Coryphantha robbinsorum

Plants usually unbranched, spine-bearing areoles with long white wool obscuring basal portion of spine. Roots diffuse or short taproots. Stems deep-seated (buried except for its apical 0.5-3 cm), 2-10 × 2-6 cm; tubercles 5-8 × 4-6 mm, moderately soft; areolar glands absent. Spines 11-20 per areole, white, largest spines dark tipped when fresh, straight; radial spines 10-20 per areole, 8-18 × 0.2-0.5 mm; central spines 0(-1) per areole, similar to largest radial spines, but porrect, ca. 8-18 mm. Flowers nearly apical, 12-29 × 10-18.5 mm; outer tepals fringed; inner tepals 14 per flower, dull yellow, frequently tinted greenish or bronze, often with midstripes of brownish or dull pink; outer filaments greenish; anthers bright yellow; stigma lobes green or yellow-green. Fruits bright orange-red or scarlet, spheric to obovoid, 6-8.5 × 3-4.5 mm, slightly juicy, quickly drying and turning brownish; floral remnant weakly persistent, often deciduous through breakage not abscission. Seeds dark brown, drying blackish, spheric, 1.3-1.4 mm, pitted. 2n = 22.

Flowering Mar-Apr; fruiting Jun-Aug. Semidesert grasslands, limestone hills; of conservation concern; 1300-1500 m; Ariz.; Mexico (Sonora).

Coryphantha robbinsorum is somewhat intermediate between the C. dasyacantha species-group, especially C. zilziana Boedeker, and the C. missouriensis species-group.

Coryphantha robbinsorum is in the Center for Plant Conservation´s National Collection of Endangered Plants.