Taxon	Illustrator ⠉
	Carnegiea gigantea Stenocereus thurberi Echinocactus horizonthalonius	John Myers John Myers John Myers

Plants erect, branched or unbranched, forming compact mounds of 30+ branches, not deep-seated in substrate (or deep-seated in E. texensis and, if plants immature or at high altitude, in E. horizonthalonius). Roots diffuse or short taproots. Stems unsegmented, gray-blue, gray-green, yellow-green, or grass green, flat-topped spheric to short cylindric, 4–40 (–45) [–250] × 8–30 [–80] cm, apical region appearing copiously woolly (shortly velvety in E. texensis) [glabrous]; ribs (7–) 8–27 [–60+], very prominent, straight (or only slightly undulate), sometimes helically curving around stems, broadly rounded to nearly keeled, rib crests uninterrupted or ± constricted between areoles; areoles widely spaced or confluent with age, nearly circular to oblong, with fertile portion as short, broad adaxial prolongation confluent with spine cluster; areolar glands absent; cortex and pith hard, not mucilaginous. Spines (5–) 7–19 per areole, straw colored, pink, red, gray, tan, or brown, conspicuously annulate-ridged, rigid, stiff; radial spines 5–14 per areole, straight to curved, 2–5 cm; central spines 1–4 per areole, straight to curved, terete, flattened, or abaxially ridged. Flowers diurnal, near stem apex, near adaxial edges of short adaxial extensions of areoles, broadly funnelform to nearly salverform (more narrowly funnelform in E. texensis); outer tepals margins entire, apically spinose; inner tepals yellow or pink to magenta, 2.4–3.2 × 0.3–1.5 cm, margins entire, serrate, toothed, or erose; ovary scaly, spineless, copiously woolly, with white or pale tan hairs from areoles hiding ovary and flower tube at anthesis; scales 8–60, margins entire or fimbriate, spine-tipped; stigma lobes 6–14 (–17), yellow, pink, or olive, 1–4.5 mm. Fruits either indehiscent or rupturing irregularly, or tardily dehiscent through basal abscission pore, whitish tan to pinkish (bright red in E. texensis), spheric to ovoid or ovoid-cylindric, 10–50 × 10–40 [–100] mm, usually nearly dry (strongly succulent in E. texensis), many scaled; axils of scales copiously woolly (wool hiding surface of fruits except in E. texensis), spineless, distal scales spine-tipped; pulp red; floral remnant persistent. Seeds reddish-brown to black, spheric, or subreniform to obovoid, 2.4–4.7 mm, shiny or dull; testa cells convex or flat. x = 11.

Distribution

sw United States, Mexico

Discussion

Species 6 (3 in the flora).

With the exception of the much-branched Echinocactus polycephalus, species of Echinocactus in the flora area are almost never branched, and will die instead of producing a lateral bud when the apex is injured. Smaller than normal fruits of Echinocactus are usually aborted and hollow as the result of infestation by insect larvae (M. Chamberland, pers. comm.).

Selected References

None.

Lower Taxa

Echinocactus horizonthalonius, Echinocactus polycephalus, Echinocactus texensis

Key

1	Inner tepals yellow; plants many branched (except in very young adults); w Arizona, s Nevada, se California, Mexico	Echinocactus polycephalus
1	Inner tepals pink, magenta, or proximally red and distally pink; plants unbranched (rarely few branched); c to s Arizona, New Mexico, Texas, Mexico	> 2
2	Ribs (7-)8(-9), crests broadly rounded, never keeled; stems flat-topped or hemispheric to stoutly cylindric; fruit surfaces hidden by abundant wool at maturity; spines and bract tips glabrous (use lens)	Echinocactus horizonthalonius
2	Ribs 13-27, crests narrow, ± keeled; stems flat-topped to hemispheric; fruit surfaces exposed at maturity, bright red; spines and bract tips densely puberulent with ± matted, strap-shaped, unicellular hairs (use at least 8× lens)	Echinocactus texensis