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Trees or shrubs, erect to trailing, usually many branched, sometimes forming clumps or mats; trunk, when present, initially segmented, appearing continuous with age, main axis determinate, usually terete. Stem segments green or sometimes reddish to purple, usually flattened, circular, elliptic, ovate, lanceolate, or obovate to oblanceolate, 2-60(-120) × 1.2-40 cm, nearly smooth to tuberculate, glabrous or pubescent; areoles usually elliptic, circular, or obovate, 3-8(-10) × 1-7(-10) mm; wool white, gray, or tan to brown, aging white or gray to black. Spines 0-15+ per areole, white, yellow to brown, red-brown to gray, or black, sometimes partly to wholly white chalky (chalkiness disappearing when wet), aging gray to dark brown to black, with epidermis intact, not sheathed, acicular to subulate, sometimes setose or with hairlike bristles, terete to angular-flattened, to 75(-170) mm, tips sometimes paler or yellow. Glochids in adaxial crescent at margin of areole, in tuft or encircling areole margin, white to yellow to brown, or red-brown, aging white to brown or red-brown. Flowers bisexual or sometimes functionally staminate, radially symmetric; outer tepals green to yellow with margins tinged color of inner tepals; inner tepals pale yellow to orange, pink to red or magenta, rarely white (unicolored) or with base of a different color (bicolored), oblong to spatulate, emarginate-apiculate; nectar chamber simple, open, not covered by proximal thickening style. Pollen yellow, grains reticulate or foveolate (opuntioid type). Fruits sometimes proliferating (sprouting from another fruit), if fleshy, green, yellow, or red to purple or, if dry, tan to gray, straight, sometimes stipitate, clavate to cylindric, ovoid, or obovoid to subspheric, 10-120 × 8-120 mm, fleshy to juicy or dry, smooth or tuberculate, spineless or spiny, sometimes burlike. Seeds pale yellow to tan or gray, generally circular to reniform, flattened (discoid) to subspheric, angular to squarish, sometimes warped, 2-7 × 2-7 mm, glabrous, commonly bearing 1-4 large, shallow depressions due to pressures from adjacent developing seeds; girdle protruding 0.3-3.5 mm, forming ridge or flat wing, or not protruding. x = 11.
Succulent shrubs, small trees, and subshrubs; stems phylloid and jointed when young, the joints fleshy, compressed and oval to broadly linguiform in our species, bearing amphigenous areoles armed with hairs, glochids, and spines, very rarely un-armed. Leaves inconspicuous, acicular and fugacious. Flowers sessile, chiefly marginal upon the young joints; perianth broadly campanulate, with a short and broad hypanthium, the segments numerous, the outer progressively shorter and less petaloid than the inner, widely spreading; stamens very numerous, the filaments much shorter than the perianth, united at different levels into a shallow glandular cup, somewhat deflexed at anthesis, sensitive and inflexed upon stimulation; ovary turbinate to cylindric, concave, the areoles prominent; style slightly longer than the stamens, terete to fusiform. Fruit a fleshy berry with numerous seeds.
Shrubs or small trees. Stems fleshy, usually many branched, terete, club-shaped, subglobose, laterally compressed; areoles with glochids and usually 1 to many spines. Leaves conic to terete, usually small, caducous. Flowers solitary, lateral or subterminal, rarely terminal, sessile. Receptacle obovoid, truncate and depressed at apex. Perianth rotate, spreading, or erect, inserted at rim of receptacle tube; segments numerous, outer ones sepaloid, inner ones petaloid. Stamens inserted in perianth throat, sensitive (except in O. cochenillifera). Ovary (pericarpel) inferior; placentas parietal. Fruit fleshy or dry, globose or ovoid, umbilicate, with areoles, glochids, and sometimes spines. Seeds encased in a white, hard, rarely hairy aril.
Shrubby or arborescent cacti with cylindrical, club-shaped, subglobose or flattened branches. Areoles tufted with barbed bristles (glochids) and usually 1–∞ longer stouter spines. Leaves small, subulate to terete, usually early deciduous. Flowers solitary, sessile; perianth rotate or erect,brightly coloured, segments ∞. Ovules circinotropous (the funicles circinnate). Berry bearing areoles, glochids, and sometimes spines. Seeds ∞, encased by the hard, white funicular aril; endosperm scanty. Widely introduced in the warmer parts of the world, some species as food-plants for the cochineal insect, some for forage or for their edible fruits.
Prostrate to arborescent shrubs. Stems jointed, the segments cylindric to globose or flattened and pad-like. Areoles bearing numerous glochids and often hairs and spines. Lvs small, subulate or terete, caducous. Fls single in an areole, usually on margins of stem segments. Sepals green, or inner sepals similarly coloured to petals. Petals green, yellow, red, or occasionally white, opening widely. Stamens much < petals. Ovary with areoles and glochids, often with spines. Berry rather dry to very juicy, sometimes spiny. Seeds with whitish bony aril.
Stems branched and jointed, the joints cylindric to flattened; spines and glochids arising from the areoles, or the plants virtually spineless; fls borne within the areoles near the tips of joints of the previous year; pet and sep rotate from the summit of the scarcely prolonged hypanthium; stamens shorter than the pet; seeds wingless. 150+, New World.
