The main objective of the book is to provide an up-to-date examination of the possible roles of plant hormones during the cytodifferentiation of xylem and phloem elements in higher plants. Various facets of vascular differentiation, as cell determination, cell cycle activity, and the biochemical events in xylogenesis, are analyzed. Furthermore, the latest information on the roles of auxins, cytokinins, gibberellins, ethylene, and abscisic acid during vascular cell formation are summarized. A theoretical discussion of the six-point hypothesis and the vascular adaptation hypothesis is included. The experimental induction of vascular differentiation under tissue culture conditions is critically appraised and a concluding chapter covers the interactions between physical factors, growth regulators, and differentiation.