The striking fact that arid or semi-arid environments provide appropriate habitats for members of the order was mentioned by various authors. A common adaptation to arid conditions are succulent leaves (e.g., Aizoaceae, Chenopodiaceae, and Portulacaceae) and/or stems. Succulent stems are often photo-synthetically active, sometimes combined with a more or less strong reduction of the leaves (Cac taceae, Didiereaceae, and Chenopodiaceae). Succulence can be combined with Crassulacean Acid Metabolism (Kluge and Ting 1978). Also the C„type of photosynthesis, adaptive in warm or hot regions, is frequent in the order (Carolin et al. 1978). Halo-phytic species are well represented especially among succulent members of the Chenopodiaceae and Aizoaceae. It is unclear, however, if the evolution of succulence started as an adaptation to saline habitats (Shmida 1985), or if succulence served as a preadaptation to such habitats.
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