16. Opuntia macrocentra Engelmann, Proc. Amer. Acad. Arts. 3: 292. 1856.
Black-spined pricklypear, purple pricklypear
Opuntia violacea Engelmann ex B. D. Jackson; O. violacea var. castetteri L. D. Benson; O. violacea var. macrocentra (Engelmann) L. D. Benson
Shrubs, erect to decumbent, to 1 m. Stem segments not easily detached, purple (particularly under stress) to green with purple near areoles and margins of stem segment, flattened, broadly obovate to subcircular, thickish, 7-20 × 6-18 cm; areoles 6-8(-10) per diagonal row across midstem segment, elliptic to circular, 3-7 × 2.5-5 mm; wool tan to whitish, aging black. Spines 0 or 1-15+ per areole, few and at distal areoles or many and on most areoles, usually erect to spreading, appearing unruly, sometimes deflexed, straight or curving, acicular, often flexible, subterete to flattened basally; longer ones reddish brown to ± black (rarely yellow to red in w Texas), or partly to wholly white, 30-120(-170) mm; reflexed spine rarely present, 1 in some areoles, short, whitish. Glochids dense in crescent at adaxial edge of areole and well-developed subapical tuft, reddish yellow, aging brown, 2-3(-6) mm. Flowers: inner tepals yellow with red basal portions, obovate-apiculate, 25-40 mm; filaments and anthers yellowish; style cream; stigma lobes green. Fruits red to purplish, obovoid to barrel-shaped, 25-40 × 20-23 mm, fleshy or ± juicy, glabrous, spineless; umbilicus 8-10 mm deep; areoles 22-44. Seeds yellowish, suborbicular to reniform, angled, 5-7 × 3.5-5 mm, sides flattened; girdle protruding 0.8-1.2 mm. 2n = 22, 44.
Flowering spring (Mar-Jun). Desert uplands, grasslands, oak woodlands, sandy desert flats, rocky hills and valleys; 900-1600 m; Ariz., N.Mex., Tex.; Mexico (Chihuahua, Coahuila, Sonora).
Opuntia macrocentra var. minor is represented by relatively short and compact shrubs with a glochid pattern of a dense crescent in the adaxial edge of the areoles like the species and a much taller tuft of spreading glochids. This variety, which grows along the western side of the Rio Grande in Big Bend, Texas, appears best interpreted as a tetraploid hybrid between tetraploid putative parents, Opuntia macrocentra and O. tortispina.