The anatomy of non-cactus stem-succulent dicots was examined in the families Apocynaceae (Plumeria, Pachypodium), Asclepiadaceae (Caralluma, Hoodia, Huernia, Pseudolithos, Stapelia, Larryleachia), Asteraceae (Senecio), Crassulaceae (Tylecodon), Euphorbiaceae (Monadenium, Euphorbia) and Vitaceae (Cissus). Some specimens have large, persistent photosynthetic leaves, others are stem-photosynthetic and “leafless”; all had succulence based on pith and cortex, not wood. These specimens had remarkably few of the adaptations that commonly occur in cacti: none had cortical bundles, none had a multilayered hypodermis of extremely thick-walled cells, none had deeply sunken stomata. Few had a thick cuticle or thick-walled epidermis. Many did have some form of collapsible cortex (either cells with plicate walls or extremely thin walls) and members of Apocynaceae and Asclepiadaceae had internal phloem or medullary bundles. Despite being desert-adapted succulents, none of the species had really wide, massively succulent bodies as is common in the Cactaceae: only one (Euphorbia obesa) had a cortex thicker than 12mm, and almost all others had cortexes less than 5mm thick.
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Vol. 22 • No. 22