Trees, shrubs, subshrubs, or somewhat vinelike, solitary to forming mats or clumps, terrestrial (sometimes deep-seated in substrate) to epiphytic or epipetric, erect to sprawling (rarely scrambling or climbing) or pendent in epiphytic or epipetric taxa, simple to many branched, usually stem succulent. Roots diffuse, taproots, or tuberlike, sometimes adventitious. Stems unsegmented or segmented, segments persistent to easily detachable; long shoots spheric to depressed-spheric or club-shaped to long cylindric, or sometimes flattened cladodes, smooth, tuberculate and/or fluted with ribs; tubercles distinct as nipple-shaped or ridgelike (to triangular or pyramidal) protuberances to coalescent as vertical ribs; ribs 2-30[-40+], if ribs 2, stems winged, if ribs 3 or more, stems ± angled; short shoots (areoles) positioned on crests of ribs, at or near tubercle apices, or in axils of tubercles, commonly bearing persistent spines, also minute, barbed, deciduous spines (glochids) in subfam. Opuntioideae, and abundant, dense hairs (wool) creating a cushionlike appearance. Leaves deciduous to persistent, vestigial or absent, spirally alternate, sessile (petiolate to subsessile in Pereskia and several genera outside the flora), terete or flat, 0-3 cm (to 10 cm in Pereskia); stipules absent. Spines flexible and hairlike or bristlelike to rigid and needlelike or nail-like, terete to angled or flat, mostly hard (rarely corky or papery). Flowers bisexual (rarely unisexual or with bisexual and pistillate flowers on separate plants), nocturnal or diurnal, 1(-several) per areole, arranged in true inflorescence only in subfam. Pereskioideae, or chains of fruits proliferating from fruit areoles (in Cylindropuntia fulgida), sessile (pedicellate in Pereskia), arising from stem areole at apex or axil of tubercle, radially symmetric [bilaterally symmetric]; flower tube 0.2-15[-30] cm; perianth epigynous (perigynous in some Pereskia), deciduous or persistent on fruit; tepals 5-50 or more, intergrading gradually from bractlike or sepal-like outer tepals to petal-like inner tepals; stamens usually 50-1500+ [sometimes fewer], decurrent on inner surface of flower tube; true ovary sunken in stem with tubercles present or absent, areoles conspicuous to obscure or absent; subtending scales persistent or deciduous, sometimes absent; spines present or absent, glochids present only in subfam. Opuntioideae; pistils compound, 1-locular; placentas parietal, 3-14[-20+]; style 1; nectary usually forming chamber around base of style; stigma lobes 3-14[-20+], 1 per placenta. Fruits basically berrylike (variable in succulence), deciduous or long persistent, indehiscent or dehiscent, succulent or leathery, sometimes promptly drying. Seeds (0-)5-3000+, yellowish, reddish, brown, black, or appearing tan or whitish (dark testa completely covered by pale, tough, glabrous or rarely pubescent, tight-fitting aril or "funicular envelope" in subfam. Opuntioideae), pyriform, obovoid, lenticular-reniform, or nearly circular, 0.4-12 mm diam.; testa glossy or dull; rarely with corky arillate appendages (strophioles in Mammillaria tetrancistra). x = 11.
Fleshy perennials, shrubs, trees or vines, terrestrial or epiphytic. Stems jointed, terete, globose, flattened, or fluted, mostly leafless and variously spiny. Leaves alternate, flat or subulate to terete, vestigial, or entirely absent; spines, glochids (easily detached, small, bristlelike spines), and flowers always arising from cushionlike, axillary areoles (modified short shoots). Flowers solitary, sessile, rarely clustered and stalked (in Pereskia), bisexual, rarely unisexual, actinomorphic or occasionally zygomorphic. Receptacle tube (hypanthium or perianth tube) absent or short to elongate, naked or invested with leaflike bracts, scales, areoles, and hairs, bristles, or spines; perianth segments usually numerous, in a sepaloid to petaloid series. Stamens numerous, variously inserted in throat and tube; anthers 2-loculed, dehiscing longitudinally. Ovary (pericarpel) inferior, rarely superior, 1-loculed, with 3 to many parietal (rarely basal) placentas; ovules usually numerous; style 1; stigmas 2 to numerous, papillate, rarely 2-fid. Fruit juicy or dry, naked, scaly, hairy, bristly, or spiny, indehiscent or dehiscent, when juicy then pulp derived from often deliquescent funicles (except in Pereskia). Seeds usually numerous, often arillate or strophiolate; embryo curved or rarely straight; endosperm present or absent; cotyledons reduced or vestigial, rarely leaflike.
Ovary inferior, 1-celled, with parietal many- or rarely few-ovuled placentas; stigma usually radiate
Succulent herbs and shrubs of diverse habit, often very spiny, and usually with much reduced leaves
Flowers hermaphrodite, actinomorphic, often handsome, but very small in the African species (Rhipsalis)
Calyx-tube adnate to the ovary and often produced beyond; lobes few to many, or reduced to minute teeth
Anderson, E. F. 2001. The Cactus Family. Portland. Barthlott, W. and D. R. Hunt. 1993. Cactaceae. In: K. Kubitzki et al., eds. 1990+. The Families and Genera of Vascular Plants. 4+ vols. Berlin etc. Vol. 2, pp. 161-197. Benson, L. D. 1969b. Cactaceae. In: C. L. Lundell. 1942-1969. Flora of Texas. 3 vols. in parts. Dallas and Renner, Tex. vol. 2, pp. 221-317. Benson, L. D. 1982. Cacti of the United States and Canada. Stanford. Gibson, A. C. and P. S. Nobel. 1986. The Cactus Primer. Cambridge, Massachusetts. Hunt, D. R., comp. 1999. CITES Cactaceae Checklist, ed. 2. Kew. Hunt, D. R. and N. P. Taylor, comps. 1990. The genera of Cactaceae: Progress towards consensus. Bradleya 8: 85-107. Wallace, R. S. 1995. Molecular systematic study of the Cactaceae: Using chloroplast DNA variation to elucidate cactus phylogeny. Bradleya 13: 1-12. Wallace, R. S. and A. C. Gibson. 2002. Evolution and systematics. In: P. S. Nobel, ed. 2002. Cacti: Biology and Uses. Berkeley. Pp. 1-21.