Stem succulents are fleshy succulent columnar shaped plants which conduct photosynthesis mainly through stems not leaves. They are related by form, but not by evolution. They evolved to have similar forms and physiological characteristics by convergent evolution. Examples are tall thin Euphorbias from deserts and arid regions of southern African and Madagascar, similarly shaped cacti from North America and South America, which occupy a similar xeric evolutionary niche, and members of two genera of the family Asclepiadaceae (Hoodia and Stapelia). Shared features are a succulent stem that stores water and conducts photosynthesis, protective spines or thorns, leaves absent or highly reduced, and use of CAM photosynthesis (an opening of stomata and fixing CO2 almost exclusively at night).