Cactus cylindricus James; Opuntia arborescens Engelmann; O. vexans Griffiths

Trees, with short trunks, openly branched, 3(-5) m. Stem seg-ments 12-40 cm; tubercles widely spaced. Spines and sheaths usually tan to dirty white or ± yel-low, sometimes absent. 2n = 22.

Flowering spring-summer (May-Aug). Deserts, grasslands, pinyon-juniper woodlands, hills, plains; (800-)1100-1800(-2200) m; Ariz., Colo., Kans., N.Mex., Okla., Tex.; Mexico (Chihuahua, Coahuila, Durango, Nuevo León, San Luis Potosí, Tamaulipas, Zacatecas).

A dominant cholla of the Chihuahuan Desert, Cylindropuntia imbricata var. imbricata is wide- ranging and variable in several characters; it is generally shorter and more spiny northward (there often referred to as Opuntia arborescens). The species appears to be spreading northeastward in Oklahoma and Kansas as a result of cattle ranching activities. In Arizona, New Mexico, and Mexico, var. imbricata intergrades with C. spinosior. Northward, var. imbricata hybridizes with C. whipplei (= C. ×viridiflora).