This research examined the hypothesis that as cacti evolve to the leafless condition, the stem epidermis and cortex become more leaflike and more compatible with a photosynthetic role. All cacti in the relict genus Pereskia have non-succulent stems and broad, thin leaves. All members of the derived subfamily Cactoideae are 'leafless', having an expanded cortex that is the plant's only photosynthetic tissue. In Pereskia, leaves have a high stomatal density (mean: 50.7 stomata mm-2 in the lower epidermis, 38.1 mm-2 in the upper epidermis), but stems have low stomatal densities (mean: 11.3 mm 2, three of the species have none). Stems of Cactoideae have a high stomatal density (mean: 31.1 mm-2, all species have stomata). The outer cortex cells of stems of Cactoideae occur in columns, forming a palisade cortex similar to a leaf palisade parenchyma. In this palisade cortex, the fraction of tissue volume available for gas diffusion has a mean volume of 12.9%, which is identical to that of Pereskia leaf palisade parenchyma. Pereskia stem cortex is much less aerenchymatous (mean: 5.3% of cortex volume). Cactoideae palisade cortex has a high internal surface density (0.0207 cm2 cm-2 which is higher than in Pereskia stem cortex (0.0150 cm2 cm-3) but not as high as Pereskia leaf palisade parenchyma (0.0396 cm2 cm-3). Pereskia stem cortex has no cortical bundles, but Cactoideae cortexes have extensive networks of collateral vascular bundles that resemble leaf veins. © 1991 Annals of Botany Company.
