Leaf-like Structure in the Photosynthetic, Succulent Stems of Cacti

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,…