3. Cylindropuntia x tetracantha (Toumey) F. M. Knuth in C. Backeberg and F. M. Knuth, Kaktus-ABC. 124. 1935 (as species).
Opuntia tetracantha Toumey, Gard. & Forest 9: 432. 1896; O. kleiniae de Candolle var. tetracantha (Toumey) W. T. Marshall
Shrubs, openly branched, 0.5-1.3 m. Stem segments usually alter-nate, green to gray-green (or reddish), 5-28 × 0.7-1.4 cm; tubercles moderately prominent, narrowly elongate, 1.5-3 cm; areoles subcircular, 1.5-3 mm in diam.; wool tan, aging gray. Spines (0-)1-4 per areole, in all but proximal areoles of stem segment, usually all deflexed, yellow with whitish to pinkish basal coating, moderately stout to acicular, the longest 1.5-2.8 cm; sheaths whitish at bases to golden at tips. Glochids in adaxial tuft and marginal about areole, yellow, 1-3 mm. Flowers: inner tepals greenish bronze or red to magenta, spatulate- to obovate-apiculate, 10-15 mm; filaments reddish; anthers light yellow; color of style and stigma lobes unknown . Fruits commonly sterile, often proliferating, green, becoming yellow with red blush to red, obconic to obovate-stipitate, 15-23 × 10-20 mm, fleshy, moderately tuberculate; umbilicus deep, 3-4.5 mm; areoles 10-18, a few producing 1(-2) short spines. Seeds tan, angularly subcircular to broadly oval, warped, 3-4 × 3-3.5 mm; girdle broad, lumpy, little protruding. 2n = 22.
Flowering spring (Apr-May). Sonoran desert scrub, flats, hills; 500-1000 m; Ariz.
Cylindropuntia ×tetracantha (= C. acanthocarpa × C. leptocaulis) is an uncommon hybrid; sympatric hybrids long confused with and usually included in C. ×tetracantha involve C. leptocaulis × C. spinosior, which have spineless, reddish, and strongly tuberculate fruits and long-spreading, subwhorled branches, and C. leptocaulis × C. versicolor, which have spineless, reddish, smooth, and enlarged fruits and a less spreading habit.