The genus Euphorbia(Euphorbiaceae) contains nearly 2000 species, of which as many as a third are succulent. The majority of the succulent Euphorbiaoccurs in Africa and lower Asia. Within the New World there only about 15 succulent species. Based on a broad molecular phylogenetic analysis of Euphorbiaand related genera, it appears that these comparatively few species represent at least five independent derivations of this habit, none of which are closely related to the Old World taxa. Succulence in the New World species is achieved by the proliferation of water-storing parenchyma in the pith and/or cortex. Periderm development is delayed, the epidermis remains intact, and the stem stays green and photosynthetic for many seasons. In addition, there is a trend towards microphylly, and most species possess minute, caducous leaves, and the stem is the chief photosynthetic organ. Despite this convergence to reduced-leafed, green-stemmed succulents, there are many differences at the cellular level. Chlorenchyma organization varies from neatly layered rows of narrow palisade cells to unlayered zones of wide spheroidal cells. Tanniniferous cells are present or lacking. The vasculature also differs, and leaf traces that descend many centimeters through the cortex occur in one species. The vascular cambium is round in most taxa but is variously angled in others. In addition, there is diversity in secondary xylem. Some species posses highly lignified wood lacking axial parenchyma, while the wood of other species is less lignified with well-developed axial parenchyma, this sometimes occurring in bands. Although both the molecular phylogenetics and the many anatomical differences suggest the repeated evolution of stem succulence within Euphorbia,it is still uncertain what promotes this trend.

Key words: anatomy, Euphorbia, Euphorbiaceae, succulence