize an object, all its branches contract spirally.
I have stated that, after a tendril has come into contact with a stick, in about half an hour it bends round it; but I repeatedly observed, as with B. speciosa and its allies, that it again loosed the stick: sometimes it seized and loosed the same stick three or four times. Knowing that the tendrils avoided the light, I gave them a glass tube blackened within, and a well-blackened zinc plate: the branches curled round the tube and abruptly bent themselves round the edges of the zinc plate; but they soon recoiled, with what I can only call disgust, from these objects, and straightened themselves. I then placed close to a pair of tendrils a post with extremely rugged bark; twice the tendrils touched it for an hour or two, and twice they withdrew; at last one of the hooked extremities curled round and firmly seized an excessively minute projecting point of bark, and then the other branches spread ​themselves out, following with accuracy every inequality of the surface. I then placed a post without bark, but much fissured, and the points of the tendrils crawled into all the crevices in a beautiful manner. To my surprise, I observed that the tips of immature tendrils, with the branches not yet fully separated, likewise crawled, just like roots, into the minutest crevices. In two or three days after the tips had thus crawled into the crevices, or after their hooked ends had seized some minute point, the final process, now to be described, commenced.
This process I discovered by having accidentally left a piece of wool near a tendril. I then bound a quantity of flax, moss, and wool (the wool must not be dyed, for these tendrils are excessively sensitive to some poisons) loosely round sticks, and placed them near tendrils. The hooked points soon caught the fibres, even loosely floating fibres, and now there was no recoiling; on the contrary, the excitement from the fibres caused the hooks to penetrate the fibrous matter and to curl inwards, so that each hook firmly caught one or two fibres, or a small bundle of them. The tips and the inner surfaces of the hooks now began to swell, and in two or three days could be seen to be visibly enlarged. After a few more days the hooks were converted into whitish, irregular balls, rather above the $^1$⁄$_{20}$th of an inch in diameter, and formed of coarse cellular tissue, which sometimes wholly enveloped and concealed the hooks themselves. The surfaces of these balls secrete some viscid resinous matter, to which the fibres of the flax, &c. adhere. When a fibre has become fastened to the surface, the cellular tissue does not grow directly beneath it, but continues to grow closely on each side; so that when several adjoining fibres, though excessively thin, were caught, so many crests of cellular matter, each not as thick as a human hair, grew up between them, and these, arching over on both sides, grew firmly together. As the whole surface of the ball continues to grow, fresh fibres adhere and are enveloped; so that I have seen a little ball with between fifty and sixty fibres of flax crossing at various angles, all imbedded more or less deeply. Every gradation in the process could be seen—some fibres merely sticking to the surface, others lying in more or less deep furrows, or deeply imbedded, or passing through the very centre of the cellular ball. The imbedded fibres are so closely clasped that they cannot he withdrawn. The cellular outgrowth has such a tendency to unite, that two balls produced from two branches sometimes grow into a single one.
​On one occasion, when a tendril had curled round a small stick, half an inch in diameter, an adhesive disk was formed; but generally the tendrils can do nothing with smooth sticks or posts. If, however, the tip of any one branch can curl round the minutest projecting point, the other branches will form disks, especially if they can find crevices to crawl into. The tendril quite fails to attach itself to a brick wall.
I infer that the disks or balls secrete some resinous adhesive matter, from the adherence of the fibres to them, but more especially from such fibres becoming loose after immersion in sulphuric ether, which likewise removes small, brown, glistening points that can generally be seen on the surface of the older disks. If the hooked extremities of the tendrils touch nothing, the cellular outgrowth, as far as I have seen, never commences; but temporary contact during a moderate time causes small disks to be formed. I have seen eight disks developed on one tendril. After the development of the disks, the tendrils, which now become spirally contracted, likewise become woody and very strong. A tendril in this state supported nearly seven ounces, and would apparently have supported a considerably greater weight had not the fibres of flax to which the disks were attached yielded.
From the facts above given, I infer that though the tendrils of this Bignonia can occasionally adhere to smooth cylindrical sticks and often to rugged bark, yet that they are specially adapted to climb trees clothed with lichens, mosses, or with Polypodium incanum, which I hear from Professor Asa Gray is the case with the forest-trees where this Bignonia grows. Finally, it is a highly remarkable fact that a leaf should become metamorphosed into a branched organ which turns from the light, and which can by its extremities either crawl like roots into crevices, or seize hold of minute projecting points, these extremities subsequently forming cellular masses which envelope by their growth the finest fibres and secrete an adhesive cement.
Eccremocarpus scaber (Bignoniaceæ).—Plants in the greenhouse, though growing pretty well, showed no spontaneous movements in their shoots or tendrils; but, removed to the hot-house, the young internodes revolved at rates varying from 3 h. 15 m. to 1 h. 13 m.: at this latter unusually quick rate one large circle was swept; but generally the circles or ellipses were small, and sometimes the course pursued was extremely irregular. An internode which had made several revolutions would sometimes stand ​quite still for 12 h. or 18 h., and then recommence revolving; such strongly marked interruptions in the movements I have observed in no other plant.
The leaves bear four leaflets, themselves subdivided, and terminate in a much-branched tendril. The main petiole of the leaf, whilst young, moves spontaneously by curving itself, and follows nearly the same irregular course, and at about the same rate, with the internodes. The movement to and from the stem is naturally the most conspicuous, and I have seen the chord of the curved petiole forming an angle of 59° with the stem, and an hour afterwards an angle of 106°. The two opposite petioles do not move together, and one is sometimes raised so much as to stand close to the stem whilst the other is not far from horizontal. The basal part of the petiole moves less than the distal part. The tendrils, besides being carried by the moving petioles and internodes, themselves move spontaneously, and the opposite tendrils occasionally move in opposite directions. By these several movements of the young internodes, of the petioles, and of the tendrils, all acting together, a wider space is swept for a support.
In young plants, the tendrils are about three inches in length: they bear two lateral and two terminal branches; and each branch bifurcates twice, with the tips forming blunt double hooks, having both points directed to the same side. All the branches are sensitive on all sides; and after being lightly rubbed, or after coming into contact with a stick, they bend in about 10 m. One that became, after a light rub, curved in 10 m., continued bending for between 3 h. and 4h., but subsequently in 8 h. or 9 h. became straight again. Tendrils, which have caught nothing, ultimately contract into an irregular spire, as they do also, only much more quickly, after clasping a support. In both cases the petiole bearing the leaflets, which at first is straight and inclined a little upwards, moves downwards and abruptly bends itself in the middle into a right angle; but this is more plainly seen in E. miniatus than in E. scaber. The action of the tendrils in the Eccremocarpus is in some respects analogous to that of the tendrils of Bignonia capreolata; but the whole tendril does not move from the light, nor do the hooked tips become enlarged into cellular disks. After the tendrils have come into contact with moderately thick cylindrical sticks or with rugged bark, the several branches may be observed slowly to lift themselves up, change their position, and again come into contact with them. The object of these ​movements is that the double hooks at the extremities of the branches, which naturally face in all directions, may be brought into contact with the wood. I have watched a tendril, which had bent itself at right angles abruptly round the sharp corner of a post, neatly bring every single hook into contact with both surfaces. The appearance suggested the belief, that though the whole tendril is not sensitive to light, yet that the tips are so, and that they turn and twist themselves towards any opaque surface. Ultimately the branches arrange and fit themselves very neatly to all the irregularities of the most rugged bark, so that they resemble in their irregular course a river with its branches, as engraved on a map. But when a tendril has thus arranged itself round a rather thick smooth stick, the subsequent spiral contraction generally spoils the neat arrangement, and draws the tendril from its support. So it is, but not in quite so marked a manner, when a tendril has spread itself over the rugged bark of a thick trunk; for in this case the spiral contraction of the opposite branches sometimes draws the opposed hooks firmly to their supports. Hence we may conclude that these tendrils are not perfectly adapted to seize smooth moderately thick sticks or rugged bark. When a thin stick or twig is placed near a tendril, its terminal branches wind quite round it and seize their own lower branches or main stem; and the stick is thus firmly, but not neatly, grasped. The extremities of the branches, close to the little double hooks, have a strong tendency to curl inwards, and are excited to this movement by contact with the thinnest objects. This accounts for the tendrils apparently preferring such objects as excessively thin culms of a grass, or the long flexible bristles of a brush, or the thin rigid leaves of an Asparagus, all which objects they seized in an admirable manner; for the tips of each sub-branch seized one, two, or three of the bristles, for instance, and then the spiral contraction of the several branches brought all these little parcels close together, so that thirty or forty bristles were drawn into a single bundle, and afforded an excellent support.
Polemoniaceæ.—Cobæa scandens.—This is an admirably constructed climber. The terminal portion of the petiole, which forms the tendril, was in one very fine specimen eleven inches in length, with the basal part bearing two pairs of leaflets, only two and a half inches in length. The tendril of the Cobæa revolves more rapidly and vigorously than in any other plant observed by me, with the exception of one Passiflora. It made three fine large, nearly ​circular sweeps, against the sun, each in 1 h. 15 m., and two others in 1 h. 20 m. and 1 h. 23 m. Sometimes it travels in a much inclined position, and sometimes nearly upright. The lower part moves but little, and the basal portion or petiole, which bears the leaflets, not at all; nor do the internodes revolve; so that here we have the tendril alone moving. With most of the species of Bignonia and with Eccremocarpus, the internodes, tendrils, and petioles all revolve. The long, straight, tapering main stem of the tendril of the Cobæa bears alternate branches; and each branch is several times divided, with the finer branches as thin as very thin bristles, extremely flexible, so that they are blown about by a breath of air, yet strong and highly elastic. The extremity of each branch is a little flattened, and terminates in a minute double (but sometimes single) hook, formed of hard, transparent, woody substance, and as sharp as the finest needle. On the eleven-inch tendril I counted ninety-four of these beautifully constructed little hooks. They readily catch soft wood, or gloves, or the skin of the hands. Excepting these hardened hooks, and excepting the basal part of the central stem of the tendril, every part of every branch is highly sensitive on all sides to a slight touch, and bends in a few minutes towards the touched side. By lightly rubbing several branches on different and opposite sides, the whole tendril rapidly assumes an extraordinarily crooked shape: these movements from contact do not interfere with the ordinary revolving movement. The branches, after becoming greatly curved from being touched, straighten themselves at a quicker rate than in almost any other tendril seen by me, namely, in between half an hour and an hour. After the tendril has caught any object, the spiral contraction also begins after an unusually short interval of time, namely, in about twelve hours.
Before the tendril is mature, the terminal branches cohere and the hooks are curled closely inwards: at this period no part is sensitive to a touch; but as soon as all the branches have diverged and the hooks stand out, full sensitiveness is acquired. It is a singular circumstance that the immature tendril, before becoming sensitive, begins to revolve at its full velocity: this movement must be useless as the tendril in this state can catch nothing: it is a rare instance of a want, though only for a short time, of perfect coadaptation in the structure and functions of a climbing-plant. The petiole with the tendril perfectly matured, but with the leaflets still quite small, stands at this period vertically upwards, the young growing shoot or axis being thrown to one side. The ​tendril thus standing vertically up sweeps a circle right above the stem, and is well adapted to catch some object above, and to favour the ascent of the plant. The whole leaf, with its tendril, after a short time, bends downwards to one side, allowing the next succeeding leaf to become vertical, and ultimately it assumes a horizontal position; but, before this has occurred, the tendril, supposing it to have caught nothing, has lost its powers of movement and has spirally contracted into an entangled mass. In accordance with the rapidity of all the movements, their duration is short: in a plant growing vigorously from being placed in a hot-house, a tendril only revolved for about 36 hours, counting from the period when it became sensitive; but during this period it probably made at least 27 revolutions.
When the branches of a revolving tendril strike against a stick, they quickly bend round and clasp it; but the little hooks play an important part, especially if only the extremity of the tendril be caught, in preventing its being dragged by the rapid revolving movement away too quickly for its irritability to act. As soon as a tendril has bent round a smooth stick or a thick rugged post, or has come into contact with planed wood (for it can at least temporarily adhere even to so smooth a surface as this), the same peculiar movements begin in the branchlets as have been described in those of the Bignonia capreolata and the Eccremocarpus, namely, the branchlets lift themselves up and down; those, however, which have their hooks already directed downwards remain in this position and secure the tendril, whilst the others twist about till they arrange themselves in conformity with every irregularity of the surface, and bring their hooks, originally facing in various directions, into contact with the wood. The use of the hooks was shown by giving the tendrils tubes and slips of glass to catch; for these, though temporarily seized, were afterwards invariably lost, either during the arrangement of the branches or when the spiral contraction ensued.
The perfect manner in which the branches arrange themselves, creeping like rootlets over all the inequalities and into any deep crevice, is quite a pretty sight; for it is perhaps more effectually done than by the tendrils of the former species, and is certainly more conspicuous, as the upper surfaces of the main stem and of every branch to the extreme hooks are angular and coloured green, whilst the lower surfaces are rounded and purple. I was led to infer, as in the former cases, that light guided these conforming movements of the branches of the tendrils. I made ​many trials with black and white glass and cards to prove it, but failed from various causes; yet these trials countenanced the belief. The tendril may be looked at as a leaf split into filaments, with the segments facing in all directions; hence, when the revolving movement is arrested, so that the light shines on them steadily in one direction, there is nothing surprising in their upper surfaces turning towards the light: now this may aid, but will not account for, the whole movement; for the segments would in this case move towards the light as well as turn round to it, whereas in truth the segments or branches of the tendrils not only turn their upper surfaces to the light, and their lower surfaces which bear the hooks to any closely adjoining opaque object (that is, to the dark), but they actually curve or bend from the light towards the dark.
When the Cobæa grows in the open air, the wind must aid the extremely flexible tendrils in seizing a support, for I found a mere breath sufficed to cause the extreme branches of a tendril to catch by their hooks twigs which they could not have reached by the revolving movement. It might have been thought that a tendril thus hooked only by its extremity could not have fairly grasped its support. But several times I watched cases like the following, one of which alone I will describe: a tendril caught a thin stick by the hooks of one of its two extreme branches; though thus held by the tip, it continued to try to revolve, bowing itself out to all sides, and thus moving its branches; the other extreme branch soon caught the stick; the first branch then loosed itself, and then, arranging itself afresh, again caught hold. After a time, from the continued movement of the tendril, a third branch became caught by a single extreme hook; no other branches, as things then remained, could possibly have touched the stick; but before long the main stem, towards its extremity, began just perceptibly to contract into an open spire, and thus to shorten itself (dragging the whole shoot towards the stick), and as it continued to try to revolve, a fourth branch was brought into contact. As the spiral contraction travelled down the main stem and down the branches of the tendril, all the lower branches, one after another, were brought into contact with the stick, and were wound round it and round their own branches until the whole was tied together in an inextricable knot round the stick. The branches of a tendril, though at first so flexible, after having clasped a support for a time, become rigid and even stronger than they were at first. Thus the plant is secured to its support in a perfect manner.
​Leguminosæ.—Pisum sativum.—The common Pea was the subject of a valuable memoir by Dutrochet, who discovered that both the internodes and tendrils revolved in ellipses. The ellipses are generally very narrow, but sometimes approach to circles: I several times observed that the longer axis slowly changed its direction, which is of importance, as the tendril thus sweeps a much wider circuit. Owing to this change of direction, and likewise to the movement of the stem towards the light, the successive irregular ellipses generally form an irregular spire. I have thought it worth while to annex a tracing of the course pursued by the upper internode (the movement of the tendril being neglected) of a young plant from 8.40 A.M. to 9.15 P.M. The course was traced on a hemispherical glass placed over the plant, and the dots with figures give the hours of observation; each dot was joined by a straight line: no doubt these lines, if the course had been observed at shorter intervals, would have been all curvilinear. The extremity of the petiole, where the young tendril arises, was 2 inches from the glass, so that if a pencil 2 inches long had been in imagination affixed to the petiole, it would have traced the annexed figure on the under side of the glass; but it must be remembered that the figure is here reduced one-half. Neglecting the first great sweep towards the light or window, the end of the petiole swept a space 4 inches across in one direction, and 3 inches in another. As a full-grown tendril is considerably above 2 inches in length, and as the ​tendril itself bends and revolves in harmony with the internode, a considerably wider space than that here specified (and represented one-half reduced) is swept. Dutrochet observed an ellipse completed in 1 h. 20 m.; I saw one completed in 1 h. 30 m. The direction followed is variable, either with or against the sun.
Dutrochet asserts that the petiole of the leaf spontaneously moves, as well as the young internodes and tendrils; but he does not say that he secured the internodes; when this was done, I never detected any movement in the petiole, except to and from the light.
The tendrils, on the other hand, when the internodes and petioles were secured, described irregular spires or regular ellipses, exactly like those made by the internodes. A young tendril, only 1⅛ inch in length, revolved. Dutrochet has shown that when a plant is placed in a room, so that the light enters laterally, the internodes travel much quicker to the light than from it: on the other hand, he asserts that the tendril itself moves from the light towards the dark side of the room. With due deference to this great observer, I think he was mistaken, owing to his not having secured the internodes. I took a young plant with highly sensitive tendrils, and tied the petiole so that the tendril alone could move; it completed a perfect ellipse in 1 h. 30 m.; and I then turned the plant half round, so that the opposite side faced the light, but this made no change in the direction of the succeeding ellipse. The next day I watched a plant similarly secured until the tendril (which was highly sensitive) made an ellipse in a line exactly to and from the light; the movement was so great that the tendril bent itself down at the two ends of its elliptical course into a line a little beneath the horizon, thus travelling more than 180 degrees; but the curvature was fully as great towards the light as towards the dark side of the room. I believe Dutrochet was misled by not having secured the internodes, and by having observed a plant of which the internodes and tendrils, from inequality of age, no longer curved or moved in harmony together.
Dutrochet made no observations on the sensitiveness of the tendrils; these, whilst young and about an inch in length, with the leaflets on the petiole only partially expanded, are highly sensitive; a single light touch with a twig on the inferior or concave surface near the tip caused them quickly to bend, as did occasionally a loop of thread weighing one-seventh of a grain. The upper or convex surface is barely or not at all sensitive. After bending from a touch the tendril straightened itself in ​about two hours, and was ready to act again. As soon as the tendrils begin to grow old their extremities become hooked, and they then appear, with their two or three pairs of branches, an admirable grappling instrument; but this is not really the case, for at this period the tips have generally quite lost their sensitiveness; when hooked on to twigs some were not at all affected, and others required from 18 h. to 24 h. to clasp the twigs. Ultimately the lateral branches of the tendril, but not the middle or main stem, contract spirally.
Lathyrus aphaca.—As the tendril here replaces the whole leaf (except occasionally in very young plants), the leaf itself being replaced in function by the large stipules, it might have been expected that the tendrils would have been highly organized; this, however, is not so. They are moderately long, thin, and unbranched, with their tips slightly curved: they are sensitive whilst young on all sides, but chiefly on the concave side of the extremity. They have no spontaneous revolving power, but are at first inclined upwards at an angle of about 45°, then move into a horizontal position, and ultimately bend downwards. The young internodes, on the other hand, revolve in ellipses, and carry with them the tendrils: two ellipses were completed, each in nearly 5 h.; the longer axes of these two, and of some subsequently formed ellipses, were directed at about an angle of 45° from the line of the axis of the previous ellipse.
Lathyrus grandiflorus.—The plants observed were young, and not growing vigorously, yet sufficiently so, I think, for my observations to be trusted. Here we have the rare case of neither internodes nor tendrils having any spontaneous revolving power. The tendrils in vigorous plants are above 4 inches in length, and are often twice divided into three branches; the tips are curved and are sensitive on the concave side; the lower part of the central stem is hardly at all sensitive. Hence this plant climbs simply by its tendrils being brought, through the growth of the stem, or the more efficient aid of the wind, into contact with surrounding objects, which are then effectually clasped. I may add that the tendrils, or the internodes, or both, of Vicia sativa spontaneously revolve.
Compositæ.—Mutisia clematis.—The enormous family of Compositæ is well known to include very few climbing plants. We have seen in the Table in the first Part that Mikania is a regular twiner, and Mutisia is the only genus, as far as I can learn, which bears tendrils: it is therefore interesting to discover that these tendrils, though rather less metamorphosed from their primordial ​foliar nature than most other tendrils, yet display all the ordinary characteristic movements, both those that are spontaneous and those excited by contact.
The long leaf bears seven or eight alternate leaflets, and terminates in a tendril which, in a plant of considerable size, was 5 inches in length. It consists generally of three branches, which evidently represent in a much elongated condition the petioles and midribs of three leaflets; for the branches of the tendril are exactly like the petioles and midribs of the leaflets, being square on the upper surface, furrowed, and edged with green. Moreover, in the plant whilst quite young, the green edging to the branches of the tendrils sometimes expands into narrow laminæ or blades. Each branch is curved a little downwards, and is slightly hooked at its extremity.
An upper young internode revolved, judging from three revolutions, at an average rate of 1 h. 38 m.; it swept ellipses with the longer axes directed at right angles to each other; the plant, apparently, cannot twine. The petiole which bears the tendril, and the tendril itself, are both in constant movement. But the movement is slower and much less regularly elliptical than that of the internodes; it is, apparently, much affected by the light, for the whole leaf usually sank during the night and rose during the day, moving in a crooked course to the west. The tips of the tendrils are highly sensitive on their lower surfaces: one just touched with a twig became perceptibly curved in 3 m., and another became so in 5 m.; the upper surface is not at all sensitive; the sides are moderately sensitive, so that two branches rubbed on their adjoining sides converged and crossed each other. The petiole of the leaf and the lower part of the tendril, halfway between the upper leaflet and the lowest tendril-branch, are not sensitive. A tendril after curling from a touch became straight again in about 6 h.,and was ready to react; but one that had been so roughly rubbed as to have coiled into a helix was not perfectly straight after 13 h. The tendrils retain their sensibility to an unusual age; for one borne by a leaf, with five or six fully developed leaves above it, was still active. If a tendril catches nothing, the tips of its branches, after a considerable interval of time, spontaneously curl a little inwards; but if the tendril has clasped some object, the whole length contracts spirally.
Smilaceæ.—Smilax aspera, var. maculata.—Aug. St.-Hilaire considers the tendrils which rise in pairs from the petiole as ​modified lateral leaflets; but Mohl (S. 41) ranks them as modified stipules. These tendrils are from 1½ to 1¾ inch in length, are thin, and have slightly curved, pointed extremities. They diverge a little from each other, but stand at first nearly upright. When lightly rubbed on either side, they slowly bend to that side, and subsequently become straight again. The back or convex side of a tendril placed in contact with a stick became just perceptibly curved in 1 h. 20 m., but did not completely surround the stick till 48 h. had elapsed; the concave Smilax aspera.side of another tendril became considerably curved in 2 h., and fairly clasped the stick in 5 h. As the tendrils grow old, they diverge more from each other and slowly bend towards the stem and downwards, so that they project on the opposite side of the stem to that on which they arise; they still retain their sensitiveness, and can clasp a support placed behind the stem. Owing to this movement, the plant can ascend a thin upright stick, clasping it with the tendrils which arise from the leaves placed alternately on opposite sides of the stem. Ultimately the two tendrils belonging to the same petiole, if they do not come into contact with any object, cross each other (as at B in fig. 7) behind the stem and loosely clasp it. This movement of the tendrils towards and round the stem is, to a certain extent, guided by the action of the light; for when the plant stood so that one of the two tendrils in thus slowly moving had to travel towards the light, and the other from the light, the latter always travelled, as I repeatedly observed, more quickly than its fellow. The tendrils do not contract spirally in any case. Their chance of finding a support depends on the growth of the plant, on the wind, and on their own slow backward and downward movement, which is guided, to ​a certain extent, by the movement from the light or towards any dark object; for neither the internodes nor the tendrils have any proper revolving movement. From this latter circumstance, from the slow movements of the tendrils after contact (though their sensitiveness is retained for an unusual length of time), from their simple structure and shortness, this plant shows less perfection in its means of climbing than any other tendril-bearing plant observed by me. Whilst young and only a few inches in height, it does not produce any tendrils; and considering that it grows to only about 8 feet high, that the stem is zigzag, and is furnished, as well as the petioles, with spines, it is surprising that it should be provided with tendrils, comparatively inefficient though they be. The plant might have been left, one would have thought, to climb by the aid of its spines alone, like our brambles. But, then, it belongs to a genus some of the species of which are furnished with much longer tendrils; and we may believe that S. aspera is endowed with these organs solely from being descended from progenitors more highly organized in this respect.
Fumariaceæ.—Corydalis claviculata.—According to Mohl (S. 43), both the leaves and the extremities of the branches are converted into tendrils. In the specimens examined by me all the tendrils were certainly foliar, and it is hardly credible that the same plant should produce tendrils of such widely different homological natures. Nevertheless, from this statement by Mohl, I have ranked this Corydalis amongst tendril-bearers; if classed exclusively by its foliar tendrils, it would be doubtful whether it ought not to have been placed amongst leaf-climbers, with its allies, Fumaria and Adlumia. A large majority of its so-called tendrils still bear leaflets, though excessively reduced in size; some few of them may be properly designated as tendrils, for they are completely destitute of laminæ or blades. Consequently we here behold a plant in an actual state of transition from a leaf-climber to a tendril-bearer. Whilst the plant is young, only the outer leaves, but when full-grown all the leaves, have their extremities more or less perfectly converted into tendrils. I have examined specimens from one locality alone, viz. Hampshire; and it is not improbable that plants growing under different conditions might have their leaves a little more or less changed into true tendrils.
Whilst the plant is quite young, the first-formed leaves are not modified in any way, but those next formed have their terminal leaflets reduced in size, and soon all the leaves assume the ​structure represented in the following diagram. This leaf bore nine leaflets; the lower ones are much subdivided. The terminal portion of the petiole, about 1½ inch in length (above the leaflet (f)), is thinner and more elongated than the lower part, and may Corydalis claviculata. Leaf-tendril, of natural size.be considered as the tendril. The leaflets borne by this part are greatly reduced in size, being, on an average, about the tenth of an inch in length and very narrow; one small leaflet measured one-twelfth of an inch in length and one-seventy-fifth in breadth, so that it was almost microscopically minute. All the reduced leaflets have branching nerves, and terminate in little spines like the fully developed leaflets. Every gradation can be traced, until we come to branchlets (as a and d in the figure) which show no vestige of a lamina or blade. Occasionally all the terminal branchlets of the petiole are in this latter condition, and we then have a true tendril.
The several terminal branches of the petiole bearing the ​much-reduced leaflets (a, b, c, d) are highly sensitive, for a loop of thread weighing only the one-sixteenth of a grain caused them, in under 4 h., to become greatly curved: when the loop was removed, the petioles straightened themselves in about the same time. The petiole (e) was rather less sensitive; and in another specimen, in which the corresponding petiole bore rather larger leaflets, a loop of thread weighing one-eighth of a grain did not cause curvature until 18 h. had elapsed. Loops of thread weighing one-fourth of a grain, left suspended on all the lower petioles (f to l) during several days, produced no effect. Yet the three petioles f, g, and h are not quite insensible, for when left in contact with a stick for a day or two they slowly curled round it. So that the sensibility of the petiole gradually diminishes from the tendril-like extremities to the base. The internodes are not at all sensitive, which makes Mohl's statement that they are sometimes converted into tendrils the more surprising, not to say improbable.
The whole leaf, whilst young and sensitive, stands almost vertically upwards, as we have seen is the case with many tendrils. It is in continual movement, and one that I observed swept large, though irregular, ellipses, sometimes narrow, sometimes broad, with their longer axes directed to different points of the compass, at an average rate of about 2 h. for each revolution. The young internodes also, which bear the revolving leaves, likewise revolve irregularly in ellipses and spires; so that by these combined movements a considerable space is swept for a support. If the terminal and attenuated portion of the petiole fails in seizing any object, it ultimately bends downwards and inwards, and then soon loses all its irritability and power of movement. This bending down is of a very different nature from that which occurs with the extremities of the young leaves in many species of Clematis; for these, when thus bent or hooked, first acquire their full degree of sensitiveness.
Dicentra thalictrifolia.—In this allied plant the metamorphosis of the terminal leaflets has been complete, and they are converted into perfect tendrils. Whilst the plant was young, the tendrils appeared like modified branches, so that a distinguished botanist thought this was their nature; but in a full-grown plant, there can be no doubt, as I am assured by Dr. Hooker, that the tendrils are modified leaves. The tendrils, when of full size, are above 5 inches in length; they bifurcate twice, thrice, or even four times; their extremities are hooked, but blunt. All the branches of the tendrils are sensitive on all sides, but the basal ​portion of the main stem is only slightly sensitive. The terminal branches lightly rubbed with a twig did not curve until from 30 m. to 42 m. had elapsed: they slowly became straight again in between 10 h. and 20 h. A loop of thread weighing one-eighth of a grain plainly caused the thinner branches to curve, as did occasionally a loop weighing one-sixteenth of a grain; but this latter slight weight, though left suspended, was not sufficient to cause a permanent flexure. The whole leaf with its tendril and the young upper internode together revolve vigorously and quickly, though irregularly, and sweep a wide space. The figure traced on a bell-glass was either an irregular spire or a zigzag line. The nearest approach to an ellipse was an elongated figure of 8, with one end a little open; this was completed in 1 h. 53 m. During a period of 6 h. 17 m. another shoot made a complex figure, apparently representing three and a half ellipses. When the lower part of the petiole bearing the leaflets was securely fastened, the tendril itself described similar but much smaller figures.
This species climbs well. The tendrils after clasping a stick become thicker and more rigid; but the blunt hooks do not turn and adapt themselves to the supporting surface, as is the case in so perfect a manner with some of the Bignoniaceæ and the Cobæa. In young plants 2 or 3 feet in height, the tendrils, which are only half the length of those borne by the same plants when grown taller, do not contract spirally after clasping a support, but only become slightly flexuous. Pull-sized tendrils, on the other hand, contract spirally, excepting the thick basal portion. Tendrils which have caught nothing simply bend downwards and inwards, like the extremities of the leaves of the Corydalis claviculata. But in all cases the petiole after a time becomes angularly and abruptly bent like that of the Eccremocarpus.
​Echinocystis lobata.—I made numerous observations on this plant (raised from seed sent me by Prof. Asa Gray), for here I first observed the spontaneous revolving movement of the internodes and of the tendrils; and knowing nothing of the nature of these movements, was infinitely perplexed by the whole case, and by the false appearance of twisting of the axis. My observations may now be greatly condensed. I recorded thirty-five revolutions of the internodes and tendrils; the slowest rate was 2 h., and the average, with no great fluctuations, was 1 h. 40 m. for each revolution. Sometimes I tied the internodes, so that the tendrils alone moved; at other times I cut off the tendrils whilst very young, so that the internodes revolved by themselves; but the rate was not thus affected. The course generally pursued was with the sun, but often in the opposite direction; sometimes the movement during a short time would either stop or be reversed; and this apparently resulted from the interference of the light, shortly after the plant was placed close to a window. In one instance, an old tendril, which had nearly ceased revolving, moved in one direction, whilst the young tendril above moved in the opposite direction. The two uppermost internodes alone revolve; as the internodes grow old, the upper part alone moves. The summit of the upper internode made an ellipse or circle about 3 inches in diameter, whilst the tip of the tendril swept a circle 15 or 16 inches in diameter. During the revolving movement the internodes become successively curved to all points of the compass; and often in one part of their course they were inclined, together with the tendril, at about 45° to the horizon, and in another part stood vertical. There was something in the appearance of the revolving internodes which continually gave the false impression that their movement was due to the weight of the long and spontaneously revolving tendril; but, on suddenly cutting off the tendril with a sharp scissors, the top of the shoot rose very little, and went on revolving: this false appearance is apparently due to the internodes and tendrils all curving and moving harmoniously together.
I repeatedly saw that the revolving tendril, though inclined during the greater part of its course at an angle of about 45° (in one case of only 37°) above the horizon, in one part of its course stiffened and straightened itself from tip to base, and became ​nearly or quite vertical. This occurred both when the supporting internodes were free and when they were tied up; but was perhaps most conspicuous in the latter case, or when the whole shoot happened to stand in an inclined position. The tendril forms a very acute angle with the extremity of the shoot, which projects above the point where the tendril arises; and the stiffening always occurred as the tendril approached, and had to pass in its revolving course, the point of difficulty—that is, the projecting extremity of the shoot. Unless the tendril had the power of thus acting, it would strike against the extremity of the shoot, and be arrested by it. As soon as all three branches of the tendril have begun to stiffen themselves in this remarkable manner, as if by a process of turgescence, and to rise from an inclined into a vertical position, the revolving movement becomes more rapid; and as soon as the tendril has succeeded in passing the extremity of the shoot, its revolving motion, coinciding with that from gravity, often causes it to fall into its previously inclined position so quickly, that the end of the tendril could be distinctly seen travelling like the minute hand of a gigantic clock.
The tendrils are thin, from 7 to 9 inches in length, with a pair of short lateral branches rising not far from the base. The tip is slightly but permanently curved, so as to act to a limited extent as a hook. The concave side of the tip is highly sensitive to a touch, but not so the convex side, as was likewise observed by Mohl (S. 65) with other species of the family. I repeatedly proved this difference by lightly rubbing four or five times the convex side of one tendril, and only once or twice the concave side of another tendril, and the latter alone curled inwards: in a few hours afterwards, when those which had been rubbed on the concave side had recovered themselves, I reversed the process of rubbing, and always with a similar result. After touching the concave side, the tip becomes sensibly curved in one or two minutes; and subsequently, if the touch has been at all rough, it becomes coiled into a helix. But this helix will, after a time, uncoil itself, and be ready to act again. A loop of thin thread only one-sixteenth of a grain in weight caused a temporary flexure in a tendril. One of my plants had two shoots near each other, and the tendrils were repeatedly drawn across each other, but it is a singular fact that they did not once catch each other. It would appear as if the tendrils had become habituated to the contact of other tendrils, for the pressure thus caused would apparently be greater than that caused by a loop of soft thread weighing only the one-sixteenth ​of a grain. So it would appear that the tendrils are habituated to drops of water or to rain; for artificial rain made by violently flirting a wet brush produced not the least effect on them. I repeatedly rubbed rather roughly the lower part of a tendril, but never caused any curvature; yet this part is sensitive to prolonged pressure, for when it came into contact with a stick, it would slowly bend round it.
The revolving movement is not stopped by the extremity curling after having been touched. When one of the lateral branches of a tendril has firmly clasped any object, the middle branch continues to revolve. When a stem is bent down and secured, so that its tendril depends but is left free to move, its previous revolving movement is nearly or quite stopped; but it begins to rise in a vertical plane, and as soon as it has become horizontal the revolving movement recommences. I tried this four times; generally the tendril rose to a horizontal position in an hour or an hour and a half; but in one case, in which the tendril depended at an angle of 45° beneath the horizon, the movement took two hours; in another half-hour the tendril rose to 23° above the horizon and recommenced revolving. This upward vertical movement is independent of the action of light, for it took place twice in the dark, and another time with the light coming in on one side alone. The movement no doubt is guided by opposition to the force of gravity, as in the case of the ascent of the plumules of germinating seeds.
A tendril does not long retain its revolving power; as soon as this ceases, it bends downwards and contracts spirally. But after the revolving movement has ceased the tip still retains for a short time its sensitiveness to contact, but this can be of little service to the plant.
Though the tendril is highly flexible, and though the extremity travels, under favourable circumstances, at about the rate of an inch in two minutes and a quarter, yet its sensitiveness to contact is so great that it hardly ever fails to seize a thin stick placed in its path. The following case surprised me much: I placed a thin, smooth, cylindrical stick (and I repeated the experiment seven times) so far from a tendril, that its extremity could only curl half or three-quarters round the stick; but I always found in the course of a few hours afterwards that the tip had managed to curl twice or even thrice quite round the stick. I at first thought that this was due to rapid growth; but by coloured points and measurements I proved that there was no sensible increase of ​length by growth. When a stick, flat on one side, was similarly placed, the tip of the tendril could not curl beyond the flat surface, but coiled itself into a helix, which, turning to one side, lay flat on the little flat surface of wood. In one instance a portion of tendril three-quarters of an inch in length was thus dragged on to the flat surface by the coiling in of the helix. But the tendril thus acquires a very insecure hold, and generally slips off: in one case alone the helix subsequently uncoiled itself, and the tip then passed round and clasped the stick. The formation of a helix on the flat side of a stick apparently shows us that the continued striving of the tip to curl itself closely inwards gives the force which drags the tendril round a smooth cylindrical stick. In this latter case, whilst the tendril was slowly and quite insensibly crawling onwards, I several times observed through a lens that the whole surface was not in close contact with the stick; and I can understand the onward movement only by supposing that it is slightly vermicular, or that the tip alternately straightens itself a little and then again curls inwards, thus dragging itself onwards by an insensibly slow, alternate movement, which may be compared to that of a strong man suspended by the ends of his fingers to a horizontal pole, who works his fingers onwards until he can grasp the pole with the palm of his hand. However this may be, the fact is certain that a tendril which has caught a round stick by its extreme point can work itself onwards until it has passed twice or even thrice round the stick, and has permanently grasped it.
Hanburya Mexicana.—The young internodes and tendrils of this anomalous member of the family revolve in the same manner and at about the same rate with the Echinocystis. The stem does not twine, but can ascend an upright stick by the aid of its tendrils. The concave tip of the tendril is very sensitive; after rapidly coiling into a loop from a single touch, it straightened itself in 50 m. The tendril, when in full action, stands vertically up, with the young projecting extremity of the shoot thrown a little on one side out of the way; but the tendril bears near its base, on the inner side, a short branch, which projects out at right angles, like a spur, with the terminal half bowed a little downwards. Hence, as the main vertical branch of the tendril revolves, the spur, from its position and rigidity, cannot pass over the extremity of the shoot in the same curious manner as do the three branches of the tendril of the Echinocystis by stiffening themselves at the proper point, but is pressed laterally against the young shoot in one part of the revolving course, and in another part is carried only a little ​way from it. Hence the sweep of the lower part of the tendril of the Hanburya is much restricted. Here a nice case of coadaptation comes into play: in all the other tendrils observed by me the several branches become sensitive at the same period; had this been the case with the Hanburya, the rectangular spur-like branch being pressed, during the revolving movement, against the projecting end of the shoot, would infallibly have seized it in a highly injurious manner. But the main tendril, after revolving for a time in a vertical position, spontaneously bends downwards; and this, of course, raises the rectangular branch, which itself also curves upwards; so that by these combined movements the spur-like branch rises above the projecting end of the shoot, and can now move freely without touching it; then, and not until then, it first becomes sensitive.
The tips of both branches, when they come into contact with a stick, grasp it like any ordinary tendril. In a few days afterwards the inferior surface swells and becomes developed into a cellular layer, which adapts itself closely to the wood, and firmly adheres to it. This layer is analogous to the adhesive disks formed by the tips of the tendrils in some species of Bignonia, but in the Hanburya the layer is developed along the terminal portion of the tendril, sometimes for a length of 1¾ inch, but not at the extreme tip. The layer is white, whilst the tendril is green, and near the tip it could sometimes be seen to be thicker than the tendril itself; it generally spreads a little beyond the sides of the tendril, and its edge is fringed with free elongated cells, which have enlarged globular or retort-shaped heads. This cellular layer apparently secretes some resinous cement; for its adhesion to the wood was not lessened by immersion for 24 h. in alcohol or water, but was quite loosened by the action during the same period of ether and turpentine. After the tendril has once firmly coiled itself round a stick, it is difficult to imagine of what use the formation of the adhesive cellular layer can be. Owing to the spiral contraction, which ensues after a time, whether or not the tendril has clasped any object, it was never able to remain, excepting in one instance, in contact with a thick post or a nearly flat surface; if it could have become attached to such objects by means of the adhesive cellular layer, this layer would evidently have been of service to the plant. I hear from Dr. Hooker that several other Cucurbitaceous plants have adherent tendrils.
Of other Cucurbitaceæ, I observed in Bryonia dioica, Cucurbita ovifera, and Cucumis sativa, that the tendrils were sensitive and ​revolved; in the latter plant, Dutrochet saw the movement of the tendril reversed; but whether the internodes as well as the tendrils revolve in these several species I did not observe. In Anguria Warscewiczii, however, the internodes, though thick and stiff, do revolve: in this plant the lower surface of the tendril, some time after clasping a stick, produces a coarsely cellular layer or cushion, fitting the wood, like that formed by the tendril of the Hanburya; but it was not in the least adhesive. In Zanonia Indica, which belongs to a different tribe of the family, both the forked tendrils and the internodes revolved, in periods between 2 h. 8 m. and 3 h. 35 m., moving against the sun.
Vitaceæ.—In this family and in the two following, namely, the Sapindaceæ and Passifloraceæ, the tendrils are modified flower-peduncles; so that they are axial in their nature. In this respect they differ from those of all the first described families, but perhaps not from those of the Cucurbitaceæ. The homological nature, however, of a tendril seems to make no difference in its action.
Vitis vinifera.—The tendril is thick and of great size; one from Tendril of the Vine. A. Peduncle of tendril.- a scale at its baseC. Shorter branch.site leaf B. Longer branch, with a scale at its base.D. Petiole of opposite leaf.a vine not growing vigorously out of doors, measured 16 inches in length. It consists of a peduncle (A), bearing two branches ​which diverge equally from it. One of the branches (B) has a scale at its base, and is always, as far as I have seen, longer than the other, and very often bifurcates. The several branches when rubbed become curved, and subsequently straighten themselves. After a tendril has clasped any object by its extremity, it contracts spirally; but this does not occur (Palm, S. 56) when no object has been seized. The tendrils move spontaneously from side to side; and on a very hot day one made two elliptical revolutions at an average rate of 2 h. 15 m. During these movements a coloured line, painted along the convex surface, became first lateral and then concave. The separate branches have independent movements; after a tendril has spontaneously revolved for a time, it bends from the light towards the dark: I do not give this latter statement on my own authority, but on that of Mohl and Dutrochet; Mohl (S. 77) says that in a vine planted against a wall the tendrils point towards it, and in a vineyard generally more or less to the north.
The young internodes spontaneously revolve; but in hardly any other plant have I seen so slight a movement. A shoot faced a window, and I traced its course on the glass during two perfectly calm and hot days; during ten hours on one day it described a spire, representing two and a half ellipses. I likewise placed a bell-glass over a young muscat grape in a hothouse, and it made three or four extremely minute oval revolutions each day: the shoot moved less than half an inch from side to side; and had it not made at least three revolutions during the same day when the sky was uniformly overcast, I should have attributed the motion to the varying action of the light. The extremity of the shoot is more or less bent downwards; but the extremity never reverses its curvature, as so generally occurs with twining plants.
Various authors (Palm, S. 55; Mohl, S. 45; Lindley, &c.) believe that the tendrils of the vine are modified flower-peduncles. I here give a drawing (fig. 10) of the ordinary state of a flower-peduncle in bud: it consists of the "common peduncle" (A); of the "flower-tendril" (B), which is represented as having caught a twig; and of the "sub-peduncle" (C) bearing the flower-buds. The whole peduncle moves spontaneously, like a true tendril, but in a less degree, and especially when the sub-peduncle (C) does not bear many flower-buds. The common peduncle (A) has not the power of clasping a support, nor has the corresponding part in the true tendril. The flower-tendril (B) is always longer than the sub-peduncle (C), and has a scale at its base; it sometimes ​bifurcates, and therefore corresponds in every detail with the longer scale-bearing branch (B, fig. 9) of the true tendril. It is, however, inclined backwards from the sub-peduncle (C), or stands at right angles with it, and is thus adapted to aid in carrying the future bunch of grapes. The flower-tendril (B), when rubbed, curves and subsequently straightens itself; and it can, as shown in the drawing, securely clasp a support. I have seen an object as soft as a young vine-leaf caught by one.
The lower and naked part of the sub-peduncle (C) is likewise Flower of the Vine. A. Common Peduncle.a scale at its baseC. Sub-Peduncle.osite leaf B. Flower-tendril, with a scale at its base.D. Petiole of opposite leaf.slightly sensitive to a rub, and I have seen it distinctly bent round a stick and even partly round a leaf with which it had come into contact. That the sub-peduncle has the same nature as the corresponding branch of the ordinary tendril is well shown when it bears only a few flowers; for in this case it becomes less branched, increases in length, and gains both in sensitiveness and in the power of spontaneous movement. I have twice seen sub-peduncles (C), bearing only from thirty to forty flower-buds, which had ​become considerably elongated and had completely wound round sticks, exactly like true tendrils. The whole length of another sub-peduncle bearing only eleven flower-buds quickly became curved when slightly rubbed; but even this scanty number of flowers rendered the stalk less sensitive than the other branch, that is, the flower-tendril; for the latter after a lighter rub became curved in a greater degree and more quickly than the sub-peduncle with its few flowers. I have seen a sub-peduncle thickly covered with flower-buds, but with one of the higher lateral branchlets bearing from some cause only two buds, and this one branchlet had become much elongated and had spontaneously caught hold of an adjoining twig; in fact, it formed a little tendril. The increase of length in the sub-peduncle (C) with the decreasing number of its flower-buds is a good instance of the law of compensation. Hence it is that the whole ordinary tendril is longer than the whole flower-peduncle; thus, on one and the same plant, the longest flower-peduncle (measured from the base of the common peduncle to the tip of the flower-tendril) was 8½ inches in length, whilst the longest tendril was nearly double this length, namely 16 inches.
The gradation from the ordinary state of the flower-peduncle, as represented in the drawing (fig. 10), to that of the true tendril (fig. 9) is perfect. We have seen that the sub-peduncle (C), whilst still bearing from thirty to forty flower-buds, may become somewhat elongated and partially assume all the characters of the corresponding branch of the true tendril. From this state we can trace every stage till we come to a full-sized common tendril, bearing on the branch which corresponds with the sub-peduncle one single flower-bud! Hence there can be no doubt that the tendril is a modified flower-peduncle.
Another kind of gradation well deserves notice. The flower-tendril (B, fig. 10) sometimes produces a few flower-buds; I found thirteen and twenty-two on two flower-tendrils on a vine growing against my house; in this state they retain their characteristic qualities of sensitiveness and spontaneous movement, but in a somewhat lessened degree. On vines in hothouses, so many flowers are occasionally produced by the flower-tendrils that a double bunch of grapes is the result; and this is technically called by gardeners a "cluster." In this state the whole bunch of flowers presents scarcely any resemblance to a tendril; and, judging from the facts already given, it would probably possess little power of clasping a support, or of spontaneous movement. Such flower-peduncles closely resemble in structure those borne ​by the next genus, Cissus. This genus, as we shall immediately see, produces well-developed tendrils and ordinary bunches of flowers; but there is no gradation between the two states. If the genus Vitis were unknown, the boldest believer in the modification of species would never, I suppose, have surmised that the same individual plant, at the same period of growth, would have yielded every possible gradation between ordinary flower-stalks for the support of the flowers and fruit, and tendrils used exclusively for climbing. But the vine clearly gives us this case; and it seems to me as striking and curious an instance of transition as can well be conceived.
Cissus discolor.—The young shoots show no more movement than can be accounted for by daily variations in the action of the light. The tendrils, however, revolve with much regularity, following the sun, and, in the plants observed by me, swept circles of about 5 inches in diameter. Five circles were completed in the following times:—4 h. 45 m., 4 h. 50 m., 4 h. 45 m., 4 h. 30 m., and 5 h. The same tendril continues revolving during three or four days. The tendrils are from 3½ to 5 inches in length; they are formed of a long foot-stalk, bearing two short branches, which in old plants again bifurcate. The two branches are not of quite equal length; and, as with the vine, the longer one has a scale at its base. The tendril stands vertically upwards; the extremity of the shoot is bent abruptly downwards; and this position is probably of service in keeping it out of the way of the revolving tendril.
The two branches whilst young are highly sensitive; for I found a touch with a pencil so gentle as only just to move the tendril which was borne at the end of a long flexible shoot, sufficed to cause it to become perceptibly curved in four or five minutes; the tendril became straight again in rather above one hour. A loop of soft thread weighing one-seventh of a grain was thrice tried, and caused the tendrils to become curved in 30 or 40 m.: half this weight produced no effect. The long foot-stalk is much less sensitive, for slight rubbing produced no effect; but prolonged contact with a stick caused it to bend. The two terminal branches are sensitive on all sides; if a number of tendrils be just touched on different sides, two branches of the one on their inner sides, two on their outer sides, or both branches on the same side, in about a quarter of an hour they present a curiously different appearance. If a branch be touched at the same time with equal force on opposite sides, both sides are equally stimulated and there is no movement. At the beginning of my work, and before ​examining this plant, I had observed only those tendrils which are sensitive on one side, and these when lightly pressed between the finger and thumb become curved; but on thus pinching many times the tendrils of this Cissus no curvature ensued, and I was at first falsely led to infer that they were not at all sensitive to a touch.
Cissus antarcticus.—The tendrils on a young plant were thick and straight, with the tips a little curved; when the concave surface was rubbed with some force they very slowly became curved, and subsequently became straight again. Hence they are much less sensitive than the tendrils of the last species; but they made two revolutions, following the sun, rather more rapidly, viz. in 3 h. 30 m. and 4 h. The internodes do not revolve.
Ampelopsis hederacea, or Virginian Creeper.—In this plant also the internodes do not move more than apparently can he accounted for by the varying action of the light. The tendrils are from 4 to 5 inches in length; the main stem sends off several lateral branches, which have their tips curved, as may be seen in fig. 11, A. They exhibit no true spontaneous revolving movement, but turn, as was long ago observed by Andrew Knight, from the light to the dark. I have seen several tendrils move through an angle of 180° to the dark side of a case in less than 24 hours; but the movement is sometimes very much slower. The several lateral branches often move independently of each other, and sometimes irregularly, without any apparent cause. These tendrils are less sensitive to a touch than any others observed by me: by gentle but repeated rubbings with a twig, the lateral branches, but not the main stem, became in the course of three or four hours slightly curved; but they seemed to have hardly any power of again straightening themselves. The tendrils of a plant which crawled over a large box-tree clasped several of the branches. But I have repeatedly seen the tendrils come into contact with sticks, and then withdraw from them. When they meet with a flat surface of wood, or a wall (and this is evidently what they are adapted for), they turn all their branches towards it, and, spreading them widely apart, bring their hooked tips laterally into contact with it. In effecting this, the several branches, after touching the surface, often rise up, place themselves in a new position, and again come down into contact with it.
In the course of about two days after a tendril has arranged its branches so as to press on any surface, the curved tips swell, become bright red, and form on their under-sides the well-known ​little disks or cushions, which adhere firmly to the surface. In one case these tips became slightly swollen in 38 h. after coming into contact with a brick; in another case they were considerably swollen in 48 h., and in an additional 24 h. they were firmly attached to a smooth board; and lastly, the tips of a younger tendril not only swelled but became attached to a stuccoed wall in 42 h. These adhesive disks resemble, except in colour and in being larger, those of Bignonia capreolata. When they were developed in contact with a ball of tow, fibres were separately enveloped, but not in so effective a manner as with B. capreolata. Disks are never developed, as far as I have seen, without the stimulus of at least temporary contact with some object. They are generally first formed on one side of the curved tip, the whole of which often becomes so much changed, that a line of green unaltered tissue can be traced only along the concave surface. When, however, a tendril has clasped a cylindrical stick, an irregular rim or disk is formed along the inner surface at some little distance from the curved tip; this was also observed (S. 71) by Mohl. The disks consist of enlarged cells, with smooth projecting hemispherical surfaces, coloured red, and at first gorged with fluid (see section given by Mohl, S. 70), but they ultimately become woody.
As the disks can almost immediately adhere firmly to such smooth surfaces as planed and painted wood, or to the polished leaf of the ivy, this alone would render it probable that some cement is secreted, as has been asserted to be the case (quoted by Mohl, S. 71) by Malpighi. I removed a number of disks formed during the previous year from a stuccoed wall, and placed them in warm water, diluted acetic acid and alcohol during many hours; but the attached grains of silex were not loosened: immersion in sulphuric ether for 24 h. much loosened them; but warmed essential oils (I tried oil of thyme and peppermint) in the course of a few hours completely released every atom of stone. This seems to prove that some resinous cement is secreted; the quantity secreted, however, must be small; for when a plant ascended a thinly whitewashed wall, the disks adhered firmly to the whitewash; but as the cement never penetrated the thin layer, they were easily withdrawn, together with little scales of the whitewash. It must not be supposed that the attachment is by any means exclusively effected by the cement; for the cellular outgrowth completely envelopes every minute and irregular projection, and insinuates itself into every crevice.
A tendril which has not become attached to any body, does not ​contract spirally; and in course of a week or two shrinks into the finest thread, withers and drops off. An attached tendril, on the other hand, contracts spirally, and thus becomes highly elastic; so that when the main foot-stalk is pulled, the strain is equally distributed to all the attached disks. For a few days after the Ampelopsis hederaceaA. Tendril, with the young leaf. B. Tendril, several weeks after its attachent to a wall, with the branches thickened and spirally contracted, and with the extremities developed into disks. The unattached branches have withered and dropped off.attachment of the disks, the tendril remains weak and brittle, but it rapidly increases in thickness and acquires great strength: during the following winter it ceases to live, but remains firmly attached to the stem and to the surface of attachment. In the ​accompanying diagram we may compare the differences of a tendril (B) some weeks after attachment to a wall, with one (A) from the same plant, fully grown but unattached. That the change in the nature of the tissues of the tendril, as well as the act of spiral contraction, is consequent on the formation of the disks, is well shown by any lateral branches which have not become attached; for these in a week or two wither and drop off, in the same manner as does a whole tendril when unattached. The gain in strength and durability in a tendril after its attachment is something wonderful. There are tendrils now adhering to my house which are still strong and have been exposed to the weather in a dead state for fourteen or fifteen years. One single lateral branchlet of a tendril, estimated to be at least ten years old, was still elastic and supported a weight of exactly two pounds. This tendril had five disk-bearing branches of equal thickness and of apparently equal strength; so that this one tendril, after having been exposed during ten years to the weather, would have resisted a strain of ten pounds!
Sapindaceæ.—Cardiospermum halicacabum.—In this family, as in the last, the tendrils are modified flower-peduncles. In our present plant there are no organs exclusively used for climbing like ordinary tendrils; but the two lateral branches of the main flower-peduncle have been converted into a pair of tendrils, corresponding with the single "flower-tendril" of the common vine. The main peduncle is thin, stiff, and from 3 to 4½ inches in length. Near the summit, above two little bracts, it divides into three branches. The middle one divides C. halicacabum. Upper part of the flower-peduncle with its two tendrils.and redivides, and bears the flowers; ultimately it grows half as long again as the two other modified branches. These latter are the tendrils; they are at first thicker and longer than the middle branch, but never become more than an inch in length. They taper to a point and are flattened, with the lower clasping surface destitute of hairs. At first they project straight up; but soon diverging, they spontaneously curl downwards so as to become symmetrically and elegantly hooked, as represented in the diagram. They are now, whilst the flower-buds are still small, ready for action.
The two or three upper young internodes steadily revolve; those on one plant made two circles, against the course of the sun, ​in 3 h. 12 m.; in a second plant the same course was followed, and the two were completed in 3 h. 41 m.; in a third plant the internodes followed the sun, and made two circles in 3 h. 47 m. The average rate of these six revolutions was 1 h. 40 m. The stem shows no tendency to twine spirally round a support; but the allied tendril-hearing genus Paullinia is said (Mohl, S. 4) to be a twiner. By the revolving movement, the flower-peduncles, which stand up above the end of the shoot, are carried round and round; but when the internodes were securely tied, the long and thin peduncles themselves were seen to be in continued and sometimes rapid movement from side to side. They swept a wide space, but only occasionally moved in a moderately regular elliptical course. By these combined movements one of the two short hooked tendrils, sooner or later, catches hold of some twig or branch, and then it curls round and securely grasps it. These tendrils are, however, but slightly sensitive; for by rubbing their under surfaces only a slight movement was slowly produced. I hooked a tendril on to a twig; and in 1 h. 45 m. it had curved considerably inwards; in 2 h. 30 m. it formed a ring; and in from 5 to 6 hours from being first hooked, it closely grasped the stick. A second tendril acted at nearly the same rate; but I observed one that took 24 hours before it curled twice round a thin twig. Tendrils which have caught nothing spontaneously curl, after the interval of several days, closely up into a helix. Those which have curled round some object soon become a little thicker and tougher. The long and thin main peduncle, though spontaneously moving, is not sensitive and never clasps a support. It never contracts spirally. Such contraction would apparently have been of service to the plant in climbing; nevertheless it climbs pretty well without this aid. The seed-capsules, though light, are of enormous size (hence its English name of Balloon-vine), and as two or three are carried on the same peduncle, the tendrils arising close to them may possibly be of service in preventing these balloons from being dashed to pieces by the wind. In the hothouse they served simply for climbing.
The position of the tendrils alone suffices to show their homological nature; but in two instances one of the tendrils produced at its tip a flower; this, however, did not prevent the tendril acting properly and curling round a twig. In a third case the two lateral branches which ought to have existed as tendrils, both produced flowers like the central branch, and had quite lost their tendril-structure.
​I have only seen, but was not enabled carefully to observe, one other climbing Sapindaeeous plant, namely Paullinia. It was not in flower, yet thus it bore fine long forked tendrils, differing from Cardiospermum. So that, in its tendrils, Paullinia apparently bears the same relation to Cardiospermum that Cissus does to Vitis.
Passifloraceæ.—After reading the discussion and facts given by Mohl (S. 47) on the nature of the tendrils in this family, no one can doubt that they are modified flower-peduncles. The tendrils and true flower-peduncles rise close side by side; and my son, Mr. W. E. Darwin, made sketches for me of their earliest state of development in the hybrid P. floribunda. The two organs at first appear as a single papilla which gradually divides; so that I presume the tendril is a modified branch of a single flower-peduncle. My son found one very young tendril surmounted by traces of floral organs, exactly like those on the summit of the true flower-peduncle at the same early age.
Passiflora gracilis.—This well-named, elegant, annual species differs from the other members of the group, observed by me, in the young internodes having the power of revolving. It exceeds all other climbing plants in the rapidity of its movements, and all tendril-bearers in the sensitiveness of its tendrils. The internode which carries the upper active tendril and which likewise carries one or two young immature internodes, made three revolutions, following the sun, at an average rate of 1 h. 4 m.; it then made, the day becoming very hot, three other revolutions at an average rate of between 57 and 58 m.; so that the average rate of all six revolutions was 1 h. 1 m. The apex of the tendril described ellipses, sometimes narrow and long, sometimes broad and long, with their longer axes inclined in slightly different directions. The plant can ascend a thin upright stick by the aid of its tendrils; but the stem is too stiff for it to twine spirally round a stick, even when not interfered with by the tendrils, which had been successively pinched off at an early age.
When the stem was secured, the tendrils were seen to revolve in nearly the same manner and at the same rate as the internodes. The tendrils are very thin, delicate, and straight, with the exception of the tips, which are a little curved; they are from 7 to 9 inches in length. A half-grown tendril was not sensitive; but when nearly full-grown they are extremely sensitive. A single delicate touch on the concave surface of the tip soon caused it to curve, and in two minutes it formed an open helix. A loop of soft thread weighing $^1$⁄$_{32}$nd of a grain (equal to only two ​millegrammes) placed most gently on the tip, thrice plainly caused it to curve; as twice did a bent bit of thin platina wire weighing $^1$⁄$_{50}$th of a grain; but this latter weight, when left suspended, did not suffice to cause permanent curvature. These trials were made under a bell-glass, so that the loops of thread and wire were not agitated by the wind. The movement after a touch is very rapid: I took hold of the lower part of several tendrils and then touched with a thin twig their concave tips, and watched them carefully through a lens; the tips plainly began to bend in the following times—31, 25, 32, 31, 28, 39, 31, and 30 seconds; so that the movement was generally perceptible in half a minute after the touch, but once plainly in 25 seconds. One of the tendrils which thus became bent in 31 seconds had been touched two hours previously and had coiled into a helix; thus in this interval it had straightened itself and had perfectly recovered its sensibility.
I repeated the experiment made on the Echinocystis, and placed several plants of this Passiflora so close together that the tendrils were repeatedly dragged over each other; but no curvature ensued. I likewise repeatedly flirted small drops of water from a brush on many tendrils, and syringed others so violently that the whole tendril was dashed about, but they never became curved. The impact from the drops of water on my hand was felt far more plainly than that from the loops of thread (weighing $^1$⁄$_{32}$nd of a grain) when allowed to fall on it; and these loops, which caused the tendrils to become curved, had been placed most gently on them. Hence it is clear, either that the tendrils are habituated to the touch of other tendrils and to that of drops of rain, or that they are sensitive only to prolonged though excessively slight pressure. To show the difference in the kind of sensitiveness in different plants and likewise to show the force of the syringe used, I may add that the lightest jet from it instantly caused the leaves of a Mimosa to close; whereas the loop of thread weighing $^1$⁄$_{32}$nd of a grain, when rolled into a ball and gently placed on the glands at the bases of the leaflets of the Mimosa, caused no action. Had I space, I could advance much more striking cases in plants both belonging to the same family, of one being excessively sensitive to the lightest pressure if prolonged, but not to a brief impact; and of another plant equally sensitive to impact, but not to slight though prolonged pressure.
Passiflora punctata.—The internodes do not move; but the tendrils regularly revolve. One that was about half-grown and very sensitive made three revolutions, opposed to the course of ​the sun, in 3 h. 5 m., 2 h. 40 m., and 2 h. 50 m.; perhaps it might have travelled more quickly when nearly full-grown. The plant was placed in front of a window, and I ascertained that, as with twining stems so with these tendrils, the light accelerated the movement in one direction and retarded it in the other, the semicircle towards the light being performed in one instance in 15 m., and in a second instance in 20 m. less time than that required by the semicircle towards the dark end of the room. Considering the extreme tenuity of these tendrils, the action of the light on them is remarkable. The tendrils are long, and, as just stated, very thin, with the tip slightly curved or hooked. The concave side is extremely sensitive to a touch—even a single touch causing it to curl inwards; it subsequently straightens itself, and is again ready to act. A loop of soft thread weighing $^1$⁄$_{14}$th of a grain caused the extreme tip to bend; at another time I tried to hang the same little loop on an inclined tendril, but three times it slid off; yet this extraordinarily slight degree of friction sufficed to make the tip curl. The tendril, though so sensitive, does not move very quickly after a touch, no conspicuous change being observable until 5 or 10 m. had elapsed. The convex side of the tip is not sensitive to a touch or to a suspended loop of thread. In one instance I observed a tendril revolving with the convex side of the tip forwards, and on coming into contact with a stick it merely scraped up and past the obstacle and was not able to clasp it; whereas tendrils revolving with the concave side of their tips forward promptly seize any object in their path.
Passiflora quadrangularis.—This is a very distinct species. The tendrils are thick, long, and stiff; they are sensitive to a touch only towards the extremity and on the concave surface. When a stick was so placed that the middle of the tendril came into contact with it, no curvature ensued. In the hothouse a tendril made two revolutions each in 2 h. 22 m.; in my cooler study one was completed in 3 h., and a second in 4 h. The internodes do not revolve; nor do those of the hybrid P. floribunda.
Tacsonia manicata.—Here again the internodes do not revolve. The tendrils are moderately thin and long; one made a narrow ellipse in 5 h. 20 m., and the next day a broad ellipse in 5 h. 7 m. The extremity being lightly rubbed on the concave surface, became just perceptibly curved in 7 m., clearly curved in 10 m., and hooked in 20 m.
We have seen that the tendrils in the last three families, namely ​the Vitaceæ, Sapindaceæ, and Passifloraceæ, are modified flower-peduncles. This is likewise the case, according to De Candolle (as quoted by Mold), with the tendrils of Brunnichia, one of the Polygonaceæ. In two or three species of Modecca, one of the Papayaceæ, the tendrils, as I hear from Prof. Oliver, occasionally bear flowers and fruit; so that at least they are axial in their nature.
Spiral contraction of Tendrils.—This movement, which shortens the tendrils and renders them elastic, commences in half a day or in a day or two after their extremities have caught some object. There is no such movement in any leaf-climber, with the exception of an occasional trace of it in the petioles of Tropæolum tricolorum. On the other hand, it occurs with all tendrils after they have seized some object, with the few following exceptions,—namely Corydalis claviculata, but then this plant might still be called a leaf-climber; Bignonia unguis and its close allies, and the Cardiospermum; though these tendrils are so short that the contraction could hardly take place, and would be quite superfluous; and Smilax aspera, the tendrils of which, though rather short, offer a more marked exception. In the Dicentra, whilst young, the tendrils are short and do not contract spirally, but only become slightly flexuous; the longer tendrils, however, borne by older plants contract spirally. I have seen no other exceptions to the rule that all tendrils, after clasping by their extremities a support, contract spirally. When, however, the tendril of any plant of which the stem happens to be immoveably fixed, catches some fixed object, it does not contract, simply because it cannot; this, however, rarely occurs. In the common Pea only the lateral branches, and not the central stem of the tendril, contract; and with most plants, such as the Vine, Passiflora, Bryony, the basal portion never contracts into a spire.
I have said that in Corydalis claviculata the end of the leaf or the tendril (for this part may be indifferently thus designated) does not contract into a spire. The branchlets, however, of the tendril, after they have wound round thin twigs, become deeply sinuous or zigzag; and this may be the first indication of the process of spiral contraction. Moreover the whole end of the petiole or tendril, if it seizes nothing, ultimately bends abruptly downwards and inwards, showing that its inferior surface contracts; and this may be confidently looked at as the first indication of the power of spiral contraction. For with all true tendrils when they contract spirally, it is the lower surface, as Mohl (S. 52) has remarked, which contracts. If the inferior surface of ​the extremity of a free tendril were to contract quite regularly, it would roll itself up into a flat helix, as occurs with the Cardiospermum; but if it were to contract in the least on one side, or if the basal portion were first to contract (as does occur), the long free extremity could not be rolled up within the basal part, or if the tip were held during the contraction, as when a tendril has caught some object,—in all these cases the inevitable result would be the formation not of a helix, but of a spire, such as free and caught tendrils form in the act of contraction.
Tendrils of many kinds of plants, if they catch nothing, contract after an interval of several days or weeks into a close spire; but in these cases the movement takes place after the tendril has lost its revolving power and has partly or wholly lost its sensibility, and hangs downwards; this, as we shall presently see, is a quite useless movement. The spiral contraction of unattached tendrils is a much slower process than that of attached tendrils: young tendrils which have caught a support and are spirally contracted may be constantly seen on the same stem with much older tendrils, unattached and uncontracted. In the Echinocystis I have seen a tendril with the two lateral branches clasped to twigs and contracted into beautiful spires, whilst the main branch which had caught nothing remained for many days afterwards uncontracted. In this plant I once observed a main branch after it had caught a stick become spirally flexuous in 7 h., and spirally contracted in 18 h. Generally the tendrils of the Echinocystis begin to contract in from 12 h. to 24 h. after catching something; whilst its unattached tendrils do not begin to contract until two or three or even more days have elapsed after the revolving movement has ceased. I will give one other case: a full-grown tendril of Passiflora quadrangularis which had caught a stick began in 8h. to contract, and in 21 h. several spires were formed; a younger tendril, only two-thirds grown, showed the first trace of contraction in two days after clasping a stick, and in two additional days had formed several spires; hence, apparently, the contraction does not begin in a tendril until it is grown to nearly its full length. Another young tendril of about the same age and length as the last did not catch any object; it acquired its full length in four days; in six additional days it first became flexuous, and in two more days had formed one complete spire. This first spire was formed towards the basal end of the tendril, and the contraction steadily but slowly progressed towards the apex; but the whole was not closely wound up until 21 days had ​elapsed from the first observation, that is until 17 days after the tendril was fully grown.
The best proof of the intimate connexion between the spiral contraction of a tendril and the previous act of clasping a support, is afforded by those tendrils which, when caught, invariably contract into a spire, whilst as long as they remain unattached they continue straight, though dependent, and thus wither and drop off. The tendrils of Bignonia, which are modified leaves, thus behave, as do the tendrils of the three genera of Vitaceæ, and these are modified flower-peduncles. The tendrils, however, of Eccremocarpus, which is allied to Bignonia, contract spirally even when they have caught nothing. The uncaught tendrils of the Cardiospermum, and to a certain extent those of the Mutisia, roll themselves up not into a spire, but into a helix.
The spiral contraction which ensues after a tendril has caught a support is of high service to all tendril-bearing plants; hence its almost universal occurrence with plants of widely different orders. When a shoot is inclined and its tendril has caught an object above, the spiral contraction drags up the shoot. When the shoot is upright, the growth of the internodes, subsequently to the tendrils having seized some object above, would slacken the stem were it not for the spiral contraction, which draws up the internodes as they increase in length. Thus there is no waste of growth, and the stretched stem ascends by the shortest course. We have seen in the Cobæa, when a terminal branchlet of the tendril has caught a stick, how well the spiral contraction of its branches successively brings them one after the other into contact with the stick, until the whole tendril has grasped it in an inextricable knot. When a tendril has caught a yielding object, this is sometimes enveloped and still further secured by the spiral folds, as I have seen with Passiflora quadrangularis; but this action is of little importance.
A far more important service rendered by the spiral contraction is that the tendrils are thus made highly elastic. As was previously remarked under Ampelopsis, the strain is thus equally distributed to the several attached branches of a branched tendril; and this must render the whole tendril far stronger, as branch after branch cannot separately break. It is this elasticity which saves both branched and simple tendrils from being torn away during stormy weather. I have more than once gone on purpose during a gale to watch a Bryony growing in an exposed hedge, with its tendrils attached to the surrounding bushes; and as the ​thick or thin branches were tossed to and fro by the wind, the attached tendrils, had they not been excessively elastic, would instantly have been torn off and the plant thrown prostrate. But as it was, the Bryony safely rode out the gale, like a ship with two anchors down, and with a long range of cable ahead to serve as a spring as she surges to the storm.
With respect to the exciting cause of the spiral contraction, little can be said. After reading Prof. Oliver's interesting paper on the hygroscopic contraction of legumes, I allowed a number of different kinds of tendrils to dry slowly, but no spiral contraction ensued; nor did this occur with the tendrils of the Bryony when placed in water, diluted alcohol, and syrup of sugar. We know that the act of clasping a support leads to a change in the nature of their tissues; and we call this a vital action, and so we must call the spiral contraction. The contraction is not related to the spontaneous revolving power, for it occurs in tendrils, such as those of Lathyrus grandiflorus and Ampelopsis hederacea, which do not revolve. It is not necessarily related to the curling of the tips round a support, as we see in the case of the Ampelopsis and Bignonia capreolata, in which the development of the adherent disks suffices to induce the contraction. Yet it certainly seems to stand in some close relation to the curling or clasping movement due to contact with a support; for not only does it soon follow this act, but the spiral contraction generally begins close to the curled extremity, and travels down towards the base, as if the whole tendril tried to imitate the movement of its extremity. If, however, a tendril be very slack, the whole length seems to become almost simultaneously at first flexuous and then spiral. The spiral contraction of a tendril when unattached cannot serve any of the useful ends just described; it does not occur with many kinds of tendrils which contract when attached; and when it does occur, it supervenes, as we have seen, only after a considerable interval of time. It may almost be likened to certain instinctive or habitual movements performed by animals under circumstances rendering them manifestly useless.
When an uncaught tendril contracts spirally, the spire always runs in the same direction from tip to base. A tendril, on the other hand, which has caught a support by its extremity, invariably becomes twisted in one part in one direction, and in another part in the opposite direction; the oppositely turned spires being ​separated by short straight portions. This curious and symmetrical structure has been noticed by several botanists, but has not been A caught tendril of Bryonia dioica, spirally contracted in reversed directions.explained. It occurs without exception with all tendrils which after catching any object contract spirally, but is of course most conspicuous in the longer tendrils; it never occurs with uncaught tendrils; and when this appears to have happened, it will be found that the tendril had originally seized some object and had afterwards been torn free. Commonly all the spires at one end of a caught tendril run in one direction, and all those at the other end in the opposite direction, with a single short straight portion in the middle; but I have seen a tendril with the spires alternately turning five times in opposite directions, with straight portions between them; and M. Léon has seen seven or eight such alternations. Whether the spires turn several limes in opposite directions, or only once, there are as many turns in the one direction as in the other. For instance, I gathered ten long and short caught tendrils of the Bryony, the longest with 33, and the shortest with only 8 spiral turns; and the number of turns in one direction was in every case the same (within one) as in the opposite direction.
The explanation of this curious little fact is not difficult; I will not attempt any geometrical reasoning, but will give only practical illustrations. In doing this, I shall first have to allude to a point which was almost passed over when treating of Twining-plants. If we hold in our left hand a bundle of parallel strings, we can with our right hand turn these round and round, and imitate the revolving movement of a twining plant, and the strings do not become twisted. But if we now at the same time hold a stick in our left hand, in such a position that the strings become ​spirally turned round it, they will inevitably become twisted. Hence a straight coloured line, painted along the internodes of a twining plant before it has wound round a support, becomes twisted or spiral after it has so wound round. I painted a red line on the straight internodes of a Humulus, Mikania, Ceropegia, Convolvulus, and Phaseolus, and saw it become twisted as the plant wound round a stick. It is possible that the stems of some plants by spontaneously turning on their own axes, at the proper rate and in the proper direction, might avoid becoming twisted; but I have seen no such case.
In the above illustration, the parallel strings were wound round a stick; but this is by no means necessary, for if wound into a hollow coil (as can be done with a narrow slip of elastic paper) there is the same inevitable twisting of the axis. Hence when a tendril, which is free at its end, coils itself into a spire, it must either become twisted along its whole length (and this is a case which I have never seen), or the free extremity must turn round as many times as there are spires formed. It was hardly necessary to observe this fact; but I did so by affixing little paper vanes to the extreme points of the tendrils of the Echinocystis and Passiflora quadrangularis; and as the tendril contracted itself into successive spires, the vane slowly revolved.
We can now understand the meaning of the spires being invariably turned in opposite directions in those tendrils which, having caught some object, are thus fixed at both ends. Let us suppose a caught tendril to make thirty spiral turns in one direction; the inevitable result will be that it will become thirty times twisted on its own axis. This twisting not only would require considerable force, but, as I know by trial, would burst the tendril before the thirty turns were completed. Such a case never really occurs; for, as already stated, when a tendril has caught a support and has spirally contracted, there are always as many turns in one direction as in the other; so that the twisting of the axis in the one direction is exactly compensated by that in the other. We can further see how the tendency is given to make coils in an opposite direction to those, whether turned to the right or to the left, which are first made. Take a piece of string, and let it hang down with the lower end fixed to the floor; then wind the upper end (holding the string quite loosely) spirally round a perpendicular pencil, and this will twist the lower part of the string; after it has been sufficiently twisted, it will be seen to curve itself into an open spire, with the curves running in an opposite ​direction to those round the pencil, and consequently with a straight piece of string between the opposite spires. In short, we have given to the string the regular spiral arrangement of a tendril caught at both ends. The spiral contraction generally begins at the extremity which has clasped a support; and these first-formed spires give a twist to the axis of the tendril, which necessarily inclines the basal part into an opposite spiral curvature. I cannot resist giving one other illustration, though superfluous: when a haberdasher winds up ribbon for a customer, he does not wind it into a single coil; for, if he did, the ribbon would twist itself as many times as there were coils; but he winds it into a figure of eight on his thumb and little finger, so that he alternately takes turns in opposite directions, and thus the ribbon is not twisted. So it is with tendrils, with this sole difference, that they take several consecutive turns in one direction and then the same number in an opposite direction; but in both cases the self-twisting is equally avoided.
Summary on the Nature and Action of Tendrils.—In the concluding remarks I shall have to allude to some points which may be here passed over. In the majority of tendril-bearing genera the young internodes revolve in more or less broad ellipses, like those made by twining plants; but the figures described, when carefully traced, generally form irregular ellipsoidal spires. The rate of revolution in different plants varies from one to five hours, and consequently in some cases is more rapid than with any twining plant, and is never so slow as with those many twiners, which take more than five hours for each revolution. The direction is variable even in the same individual plant. In Passiflora, the internodes of only one of the species have the power of revolving. The Vine is the weakest revolver observed by me, apparently exhibiting only a trace of a former power. In the Eccremocarpus the movement is interrupted by many long pauses. Some, but very few, tendril-bearing plants can spirally twine up an upright stick. Although the twining-power has generally been lost by tendril-bearers, either from the stiffness or shortness of the internodes, from the size of the leaves, or from other unknown causes, the revolving movement well serves to bring the tendrils into contact with surrounding objects.
The tendrils also have the power of revolving in the same manner and generally at the same rate with the internodes. The movement begins whilst the tendril is young, but is at first slow. In Bignonia littoralis even the mature tendrils moved much slower ​than the internodes. In all cases the conditions of life must be favourable for the perfect action of the tendrils. Generally both internodes and tendrils revolve together; in other cases, as in Cissus, Cobæa, and most Passifloræ, the tendrils alone revolve; in other cases, as with Lathyrus aphaca, the internodes alone move, carrying with them the motionless tendrils; and, lastly (and this is the fourth possible case), neither internodes nor tendrils spontaneously revolve, as with Lathyrus grandiflorus and the Ampelopsis. In most Bignonias, iu the Eccremocarpus, Mutisia, and the Fumariaceæ, the petioles as well as the tendrils, together with the internodes, all spontaneously move together.
The tendrils revolve by the curvature of their whole length, excepting the extremity and excepting the base, which parts do not move, or move but little. The movement is of the same nature as that of the revolving internodes. Hence, if a line be painted along that surface which at the time happens to be convex, the line becomes first lateral and then concave, and ultimately again convex. This experiment can be tried only on the thicker tendrils, which are not affected by a thin crust of dried paint. The extremities, however, of the tendrils, which so often are slightly curved or hooked, never reverse their curvature; and in this respect they differ from the extremities of the shoots of twining plants, which not only reverse their curvature, or at least become periodically straight, but curve in a greater degree than the lower portions. But, in fact, the tendril answers to the upper internode of the several revolving internodes of a twining plant; and in the former part of this paper it was explained how the several internodes move together by the whole successively curving to all points of the compass. There is, however, in many cases this unimportant difference, that the curving tendril is separated from the curving internode by a rigid petiole. There is also another difference, namely, that the summit of the shoot, which in itself has no power of revolving, projects above the point from which the tendril arises; but the summit of the shoot is generally thrown on one side, so as to be out of the way of the revolutions swept by the tendril. In those plants in which the terminal shoot is not sufficiently out of the way, the tendril, as we have seen with the Echinocystis, as soon as it comes in its revolving course to this point, stiffens and straightens itself, and, rising up vertically, passes over the obstacle.
All tendrils are sensitive, but in very various degrees, to contact with any object, and curve towards the touched side. With ​several plants a single touch, so slight as only just to move the highly flexible tendril, is enough to induce curvature. Passiflora gracilis has the most sensitive tendrils which I have seen: a hit of platina wire $^1$⁄$_{50}$th of a grain in weight, gently placed on the concave point, caused two tendrils to become hooked, as did (and this perhaps is a better proof of sensitiveness) a loop of soft, thin cotton thread weighing $^1$⁄$_{32}$nd of a grain, or about two milligrammes. With the tendrils of several other plants, loops weighing $^1$⁄$_{16}$th of a grain sufficed. The point of the tendril of the Passiflora gracilis distinctly began to move in 25 seconds after a touch. Asa Gray saw movement in the tendrils of the Cucurbitaceous genus, Sicyos, in 30 seconds. The tendrils of some other plants, when lightly rubbed, move in a few minutes; in the Dicentra in half-an-hour; in the Smilax in an hour and a quarter or a half; and in the Ampelopsis still more slowly. The curling movement consequent on a single touch continues to increase for a considerable time, then ceases; after a few hours the tendril uncurls itself, and is again ready for action. When very light weights were suspended on tendrils of several plants and caused them to curve, these seemed to become accustomed to so slight a stimulus, and straightened themselves, as if the loops had been removed. It makes no difference, as far as I have seen, what sort of object a tendril touches, with the remarkable exception of drops of water in the case of the extremely sensitive tendrils of Passiflora gracilis and of the Echinocystis; hence we are led to infer that they have become habituated to showers of rain. As I made no observations with this view on other tendrils, I cannot say whether there are more cases of this adaptation. Moreover adjoining tendrils rarely catch each other, as we have seen with the Echinocystis and Passiflora, though I have seen this occur with the Bryony.
Tendrils of which the extremities are slightly curved or bowed are sensitive only on the concave surface; other tendrils, such as those of the Cobæa (though furnished with minute horny hooks) and those of Cissus discolor, are sensitive on all sides. Hence the tendril of this latter plant, when stimulated by a touch of equal force on opposite sides, does not bend. In the tendril of the Mutisia the inferior and lateral surfaces are sensitive, but not the upper surface. With branched tendrils, the several branches all act alike; but in the Hanburya the lateral spur-like branch does not acquire (for a reason which has been explained) its sensitiveness nearly so soon as the main branch. The lower or basal part of many tendrils is either not at all sensitive or sensitive only ​to prolonged contact. Hence we see that the sensitiveness of tendrils is a special and localized capacity, quite independent of the power of spontaneously moving; for the curling of the terminal portion from a touch does not in the least interrupt the spontaneous revolving movement of the lower part. In Bignonia unguis and its close allies the petioles of the leaves, as well as the tendrils, are sensitive to a touch.
Twining plants when they come into contact with a stick, curl round it invariably in the direction of their revolving movement; but tendrils curl indifferently to either side, in accordance with the position of the stick and the side which is first touched. The clasping-movement of the extremity apparently is not steady, but vermicular in its nature, as may be inferred from the manner in which the tendrils of the Echinocystis slowly crawled round a smooth stick.
As with a few exceptions tendrils spontaneously revolve, it may be asked, Why are they endowed with sensitiveness?—why, when they come into contact with a stick, do they not, like a twining plant, spirally wind round it? One reason may be that in most cases they are so flexible and thin that, when brought into contact with a stick, they would yield, and their revolving movement would not be arrested; they would thus be dragged onwards and away from the stick. Moreover the sensitive extremities have no revolving power, and could not by this means curl round any object. With twining plants, on the other hand, the extremity of the shoot spontaneously bends more than any other part; and this is of high importance to the ascending power of the plant, as may be seen on a windy day. It is, however, possible that the slow movement of the basal and stiffer parts of certain tendrils, which wind round sticks placed in their course, may be analogous to that of twining plants. I doubt this; but I hardly attended sufficiently to this point, and it would be difficult to distinguish between a movement due to extremely dull sensitiveness and that resulting from the arrestment of the lower part together with the continued movement of the terminal part of a tendril.
Tendrils which are only three-fourths grown, and perhaps even when younger, but not whilst extremely young, have the power of revolving and of grasping any object which they may touch. These two capacities generally commence at about the same period, and fail when the tendril is full grown. But in the Cobæa and Passiflora punctata the tendrils began revolving in a quite useless manner, before they became sensitive. In ​the Echinocystis they retained their sensitiveness for some time after they had ceased revolving and had drooped downwards; in this position, even if they should seize any object, they could be of little or no use in supporting the stem. It is a rare circumstance thus to be able to detect any imperfection or superfluity in tendrils—organs which are so admirably adapted for the functions which they have to perform; but we see that they are not always absolutely perfect, and it would be rash to assume that any existing tendril has reached the utmost limit of perfection.
Some tendrils have their revolving motion accelerated and retarded in moving to and from the light; others, as with the Pea, seem indifferent to its action; others move from the light to the dark, and this aids them in an important manner in finding a support. In Bignonia capreolata the tendrils bend from the light to the dark, like a banner from the wind. In the Cobæa and Eccremocarpus the extremities alone twist and turn about, so as to bring their finer branches and hooks into close contact with any surface, or into dark crevices and holes. This latter movement is one of the best adapted exhibited by tendrils.
A short time after a tendril (with some rare exceptions) has caught a support, it contracts spirally; but the manner of contraction and the several important advantages thus gained have been so lately discussed, that nothing need be here said on the subject. Again, tendrils soon after catching a support grow much stronger and thicker, and sometimes in a wonderful degree durable; and all this shows how much their internal tissues must change. Tendrils which have caught nothing soon shrink and wither; in some species of Bignonia they disarticulate and fall off like leaves in autumn.
Any one who did not closely study tendrils of various kinds would probably infer that their action would always be uniform. This is the case with most kinds of tendrils, of which the extremities simply curl round objects of any moderate degree of thickness, and of various shapes or natures. But Bignonia shows us what diversity of action there may be in the tendrils of even closely allied species. In all the nine species of this genus observed by me the young internodes revolved vigorously; as did the petioles of nearly all, but in very unequal degrees; in three of the species the petioles were sensitive to contact; the tendrils of all are sensitive to contact, and likewise revolve, but in some of the species in a very feeble manner. In the first-described unnamed species, the tendrils, in shape like a bird's foot, are of no service ​when the stem spirally ascends a thin upright stick, but they can seize any twig or branch lying beneath them; but when the stem spirally ascends a somewhat thicker stick, a slight degree of sensitiveness in the petioles is brought into play, and they wind their tendrils round the stick. In B. unguis and B. Tweedyana the sensitiveness, as well as the power of movement, in the petioles is greatly augmented; and the tendrils and petioles are thus inextricably wound together round thin upright sticks; but the stem, in consequence, does not twine so well: B. Tweedyana, in addition, emits aërial roots which adhere to the stick. In B. venusta the tendrils have lost the bird's-foot structure, and are converted into long three-pronged grapnels; these exhibit a conspicuous power of spontaneous movement; the petioles, however, have lost their sensitiveness. The stem can spirally twine round an upright stick, and is aided in its ascent by the tendrils alternately seizing the stick some way above and then spirally contracting. In this and all the following species the tendrils spirally contract after seizing any object. In B. littoralis and B. Chamberlaynii the tendrils, which have the same structure as in B. venusta, and the non-sensitive petioles and the internodes all spontaneously revolve. The stem, however, cannot spirally twine, but ascends an upright stick by both tendrils, seizing it above. In B. littoralis the tips of the tendrils become developed into adhesive disks. In B. speciosa and B. picta we have similar powers of movement, but the plant cannot spirally twine round a stick; it can, however, ascend by clasping it with one or both of its unbranched tendrils, on their own level; and these exhibit the strange, apparently useless, habit of continually inserting their pointed ends into minute crevices and holes. In B. capreolata the stem twines in an imperfect manner; the much-branched tendrils revolve in a capricious manner, and they have the power of bending in a conspicuous manner from the light to the dark; their hooked extremities, even whilst immature, crawl into any crevice, or, when mature, seize any thin projecting point; in both cases they develope adhesive disks, which have the power of enveloping by growth the finest fibres.
In the allied Eccremocarpus the internodes, petioles, and tendrils all spontaneously revolve together; its much-branched tendrils resemble those of Bignonia capreolata, but they do not turn from the light; and their bluntly hooked extremities, which arrange themselves so neatly to any surface, do not form adhesive disks; they act best when each seizes a few thin stems, like the culms of a grass, which they afterwards draw together by their spiral ​contraction into a firm bundle. In the Cobæa the tendrils alone revolve; these are divided into many fine branches, terminating in sharp little hooks, which crawl into crevices, and are turned by an excellently adapted movement to any object that is seized. In the Ampelopsis, on the other hand, there is little or no power of revolving in any part: the branched tendrils are but little sensitive to contact; their hooked extremities cannot seize any thin object; they will not even clasp a stick, unless in extreme need of a support; but they turn from the light to the dark, and, spreading out their branches in contact with any nearly flat surface, the disks are developed. These can adhere, by the secretion of some cement, to a wall, or even to a polished surface; and this is more than the disks of the Bignonia capreolata can effect.
The formation and rapid growth of these adherent disks is one of the most remarkable peculiarities in the structure and functions of tendrils. We have seen that such disks are formed by two species of Bignonia, by the Ampelopsis, and, according to Naudin, by the Cucurbitaceous genus Peponopsis adhærens. Their development, apparently in all cases, depends on the stimulus from contact. It is not a little singular that three families so widely distinct as the Bignoniaceæ, Vitaceæ, and Cucurbitaceæ should all have species bearing tendrils with this same remarkable peculiarity. Most tendrils, after they have clasped any object, rapidly increase in strength and thickness throughout their whole length; but some tendrils, when wound round a support either by the middle or the extremity, become swollen at these points in a remarkable manner; thus I have seen the clasped portion of a tendril of the Bignonia Chamberlaynii grown twice as thick as the free basal portion, and become wonderfully rigid. In the Anguria the lower surface of the tendril, after it has wound round a stick, forms a coarsely cellular layer, which closely fits the wood, but is not adherent; in the Hanburya similar layer is developed, which is adherent; lastly, in the Peponopsis adherent disks are formed at the tips of the tendrils. These three last-named genera belong to the Cucurbitaceæ, so that, in this one family, we have a nearly perfect gradation from a common tendril to one that forms an adherent disk at its tip; the one small step which is wanted is a tendril in a state between that of the Anguria and Hanburya—that is, adherent only in a slight degree or occasionally.
Finally, it may be added that America, which so abounds with arboreal animals, as has lately been insisted on by Mr. Bates, ​likewise, according to Mohl and Palm, abounds with climbing plants; and, of the tendril-bearing plants examined by me, the most admirably constructed come from this grand continent, namely, the several species of Bignonia, Eccremocarpus, Cobæa, and Ampelopsis.
* ↑ As far as I can make out, the history of our knowledge on tendrils is as follows:—We have seen that Palm and Von Mohl observed about the same time the singular phenomenon of the spontaneous revolving movement of twining-plants. Palm (S. 58), I presume, observed likewise the revolving movement of tendrils; but I do not feel sure of this, for he says very little on the subject. Dutrochet fully described this movement of the tendril in the common Pea. Mohl first discovered that tendrils were sensitive to contact; but from some cause, probably from observing too old tendrils, he was not aware how sensitive they were, and thought that prolonged pressure was necessary to excite movement. Professor Asa Gray, in a paper already quoted, first noticed the extreme sensitiveness and rapidity of movements in the tendrils of certain Cucurbitaceous plants.
* ↑ This and the following drawings, from which the woodcuts have been engraved, were carefully made from me from living plants by my son Mr. George H. Darwin.
* ↑ Comptes Rendus, tom. xvii. 1843, p. 989.
* ↑ Leçons de Botanique, &c., 1841, p. 170.
* ↑ I am indebted to Prof. Oliver for information on this head. In the Bulletin de la Société Botanique de France, 1857, there are numerous discussions on the nature of the tendrils in this family.
* ↑ Gardeners' Chronicle, 1864, p. 721. From the affinity of the Cucurbitaceæ to the Passifloraceæ, it might be argued that the tendrils of the former are modified flower-peduncles, as is certainly the case with the tendrils of Passion-flowers. Mr. R. Holland (Hardwicke's 'Science-Gossip,' 1865, p. 105) states that "a cucumber grew, a few years ago, in my own garden, where one of the short prickles upon the fruit had grown out into a long curled tendril."
* ↑ Comptes Rendus, tom. xvii. p. 1005.
* ↑ Trans. Phil. Soc. 1812, p. 314.
* ↑ Trans. Linn. Soc. vol. xxiv. 1864, p. 415.
* ↑ See M. Isid. Léon in Bull. Soc. Bot. de France, tom. v. 1858, p. 680.
* ↑ Annales des Sc. Nat. Bot. 4th series, tom. xii. p. 89.