XYLOGENESIS, GENETIC AND ENVIRONMENTAL-REGULATION - A REVIEW

A critique is provided of the physical and chemical control of primary and secondary xylem development in terms of mechanics, genetics, phyl ogenetics, and the larger field of plant physiology. Strengths and wea knesses of the phytohormone theory of vascular development are analyze d. Homeobox genes, sub-cellular phytohormone localization, anatomical responses to varied phytohormone ratios and dosages, polar auxin trans port, second messengers, radial fluxes in water potential, intercellul ar signalling, lignin biochemistry, and the phylogenetic position of b ryophytes in relation to xylogenesis are identified as some areas for future research. Homeodomain proteins are addressed in terms of cambia l initials and cell-fate determination, and other genetic and environm ental factors controlling differentiation of diverse cellular phenotyp es are reviewed. As a 'continuum hypothesis', it is proposed that the extent of secondary wall sculpturing during tracheary element differen tiation is a function of the duration of homeotic gene expression.