Foliar and axial aspects of vascular differentiation: Hypotheses and evidence

A comparison is made between foliar and axial vascular differentiation. Current thoughts and new evidence are presented on the role of hormones in controlling the differentiation of vascular tissues in organized and tumorous tissues, focusing on the role of auxin and cytokinin in controlling phloem and xylem relationships, vessel size and density, cambium sensitivity, vascular adaptation and xylem evolution in deciduous hardwood trees. The possible role of wounding is also considered. A new hypothesis, namely, the leaf-venation hypothesis, is proposed to explain the hormonal control of vascular differentiation in leaves of dicotyledonous plants. Experimental evidence in support of the hypothesis is presented 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 application.