REPLACEMENT OF LEAF PRIMORDIA WITH IAA IN THE INDUCTION OF VASCULAR DIFFERENTIATION IN THE STEM OF <i>COLUCUS</i>

Abstract

S ummary Removal of a leaf of Coleus before or early in the differentiation of leaf traces severely reduced vascular differentiation in the subjacent internode. Auxin (IAA) in anion exchange beads (applied on 2 out of every 3 d for 2 to 4 weeks) restored normal structure with respect to arrangement of leaf traces and their composition, size, and rate of development. That IAA is the likely leaf‐limiting factor for vascular differentiation in situ is suggested by the formation of normal quantities of procambium, phloem and xylem under the influence of IAA after leaf excision. However, some peculiarities were induced as well. Plants treated with IAA showed earlier xylem maturation in side bundles, earlier and more frequent differentiation of xylem opposite putative phloem‐only bundles, and corner‐trace xylem connections at the node of IAA application. These can be accounted for by a lack of well‐defined channelling of IAA through petiolar tissues when IAA was applied to the cut surface after leaf excision. Phloem‐only bundles are regarded as an early auxin‐deficient stage in collateral bundle formation, which depends on auxin availability to the bundle for fulfillment.