Leaflike structure in the photosynthetic, succulent stems of cacti.

Abstract

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.