Cactaceae subfam. Opuntioideae
Outlines Bot. 2: 742, 1130. 1835.
Trees or shrubs, sometimes forming clumps or mats, sometimes geophytes, trailing to erect. Roots diffuse or tuberlike. Stems segmented throughout or only in ultimate branches, succulent (less noticeably so in some Cylindropuntia), often woody, especially toward base, smooth or tuberculate; areoles cushionlike, circular or nearly so (to linear), usually bearing conspicuous spines and always bearing glochids. Spines slender to stout (to hairlike), terete to strongly flattened, usually smooth, sometimes barbed or roughened, epidermis intact or partly to wholly separating from body of spine as sheath that hangs onto spine. Leaves of brief duration, promptly deciduous, present only during initial growth of stem segments and flowers [persistent], conic or somewhat flattened, succulent. Flowers diurnal (opening late afternoon or night in Cylindropuntia fulgida), bisexual (sometimes functionally staminate or pistillate), solitary in areoles [terminal], radially or bilaterally symmetric (flower curved and/or the ovary flattened), sessile, rotate, cup-shaped, or salverform; flower tube epigynous, short adnate to extension of stem segment surrounding ovary; nectary at base of style, open or sometimes covered by outgrowths of proximal portion of style base or of flower tube wall as specialized nectar chamber. Fruits indehiscent, cylindric, ellipsoid, ovoid, or subspheric, sometimes clavate, fleshy, juicy (bleeding), or quickly drying; perianth and contained flower parts shriveling and abscising basally as single unit including floral cup, leaving deep to almost flat scar (umbilicus) atop fruit. Seeds 0 (sterile) or 1–400+, yellowish, tan, gray, or brown, flattened to subspheric, 2–7 mm, each completely enclosed by bony funicular envelope, glabrous or sometimes pubescent, its vascular bundle forming girdle around seed, sometimes enlarged and protruding as ridge or wing.
Throughout New World from near Arctic Circle to Patagonia, introduced especially in tropical, subtropical, and warm-temperate regions almost worldwide.
Genera 16, species ca. 350 (5 genera, 66 species in the flora).
All genera of Opuntioideae in the flora have been combined into the genus Opuntia at various times. Recent research findings in morphology, anatomy, palynology, seed characteristics, host-herbivore relations, and chloroplast and nuclear DNA make untenable the maintenance of the single genus Opuntia. Unlike some genera of subfam. Cactoideae, the segregate genera of Opuntioideae are not known to produce intergeneric hybrids.
Identification of the species within subfam. Opuntioideae is difficult, in part because of widespread phenotypic plasticity, interspecific hybridization, polyploidy, and apomixis (clonal seeds and stems), which play important evolutionary roles, particularly in Cylindropuntia and Opuntia. Habit descriptions, color photographs (including close-ups), and meiotic chromosome counts are very helpful in identification of species. Also, good herbarium specimens require at least two or three consecutive stem segments, flowers and/or fruits, and detailed notes on all fresh flower parts and fruits as to color (particularly inner tepals, filaments, fresh stigma lobes, and fruits), shape, and size (because of extensive shrinkage on drying). Spine characters are generally based on well-developed areoles, mostly in distal portions of stem segments. Yellow spines usually turn dark red to black with age, including those on herbarium sheets.
Pinkava, D. J. 2002. On the evolution of the continental North American Opuntioideae (Cactaceae). In: D. R. Hunt and N. P. Taylor, eds. 2002 Studies in the Opuntioideae (Cactaceae). Sherborne. Pp. 59–98.