Leaf and Stem Anatomy of Tradescantia fluminensis, Vell.

Abstract

Summary. 1The investigation was started in the hopes of throwing some light upon the problem of sap supply to the upper parts of leaves which grow by a basal meristem, but has been extended as a developmental study of the shoot. 1The scattered bundles of the stem may be classified into four groups–(a) medullary, (b) perimedullary, (c) cortical, (d) peripheral. At the nodes the bundles of the different series are in connection. 1up the second internode as the perimedullary bundles which constitute the leaf-trace. The differentiation of the protoxylem of the primary longitudinal veins of the leaf takes place basifugally, but when this differentiation reaches the leaf-apex, the direction of sap-flow is reversed, with the result that the secondary longitudinal and transverse veins develop basipetally in the leaf, and lead to the basipetal development of the cortical and peripheral stem-bundles. 1The internode grows by an intercalary basal meristem–the disappearance of which may be traced in a basipetal direction by the gradation in cell size and the micro-chemical reactions of the cell-walls in an elongating internode. The peripheral bundles develop in the region of the peripheral meristem, and in the course of development become surrounded by an endodermis, which is later converted into a sclerenchymatous bundle-sheath, continuous with the sclorenchyma sheath running round the stem. 1The leaf primordium consists of an incomplete ring of tissue which later differentiates into sheath and lamna. The differentiation of the leaftissues follows the same general plan as the vascular development, taking place chiefly in a basipetal direction. The effect of etiolation is to produce relatively narrower leaves. 1The following points are discussed as being of importance in the development of the shoot in Tradescantia, and as probably bearing upon the Monocotyledon in general:– (a). The absence of secondary meristems and consequent importance of the portions of the primary meristem, isolated as intcrcalary stem and leaf meristems. (a). The zonal upgrowth of a few leaves from intercalary rings of meristom. (b). The reversal in the direction of sap-flow from the apex of the lamina with its effect upon the irrigation of the basal meristem of leaf and internode, the formation of leaf auricles, and finally in the development of the peripheral stem-bundles as alternative channels of vascular supply to younger parts of the shoot when the older channels are ruptured by clongation of the internode. (c). The zonal distribution of meristem accounts for the continued growth of partly developed leaves under etiolation conditions. No new leaf initials separate from the apex during growth in darkness. 7It is shown that the change in shape of the leaves of Alisma, when grown in darkness, might be anticipated from a considoration of the developmental conditions under which the expanded lamina is formed.