A critique is provided of the physical and chemical control of primary and secondary xylem development in terms of mechanics, genetics, phylogenetics, and the larger field of plant physiology. Strengths and weaknesses of the phytohormone theory of vascular development are analyzed. Homeobox genes, sub-cellular phytohormone localization, anatomical responses to varied phytohormone ratios and dosages, polar auxin transport, second messengers, radial fluxes in water potential, intercellular signalling, lignin biochemistry, and the phylogenetic position of bryophytes in relation to xylogenesis are identified as some areas for future research. Homeodomain proteins are addressed in terms of cambial initials and cell-fate determination, and other genetic and environmental factors controlling differentiation of diverse cellular phenotypes are reviewed. As a 'continuum hypothesis', it is proposed that the extent of secondary wall sculpturing during tracheary element differentiation is a function of the duration of homeotic gene expression.
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