The structure of photosynthetic succulent stems in plants other than cacti

In an investigation of succulence as an adaptation to water-stressed desert environments, we studied the anatomy of photosynthetic succulent stems in 28 species from Apocynaceae, Asclepiadaceae, Asteraceae, Crassulaceae, Didieriaceae, Euphorbiaceae, Geraniaceae, and Vitaceae. Only species with enlarged pith or cortex were examined, not species with enlarged parenchymatous wood. The numbers of species with particular characters are as follows: pith is the most voluminous tissue - 5 species; cortex is the most voluminous tissue - 22; cortical bundles present - 0; medullary bundles present - 4; palisade cortex present - 16; epidermis with thick or lignified walls - 0; epidermis outer wall somewhat thickened - 6; multiple epidermis present - 0; cuticle > 2 mum thick - 4; cuticle < 1 mu m thick - 3; hypodermis cells with very thick walls - 0. These species lack many features typically considered xeromorphic and associated with desert-adapted plants; they also lack many features present in most cacti. Absence of cortical bundles in all these noncactus succulents may be especially important: none has a particularly thick cortex ( most are < 5 mm thick), all are relatively small plants, and absence of cortex vascularization may prevent them from evolving to be truly voluminous. Also, none has truly large shoot apical meristems such as occur in cacti with broad shoots.