Plants stem succulents, unbranched [to several branched from base], mostly low and deep-seated in substrate [taller in Mexican species]. Roots diffuse. Stem unsegmented, dark green or gray-green, hemispheric or depressed-spheric [to spheric or short cylindric], (2.5–)6–15(–60)[–100] × 6–10(–20) cm, speckled [or entirely hidden] by numerous tufts of dense, whitish, multicellular hairs less than 0.5 mm; ribs [4–]8–10, crests uninterrupted, straight [to sinuous and/or helically curving around stem], broad and nearly flat or rounded [sharp or keeled]; areoles distinct, [2–]6–9[–20] mm apart along ribs, circular; areolar glands absent; cortex and pith hard, not mucilaginous. Spines absent [1–25 per areole in some Mexican species]. Flowers diurnal, near stem apex, at adaxial edge of areoles, funnelform, 4.5–5.4[–8] × 3.8–5.2[–6] cm; inner tepals yellow, proximally red [all yellow], 25 × 6–10[–12] mm, margins entire; ovary sparsely to densely scaly, axils spineless long hairy with arachnoid trichomes, distal scales spine-tipped; stigma lobes [8–]10–12, yellow [pale yellowish], 4 mm. Fruits indehiscent or splitting irregularly, green or pinkish to red, ovoid to spheric, 15–20 × 12 mm, initially fleshy, drying immediately after ripening, sparsely to densely scaly with spine-tipped scales; axils of scales long woolly, spineless; floral remnant persistent. Seeds dark brown to blackish, appearing hollow or bowl-shaped from strongly expanded, inrolled rim around sunken hilum, 2–3 mm in greatest dimension, nearly smooth; testa cells very slightly convex. x = 11.
sw United States, Mexico.
Species 4–5 (1 in the flora).
Astrophytum is most often recognized as a distinct genus. Chloroplast DNA evidence (C. A. Butterworth et al. 2002), however, confirms that it is among the closest relatives of Echinocactus. Unlike Echinocactus, the stem surfaces of Astrophytum species are speckled by numerous tiny white tufts of minute, matted hairs in addition to the regularly spaced woolly areoles.