Arizona fishhook cactus, Graham’s fishhook cactus, Graham’s nipple cactus

Mammillaria grahamii var. oliviae (Orcutt) L. D. Benson; Mammillaria microcarpa Engelmann; Neomammillaria milleri Britton & Rose

Plants branched or unbranched; branches 0-9(-17). Roots diffuse; upper portion not enlarged. Stems spheric to cylindric, usually (4-)5-16(-30) × (2.3-)3.5-6.8 cm, firm; tubercles 4.8-12(-15) × 3.5-7 mm; axils appearing naked; cortex and pith not mucilaginous; latex absent. Spines (19-)26-33(-38) per areole, glabrous; radial spines 17-35 per areole whitish or pale tan, bristlelike, 6-12 × 0.1-0.15 mm, stiff; central spines (2-)3-4 per areole, 1-3(-4) spines at least hooked (uncommonly 0 hooked), reddish to purplish brown to almost black (rarely golden brown), abaxial 1 porrect, others inconspicuous, appressed against radial spines, hookless porrect spines (3-)9.5-25 × 0.1-0.5 mm; subcentral spines 1-3 per areole, adaxial to central spines, sometimes transitional to central spines, usually straight and barely distinguishable from radial spines. Flowers ca. 2 × 1.8-3.5(-4.5) cm; outermost tepal margins minutely fringed; inner tepals bright rose-pink or rose-purple, 10-16 × 4-8 mm; stigma lobes yellow-green to green, 3-7 mm. Fruits green, turning bright red, scarlet, or carmine (rarely yellowish), elongating until clavate after color change to red is complete, 12-29 × 5-8 mm, juicy only in fruit walls; floral remnant persistent. Seeds black, 0.8-1 × 0.7-0.9 mm, pitted; testa hard, brittle; anticlinal cell walls straight; interstices conspicuously wider than pit diameters; pits bowl-shaped. 2n = 22.

Flowering Apr-Sep; fruiting Sep-Mar. Chihuahuan and Sonoran desert scrub, grasslands, interior chaparral, oak woodlands, alluvial slopes, hills, canyons, silty, sandy, gravelly, or rocky soils of igneous or calcareous origin; 80-1400 m; Ariz., Calif., N.Mex., Tex.; Mexico (Chihuahua, Sonora).

Mammillaria grahamii is geographically variable. Past attempts to distinguish larger or western individuals as M. microcarpa have proven arbitrary.

All spine hooks on a plant may be oriented in the same direction, a trait sometimes mistakenly said to be limited to Mammillaria mainiae. Plants with short, straight central spines (rarely a mixture of both hooked and straight spines on the same stem) occur in some populations; they have been called M. oliviae or M. grahamii var. oliviae. The name Mammillaria microcarpa Engelmann has been widely used but was not validly published until after M. grahamii.