Water-storing and Cavitation-preventing Adaptations in Wood of Cacti

Abstract

Ancestral cacti presumably had abundant, fibrous, heavily lignified wood, similar to that present in the relictual, leaf-bearing genus Pereskia. 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, have thick lignified walls, and axial parenchyma is only scanty paratracheal. Aside from a diversity of vessel diameters, there seems to be little protection against cavitation during water-stress, and little water-storage capacity. This strong wood permits the plants to be tall and to compete for light in their tree-shaded semi-arid habitats. In other evolutionary lines, the wood lacks fibres, and almost all cells have thin, unlignified walls. Vessels occur in an extensive matrix of water-storing parenchyma, and tracheids are also abundant, constituting over half the axial tissue in some species. 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 produce a 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.