Vestured Pits in Echium (Boraginaceae): Island Woodiness Revisited

Abstract

Vestured pits are present on pits of secondary xylem vessels of all studied Echium species, roots as well as stems. Variations in vesturing presence do occur in the genus (across wide circular pit cavities; along margins of elliptical pit apertures; aggregated to various degrees; variously abundant), but these are difficult to define precisely and are related primarily to organography and ecology rather than to the taxonomic system. The Macaronesian species have been reported to form a single clade. Wood anatomical features other than vesturing are also closely keyed to species ecology and, in particular subclades, growth forms. Woodiness, exemplified by most species of the insular clade of Echium , has been claimed to have evolved as a mechanism to promote outcrossing on islands. This alleged indirect selection is questioned here. The bases for secondary woodiness on island areas are multiple and are reviewed here (lessened temperature and moisture extremes; retention of branches instead of reproducing by seed as a form of economy; lack of mammalian herbivores; superior dispersal and colonization abilities of less woody clades; ability to occupy geologically new or recently altered areas). These features may, individually and collectively, serve to increase the ability of species to become woody on islands. Vesture presence can be cited for some groups that have radiated well on Macaronesian islands (Boraginaceae, Brassicaceae) as well as on certain major land areas ( Acacia and Eucalyptus on Australia all have vestured pits on vessels). Plant species can be viewed as having an umbrella of features relevant to woodiness and growth form, with thresholds in tolerance of cold, drought, etc. In an island environment, there is alleviation, moderation, and transcendence of these thresholds and restrictions so that survival of vegetative structures into the next growing season is achieved. Advantageous radiation into new growth forms is made possible by this extension of the growing season.