THE ROLE OF SUBSIDIARY TRACE BUNDLES IN STEM AND LEAF DEVELOPMENT OF THE DICOTYLEDONEAE

Abstract

Subsidiary bundles are subdivisions of a leaf trace that do not arise as independent divergencies from a parent trace. They normally originate in the node and develop either basipetally in the stem, acropetally in the leaf, or both. The literature describing different nodal types is examined, and it is noted that multiple bundles exit the stem through a common gap, particularly in many unilacunar and trilacunar species. These extra-trace bundles are considered subsidiaries, and it is suggested that many of them might differentiate basipetally in the stem from a nodal initiation site. Acropetal subsidiary bundles originate by subdivision of trace bundles in the node where they are also reoriented and mixed before passing through the petiole and redistributed as veins in the lamina. Acropetal and basipetal subsidiary bundles must develop in synchrony to maintain functional continuity. Basipetal subsidiary bundles presumably integrate the vasculature of the stem and contribute to vascular redundancy; acropetal bundles perform similar functions in the lamina. The question is raised whether subsidiary bundles might perform a regulatory role in leaf development.