CHANGES IN INTERCELLULAR RELATIONSHIPS DURING THE GROWTH AND DIFFERENTIATION OF LIVING PLANT TISSUES

THE PROCESSES of growth and differentiation in multicellular plants involve changes not only in individual cells but in intercellular relationships. Young meristematic cells, all much alike in size and character, soon become differentiated into a wide variety of elements. Some remain small, others increase in volume even to several thousandfold. Some become elongated, others flattened, and others modified in various ways. How the changes which take place in one cell are related to those in neighboring cells is evidently an important problem, since it involves the more general question as to the developmental relations of cell and organism. As to how the intercellular readjustments made necessary by differentiation are brought about there is no general agreement. Several possibilities suggest themselves. (1) Cells might slide along the surfaces of their neighbors and thus alter their positions. This commonly occurs in animal development, where cells may migrate for some distance. Ever since the work of Krabbe (1886) and other early students of the problem, a number of botanists have presented evidence that such "sliding growth" occurs in plant tissues, though this has been seriously questioned by others. (2) Some cells might cease division and begin expansion while their neighbors were still dividing, and thus become relatively large and make contacts with an increased number of cells. This method is certainly operative in many cases, though by no means in all. (3) Adjustments might be brought about by differential growth within cells, one portion of a cell exactly keeping pace with a rapidly growing neighbor cell and another portion with a more slowly growing cell, so that different parts of the same cell would thus grow at different rates. Such unequal or localized growth certainly takes place in root hairs, tyloses, and other tissues where one portion of the cell is free, but is more difficult to demonstrate in cells which are surrounded by others. (4) For a brief period when cells are very young and their walls mav still be subject to surface forces, readjustments in length and relative position of the walls may take place which do not involve either sliding or differential growth in the strict sense of these terms. It is often difficult to determine by a study of sections of killed material alone (the method heretofore chiefly employed) which of these possibilities of intercellular readjustment are actually operative. To obtain convincing evidence, it is obviously necessary to make successive observations of a group of cells in their living condition as they grow and differentiate. The purpose of the present paper is to bring 1 Received for publication June 7, 1939. This investigation was carried on by the aid of a grant from the Special Fund for Research of Columbia University. forward such evidence from a study of the developing surface layer of the root in a number of species. METHODS.-One of the authors (Sinnott, 1939) has recently described a technique by which it is possible to study living plant meristems directly. Seeds of certain small-seeded grasses are grown on damp lens paper in moist chambers. The terminal root meristems are sufficiently translucent so that the cells of the outermost layer and the one directly beneath it can be observed under high magnifications. The members of a group of young meristematic cells, and of all their descendents, may be identified in this way from their earliest condition near the extreme tip of the root, through the period of cell division and cell enlargement until their final size is established and differentiation is complete, a process occupying from 24 to 48 hours. By successive camera lucida drawings made at intervals, a complete recor.d may thus be obtained of every cell in an entire cell lineage, both as to its own changes and as to its relations with neighboring cells. Roots of Phleum pratense, Poa trivialis, Sporobolus cryptandrus, and Chloris gayana were the ones chiefly studied. Aside from the fact that it can be observed in the living condition, such material has certain other advantages. The cells are rectangular and in regular longitudinal rows. Cell divisions in the surface layer (the tissue chiefly studied) are all anticlinal and with rare exceptions are at right angles to the long axis of the root. Cell enlargement is entirely in the longitudinal direction. A considerable degree of differentiation occurs, for the cells which are to develop root hairs are in many respects unlike those which do not do so. In Phleum and Poa this differentiation is more pronounced and occurs earlier than in Sporobolus and Chloris (Sinnott and Bloch, 1939). The exact points of origin of new cell walls, the increase in size of individual cells, and any alterations in the relationships between cells, can thus be studied under the simplest of conditions and can be expressed in quantitative terms. RESULTS.-The basic results can be set forth most directly by presenting a series of drawings of the same group of cells at successive intervals from early development to maturity. This has been done for a group of surface cells in two roots of Phleum pratense in figures 1, A-J, and 2, A-D. In the first drawing in each root every cell is given a number, and since a region was always chosen in which one or more distinctively shaped cells were present, the original cells or their descendants can always be identified in later drawings. After division has ceased, all the cells elongate greatly, but they show no change in their transverse dimensions. It will be observed that numerous cell divisions occur and that following the last division the apical daughter cell ordinarily develops a root hair but that the basal one