COMPARATIVE DIFFERENTIATION IN THE AIR ROOTS OF MONSTERA DELICIOSA

THE PARADOX of plant development is the origin of differences, often profound ones, among cells which are presumably identical in their genetic constitution. How this differentiation arises; as cells progress from a relatively uniform embryonic state to their specialized mature condition, is one of the major problems of morphogenesis. No solution for it has yet been found, but a careful study of the various phenomena and of particular instances of cellular differentiation may suggest lines of attack upon it and serve to clarify the issues which are involved. Cells descended from very similar embryonic initials may come to differ in size, form, wall character and physiological behavior. Some differences arise very early, often among the first divisions in embryonic tissue. Thus in axes which grow by means of an apical cell, the successive daughter cells and their derivatives arise in regular sequence and almost from the first differ not only in position but in other characters, so that the origin of the various tissues may be traced in a progressive series of cell lineages. In more homogeneous tissues, the daughter cells from successive divisions may show no visible differences until the last, or almost the last, division. Thus, in the growing surface layer of many roots, the cells which will bear root hairs and those which will not are set apart at the last division, which results in two markedly dissimilar cells. In still other cases, especially in the fundamental tissues, cell differences do not begin to appear until after division has ceased and the cells approach maturity. The purpose of the present paper is to study the development of an organ where all these types of differentiation occur in order to determine, if possible, what factors may be concerned in the process. MATERIAL AND METHODS.-Air roots of the aroid Monstera deliciosa provide good material for such a study since their tissues show a high degree of cellular specialization but consist of cells laid down in regular longitudinal rows. Portions of the root were studied from the tip to a point several centimeters back, where the tissues were mature. Both fast and slowly growing roots, and narrow and wide ones, were examined. Material was killed in CRAF and sectioned transversely and longitudinally. RESULTS.-As in most roots, at the very tip is a complex of embryonic cells from which the regular longitudinal rows of dividing cells, so characteristic of root development, arise. These originate by the early establishment of uniform planes of division which are ultimately transverse to the axis of the root. From the beginning the cell rows near the center of the axis, which will give rise to the stele, 1 Received for publication March 9, 1946. are narrower than the more peripheral ones, which form the cortex. Since there is a group of apical cells instead of a single one, it is not always easy to find the differential divisions which set apart the two tissue systems, but such obviously must occur. Cell division persists longer in certain cell rows than in others. In the outer 'or piliferous layer, cells do not cease division until about 3000-4000 micra back of the root apex. The last divisions are often oblique, resulting in alternation between broadbased and narrow-based cells (fig. 3). The latter appear somewhat more embryonic, contain larger nuclei and are the first to form root hairs (fig. 4), though these hairs are ultimately produced by all surface cells under favorable conditions. Just below the piliferous layer is the hypodermis or exodermis, a single cell layer in which divisions persist to a point about 2500-3500 micra from the apex. All divisions here are equal until the final one which may sometimes occur as close as 2200 micra from the apex and produces two markedly different daughter cells (fig. 3). In each case the basal one (away from the tip) is much smaller than the apical, although the nuclei are approximately equal. Both cells increase considerably in volume before maturity (which is reached about 10 mm. from the apex), but the size ratio between them remains constant (fig. 4). The smaller, possessing a relatively large and more deeply staining nucleus and denser cytoplasm, develops into one of the characteristic "short cells" of the mature hypodermis. The other becomes a much larger cell. In the cortical tissue between hypodermis and stele, cell division ceases much earlier, at about 1400-2800 micra from the root apex. In the final divisions each cell forms two similar ones and one or both daughter cells again divide, so that a longitudinal series or file of three or four cells is formed, which for a time can readily be distinguished from similar adj acent files. All cells now elongate. In many files of the cortex (though not in all) the last division on the basal side (away from the tip) is unequal and produces a smaller basal daughter cell and a larger apical one. These unequal divisions may occur as close as 700 micra from the root apex and continue for some time. The file as a whole thus usually consists of three larger cells with a terminal smaller one at the basal end (fig. 1). Not infrequently the small cell divides into two. This tissue thus resembles the hypodermis in the basally polarized and unequal character of many of its last divisions, which result in two cells, different in size but with approximately equal nuclei. Here, however, the small basal cells develop very differently from similar ones in the hypodermis, for they grow into theremarkable long and branching trichosclereids which form a mechanical system in the