Fifty species of Opuntioideae were examined to establish 1) the range of anatomical variation occurring in this subfamily, 2) characters these plants share with other desert-adapted plants, and 3) information about phylogenetic relationships within Cactaceae. Only three characters are universally present in stem-photosynthetic, stem-succulent plants: 1) a persistent epidermis, 2) a thickened cortex, and 3) a palisade cortex. A multiseriate hypodermis with extremely thick walls and crystals is almost universally present in Opuntioideae and Cactoideae but is not uniformly present in non-cactus succulents. Opuntioideae lack many features common in Cactoideae: they have no extremely thick cortex, no extensive system of cortical bundles, no collapsible cortex cells, and no medullary bundles. Features common to Pereskia, Cactoideae and Opuntioideae, which may be relictual in the family, are 1) a tendency to form multiple epidermis, 2) a multiseriate, thick-walled hypodermis with druses in at least one layer, 3) mucilage cells, 4) an unvascularized cortex (Blossfeldia in Cactoideae), 5) phloem fiber caps, 6) fibrous wood, 7) narrow rays, 8) scanty paratracheal parenchyma, 9) an unvascularized pith (some Cactoideae have medullary bundles), and 10) indeterminate growth (Pereskiopsis in Opuntioideae). Within Opuntioideae, Pereskiopsis has the greatest number of these characters; it may most closely resemble the ancestors to the Opuntioideae.
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