Convergent Adaptive Morphology of a Sonoran Desert Cactus (peniocereus Striatus) and an African Spurge (euphorbia Cryptospinosa)

Peniocereus striatus (Cactaceae) and Euphorbia cryptospinosa (Euphorbiaceae) both produce slender pencil-like stems with stomata restricted to series of furrows separated by bark-covered ribs. In each species, as the stems dehydrate, the ribs come together to close off the furrows. The unusual structure of these stems in two unrelated succulent genera is a remarkable example of evolutionary convergence. The paper reports on the comparative anatomy of the stems and the adaptive strategies of the slender stems in relation to surfacelvolume ratios.