WATER-STORING AND CAVITATION-PREVENTING ADAPTATIONS IN WOOD OF CACTI

Ancestral cacti presumably had abundant, fibrous, heavily lignified wood, similar to that present in the relictual, leaf-bearing genusPereskia.During the evolutionary radiation of the subfamily Cactoideae, diverse types of bodies and woods arose. Several evolutionary lines have retained an abundant, fibrous wood: all wood cells, even ray cells, havethick lignified walls, and axial parenchyma is only scanty paratrachéal. Aside from a diversity of vessel diameters, there seems to be little protection against cavitation during water-stress, and little water-storage capacity. This strongwood permits the plants to be tall and to compete for light in their tree-shaded semi-arid habitats. In otherevolutionary lines, the wood lacks fibres, and almost all cells have thin, unlignified walls. Vessels occur in an extensivematrix of water-storing parenchyma, and tracheids are also abundant, constituting over half the axial tissue in somespecies. There is excellent protection against cavitation, but little mechanical support for the plant body; however, these plants are short and occur in extremely arid, unshaded sites. Scandent, vinelike plants of two genera producea dimorphic wood—while their shoots are extending without external support, they produce fibrous, lignified wood, but after leaning against a host branch, they produce a parenchymatous, unlignified wood. © 1993 Annals of Botany Company.