Vascular Anatomy of the Seedling of Melilotus alba

Abstract

The primary root of M. alba is characterized by a triarch, radial protostele. The anatomical transition from the typical root structure to that of the stem begins in the lower hypocotyl. In this region diarchy or tetrarchy replaces the triarch arrangement of the root. If tetrarchy occurs, it is replaced at a higher level in the hypocotyl by a diarch condition. In the upper hypocotyl a conspicuous pith is present and the transition is continued in the formation of cotyledonary traces consisting of the pro-toxylem strands of the axial plane together with associated metaxylem and phloem. Each trace appears as 2 collateral strands of phloem and metaxylem, with several protoxylem elements between the 2 strands, thus approaching a mesarch arrangement of the xylem. In the petiole of the cotyledon, at the base of the blade, the bundle assumes a typical endarch, collateral form. A continuous, primary vascular system thus exists from root, through hypocotyl, into cotyledons. In the upper hypocotyl in the young seedling, six endarch collateral traces differentiate from the meristematic tissue of the stele. The primary xylem of these bundles appears to have no direct connection with the primary exarch strands of lower hypocotyl and root. Three of these traces supply the unifoliate leaf and 3 the 1st trifoliate leaf. In the 3d internode, a typical square stem, characterized by a dictyostele and endarch, collateral bundles ia developed. The similarity between the method of transition from root to stem structure in M. alba and the types described by others for M. arvensis, Medicago, Tri-gonella, Ononis, and Trifolium indicates a close structural and phylogenetic relationship between these spp. in the Trifolieae.