Vascular System of Young Plants of Medicago sativa

Abstract

The primary vascular root system is triarch or, more rarely tetrarch. Lateral roots arise directly opposite the xylem strands. Triarchy in the root, when present in the lower portion, changes to tetrarchy in the hypocotyl region, although the 4th ray may be an abortive one. Each cotyledonary trace is made up of a polar xylem strand and a part of each lateral xylem strand. Cotyledonary traces, as they pass into the cotyledons, are composed of triad bundles, the median portion being a polar protoxylem strand. The median protoxylem strand of the cotyledonary trace disappears as the triad bundle approaches the cotyledon lamina. The 2 remaining flanking bundles fuse to form the collateral midrib bundle of the cotyledon blade. The primary system of the root, hypocotyl, and cotyledons forms a complete circulatory system of its own, independent of subsequent plumular development. It recapitulates the phases of evolutionary development as described by Chauveaud in his phanerogamic cycle. When the plumular elements develop, slightly later, they are collateral and endarch and are directly continuous with secondary elements formed by a cambium in the region of the hypocotyl. The first plumular vessels to form are those which become traces to the first and second leaves. These leaves arise at alternate nodes above the cotyledonary node. Later, the formation of a complete cylinder of bundles in the stem obscures the priority of the emergence of these first trace bundles. Secondary tissues early form a continuous conducting system through root and shoot. The primary tissues become imbedded in secondary tissues except at the tip of the root where they are laterally supplemented by secondary tissue. The pattern of the vascular system in the stem is comparatively simple, being the result of 3 foliar traces alternating from node to node and of 2 branch traces supplying the branch which forms in the axil of each leaf. 6 stem bundles are principally involved in bud and leaf trace formation. These bundles partially lose their identity at the nodes because of anastomoses of all the bundles on the side of the stem where the leaf is forming.