Plants erect, unbranched [or branched], deep-seated in the substrate [or somewhat subterranean for whole seasons]. Roots taproots. Stem unsegmented, gray-green (yellow-green or purplish with age or stress), above-ground portion flat, concave, or weakly hemispheric, usually flush with soil surface and cryptic, strongly tuberculate, 0-2(-10) × [3-]5-10(-15) cm, hard, rigid, tough skinned [thin skinned in A. agavoides of Mexico]; tubercles arranged in rosettes or mosaics, ± triangular, 8-20[-60] × [3-]11-25 mm, hard, exposed faces of tubercles strongly differentiated from sides [except in some Mexican species], prominently fissured [wrinkled, roughened, or nearly smooth]; areoles elongate [circular and axillary, circular and subapical, or 2-parted], forming a wide woolly groove on each tubercle; areolar glands absent; cortex and pith not mucilaginous, "mucilage" restricted to elongate cavities. Spines absent [sporadic and rudimentary in some Mexican taxa]. Flowers diurnal, borne in axils of tubercles near stem apex, broadly funnelform to almost salverform, 1.5-5 × 1.5-5 cm; outer tepals brownish or greenish with pink tinge, 12-35 × 5-9 mm, margins entire; inner tepals pink or magenta [white or yellow], 13-34 × 4-10 mm, margins entire; ovary smooth (scales, hairs, and spines absent); stigma lobes 5-10, white, 1.2-5 mm. Fruits indehiscent (very rarely explosively dehiscent), white or cream to pale greenish [reddish], spheric to clavate or cylindric, proximally or almost completely buried in copious wool of stem apex, 10-25 × 5-10 mm, initially fleshy, drying and collapsing a few days after ripening, scales and spines absent; pulp white to pale greenish; floral remnant apparently persistent. Seeds black, spheric to obovoid, 1.2-1.6(-2.5) mm, minutely tuberculate, shiny; testa cells strongly convex (conspicuous with lens). x = 11.
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