PRIMARY VASCULAR DIFFERENTIATION IN PLANTS

SUMMARY During the last ten years the differentiation of the primary vascular tissues has been intensively studied in plants developing normally and in those that were treated experimentally. The untreated plants were mainly seed plants. Several critical studies of dicotyledons and gymnosperms have shown that the procambium in vegetative shoots is delimited among the derivatives of the apical meristem in continuity with the vascular tissue in the mature part of the axis; that is, it differentiates acropetally. This procambium appears in the form of strands positionally related to leaves, that is, as leaf traces. The differentiation of procambial cells into phloem cells occurs acropetally and generally continuously. The first xylem, on the other hand, matures in or near a leaf and then differentiates basipetally in the axis and acropetally in the leaf. In the roots of seed plants procambium, phloem and xylem differentiate acropetally and continuously. The procambium shows initial unity in the root-hypocotyl-cotyledon system of the embryo and seedling, but the course of differentiation of the xylem and the phloem does not necessarily duplicate the initial course of the procambium. The procambial connexion between the epicotyl and the hypocotyl may also show initial continuity. The establishment of vascular connexion between an axillary or adventitious bud and the parent organ varies in relation to the time of development of buds. Those that develop close to the apical meristem of the parent shoot may show initial vascular continuity with the latter. Others are usually separated by vacuolated parenchyma from the vascular tissue of the parent organ and may initiate their procambium independently. The information on vascular differentiation in vascular cryptogams is meagre. There is evidence, however, that in microphyllous representatives the main part of the vascular system of the shoot differentiates independently of the leaves. The experimental studies that were used to interpret primary vascular organization consisted of removal of young leaf primordia, of partial isolation of apical meristems by vertical incisions, and of division of apices into sectors by radial longitudinal cuts. These studies were carried out on ferns and dicotyledons. The defoliated parts of axes had vascular systems in the form of relatively compact cylinders or even solid cores. The partially isolated apical meristems gave rise to shoots with normal leaf arrangements. The initial vascular connexion of these shoots with the axis below occurred, in the dicotyledons, by a basipetally differentiating procambium. In the ferns such connexion was usually absent. The dissected shoot apices regenerated one or more new shoots. The manner of connexion of these regenerated shoots with the axis was apparently more complex and more varied than in the partially isolated shoots. The deductions drawn from the experimental studies regarding primary vascular organization were, first, that the apical meristem of the shoot induces the development of the primary vascular system; secondly, that this induction occurs independently of the leaves but that leaf development may later affect the form of the differentiating vascular system; and, thirdly, that the vascular system of the stem of megaphyllous plants consists not only of leaf traces but of cauline tissue as well. These deductions are for the most part hypothetical because they are not based on critical histogenetic evidence.