The vascular tissue of higher plants has been an object of continuous detailed study since the invention of the light microscope. As relevant new physical, chemical and biochemical techniques and concepts have appeared over the years, they have regularly and immediately been applied to this field of enquiry. The reasons are not far to seek. Vascular tissue provides the long-distance transport system for water and nutrients within the plant, and an understanding of the structure and development of the pathways within it is essential for an under standing of growth in all higher plants, including crop plants. Moreover, parts of the vascular tissue - the whole of the xylem and the fibers of the proble- have been, and still are, in high demand commercially for their unique properties and as the only renewable structural material on earth. There are, in addition, more subtle ways in which xylogenesis, particularly as it allows the development of tree species, contributes to the environment. A few years ago, while walking in the foothills of the Pyrenees, I found all this expressed more elegantly than I can express it, in a plea carved in wood at the entrance to a forest: Homme!! Je suis la chaleur de ton foyer par les froids nuits d'hiver, l'ombrage ami lorsque brule Ie soleil d'ete, l'eau des rivieres et des sources. Je suis la charpente de ta maison, la planche de ta table.

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1 Xylem and Phloem Differentiation in Perspective.- 1.1 Introduction.- 1.2 Procambium Initiation in Embryos.- 1.3 General Mechanisms of Programming.- 1.4 Quiescent Center.- 1.5 Quantal Mitosis and Differentiation.- 1.6 Multiple Gene Copies and Xylogenesis.- 1.7 Procambium to Cambium.- 1.8 Maintenance and Extension of the Vascular System.- 2 Hormonal Aspects of Vascular Differentiation.- 2.1 Some Characteristics of Plant Hormones.- 2.2 Auxins.- 2.3 Cytokinins.- 2.4 Gibberellins.- 2.5 Ethylene.- 2.6 Polyamines.- 2.7 Inhibitors.- 2.8 Summary.- 3 Vascular Differentiation Within the Plant.- 3.1 Major Problems.- 3.2 Control of Vascular Differentiation by Auxin Flow.- 3.3 Role of Cytokinin in Vascular Differentiation.- 3.4 Relation Between Phloem and Xylem Differentiation.- 3.5 Graniferous Tracheary Elements.- 3.6 Control of Conduit Size and Density.- 3.7 Sites of Xylem Initiation and Rates of Maturation in Relation to Conduction.- 3.8 Role of Gibberellin and the Control of Fiber Differentiation...- 3.9 Transfer Cells.- 3.10 Vascular Adaptation.- 4 Evidence from Wound Responses and Tissue Cultures.- 4.1 Rationale for Using Wounds and Tissue Cultures to Study Vascular Differentiation.- 4.2 Coleus Stem Wounds and Cultured Internodal Explants.- 4.3 Root and Shoot Wounds of Pisum.- 4.4 Vascular Differentiation in Other Wound Systems.- 4.5 Induced Xylogenesis in Lactuca Pith Parenchyma.- 4.6 Xylogenesis in Explants from Storage Organs.- 4.7 Differentiation in Suspension Cultures Including the Zinnia System.- 4.8 Cell Wall Metabolism During Xylem Differentiation.- 4.9 Some Final Thoughts on Tissue Culture Systems.- 5 Physical Factors, Hormones, and Differentiation.- 5.1 Identification of the Physical Factors Influencing Vascular Differentiation.- 5.2 Temperature.- 5.3 Water.- 5.4 Light.- 5.5 Gases.- 5.6 Mechanical Stress.- 5.7 Acidity.- Epilogue.- References.- Appendix: Abbreviations.