Foliar and axial aspects of vascular differentiation: hypotheses and evidence

A comparison is made between foliar and axial vas-cular differentiation. Current thoughts and new evi-dence are presented on the role of hormones in con-trolling the differentiation of vascular tissues in or-ganized and tumorous tissues, focusing on the role of auxin and cytokinin in controlling phloem and xylem relationships, vessel size and density, cam-bium sensitivity, vascular adaptation and xylem evolution in deciduous hardwood trees. The possible role of wounding is also considered. A new hypoth-esis, namely, the leaf-venation hypothesis, is proposed to explain the hormonal control of vascular differ-entiation in leaves of dicotyledonous plants. Experi-mental evidence in support of the hypothesis is pre-sented showing that hydathodes, the water-secreting glands, are the primary sites of auxin synthesis during leaf morphogenesis. Vessel element patterns similar to those found in hydathodes were experimentally induced by exogenous auxin appli-cation. Key words: Vascular differentiation; Xylem and phloem development; Venation pattern formation; Leaf morphogenesis; Auxin and cytokinin synthesis; Ethylene and wound response; Cambium sensitiv-ity; Arabidopsis thaliana; Cucumis sativus; Nicotiana tabacu