Plants mostly erect (rarely decumbent or prostrate), branched or unbranched, deep-seated in substrate or not. Roots diffuse or taproots (adventitious from offsets in M. thornberi and M. prolifera). Stems unsegmented, green to gray-green, sometimes purplish under stress, spheric to cylindric or turbinate, often flat-topped, 1–15(–25) × 1.8–12(–20) cm, firm or flaccid; tubercles distinct, not confluent into ribs, pyramidal, conic, or cylindric, 3–25 × 2–9 mm; areoles of 2 kinds: vegetative areoles (spine clusters) at tips of tubercles; reproductive areoles in axils of tubercles, woolly, bristly, or naked; areolar glands absent; cortex and pith usually not mucilaginous, instead containing latex (absent in M. grahamii). Spines [2–]5–80(–90) per areole, of every color that cactus spines can be, hairlike, bristlelike, or needlelike, glabrous or plumose, (0.5–)2–25(–31) × 0.01–0.6 mm; radial spines (6–)10–80 per areole, straight to curved or crinkly bristles, (0.6–)3–25 mm; central spines 0–several (indefinitely numerous and intergrading with radial spines in M. lasiacantha), straight, curved, or hooked, terete. Flowers diurnal, in ring distant from stem apex (or nearly apical at anthesis forming a ring around new growth, subsequent apical growth displacing fruits even farther away from apex), in axils of tubercles, unconnected to spine clusters, funnelform, campanulate, or rotate, 0.9–4(–5.2) × 0.6–3.5(–7.5) cm; outer tepal margins entire or fringed; inner tepals yellow, white, rose-pink, magenta, or maroon, 4–30 × 1.5–8.5 mm; ovary lacking scales and spines; stigma lobes cream, yellow, red, pink, or brownish green, 0.3–8 mm. Fruits indehiscent, usually pink, bright red, or greenish, green and barrel-shaped when seeds mature, sometimes becoming colored and clavate or cylindric to ovoid, 5–30(–40) × (2–)4–9(–26) mm, usually juicy; scales and spines absent (or rudimentary); floral remnant persistent to quickly deciduous. Seeds black, brown, reddish, or yellowish (with tan, corky strophioles in M. tetrancistra), 0.8–1.5 × 0.6–1.4 mm, usually pitted or raised-reticulate (with additional wrinkling in M. tetrancistra) [impressed-reticulate M. candida of Mexico], often shiny; testa cells flat to concave, walls straight to sinuate. x = 11.

Distribution

sw United States, Mexico, West Indies, Central America.

Discussion

Species ca. 164 (14 in the flora).

The greatest diversity in Mammillaria occurs in Mexico, in rocky sites bordering, but not in, semidesert. Some latex-bearing species, such as M. lasiacantha and many Mexican taxa, have their latex ducts deep inside the cortex, not in the tubercles. When preserving specimens outside the flora area, location, color, and viscosity of latex should be carefully recorded after plants have been cut and the latex allowed to ooze from its ducts for a few minutes. The latex ranges from sticky and white to less viscous and translucent. Tubercle length in descriptions refers to the distance the tubercle protrudes or projects outward from the stem axis. Fruits with mature, viable seeds sometimes remain on the plants for months before or after ripening. The time given in each phenology statement refers to the time of first ripening of the fruit (not seeds). Seeds or dried remains of fruits often may be found deep in the axils of the tubercles, hidden by spine clusters or even pulled below the level of the soil.

Selected References

Craig, R. T. 1945. The Mammillaria Handbook….Pasadena.

Hunt, D. R. 1987. A New Review of Mammillaria Names. Botley. [Reprinted from Bradleya 1–5, 1983–1987.]

Pilbeam, J. 1981. Mammillaria: A Collector’s Guide. London.

Reppenhagen, W. 1989. Die Gattung Mammillaria nach dem heutigen Stand meines Wissens, 1988. Frankenthal.

Lower Taxa

Mammillaria dioica, Mammillaria grahamii, Mammillaria heyderi, Mammillaria lasiacantha, Mammillaria macdougalii, Mammillaria mainiae, Mammillaria meiacantha, Mammillaria pottsii, Mammillaria prolifera, Mammillaria sphaerica, Mammillaria tetrancistra, Mammillaria thornberi, Mammillaria viridiflora, Mammillaria wrightii

Key

1	Central spines hooked	> 2
1	Central spines, when present, straight or curved throughout their lengths	> 8
2	Outer tepals long fringed; seeds large, 1.3 -2.4 mm; cortex and pith mucilaginous (slimy); stems soft and flabby	> 3
2	Outer tepals entire or short fringed; seeds small, 0.8 -1.2 mm; cortex and pith not mucilaginous, cut surfaces almost dry to the touch; stems relatively firm	> 5
3	Spine clusters appearing to have more than 1 series of radial spines, one superimposed on the other, as well as hooked central spines; seeds black, with big, tan, corky strophioles	Mammillaria tetrancistra
3	Spine clusters with only 1 series of radial spines in addition to hooked central spines; seeds black or chocolate brown, without conspicuous strophioles	> 4
4	Inner tepals rose or magenta	Mammillaria wrightii
4	Inner tepals white, pale greenish, or pale rose-pink	Mammillaria viridiflora
5	Axillary bristles present on stem; inner tepals whitish or yellowish; stigma lobes yellow, greenish yellow or brownish green; sw California, Mexico	Mammillaria dioica
5	Axillary bristles absent on stem; inner tepals brilliant pink or purple, or if mostly white then stigma lobes red; stigma lobes red or green; e edge of California e to Texas, Mexico	> 6
6	Stems 2-3.5 cm diam.; plants branched prolifically from base and every branch with independent root system, connections to rest of clone ephemeral, resulting in dense clumps of independently rooted stems; stigma lobes red	Mammillaria thornberi
6	Stems greater than 3.5 cm diam.; plants unbranched or, if branched, then centrally taproots; branches, when present, never (or very rarely and tardily) rooting; stigma lobes red or green	> 7
7	Stigma lobes green or yellow-green; outermost tepals minutely fringed; fringes less than 0.15 mm; spines (19-)26-33(-38) per areole	Mammillaria grahamii
7	Stigma lobes bright red to red-purple; outermost tepals conspicuously fringed; fringes 0.4 mm; spines ca. 9-16 per areole	Mammillaria mainiae
8	Central spines 0 or indefinitely numerous (depending on interpretation), not differentiated from radial spines; spines (26-)40-60(-90) per areole; stems (1.4-)2-4(-7) cm diam	Mammillaria lasiacantha
8	Central spines (0 or)1-12 per areole, at least 1 strongly differentiated from radial spines, if central spines absent, then spines 10 or fewer per areole and mature stems mostly greater than 7.5 cm diam	> 9
9	Radial spines hairlike, soft, white to pale yellow, conspicuously contrasting with central spines	Mammillaria prolifera
9	Radial spines bristlelike or needlelike, relatively hard, white, yellowish, brown, or almost black, not contrasting with rigid, needlelike central spines	> 10
10	Inner tepals bright yellow; flowers 50-70 mm diam.; stems many branched, soft and flabby, low, nearly spheric	Mammillaria sphaerica
10	Inner tepals greenish yellow, white, pink to purplish, maroon, or rusty; flowers 6-35(-45) mm diam.; stems few branched or unbranched, firm, either cylindric or low and flat-topped (spheric in young M. grahamii)	> 11
11	Central spines 6-12 per areole; spines (33-)43-60 per areole; inner tepals maroon, rusty, or purplish red; flowers 6-10 mm diam	Mammillaria pottsii
11	Central spines (0-)1-4 per areole; spines (6-)7-33(-38) per areole; inner tepals greenish yellow, white, pinkish, rose-purple, or lavender; flowers 15-35(-45) mm diam	> 12
12	Stems spheric to cylindric, (2.3-)3.5-6.8 cm diam., latex absent; inner tepals bright rose-pink or rose-purple; spines (19-)26-33(-38) per ar- eole; seeds black	Mammillaria grahamii
12	Stems low, flat-topped, usually 7.5-25 cm diam., latex present, sticky, white; inner tepals greenish yellow, white, cream, or pale pink, with midstripes of pink, lavender, or brown; spines (6-)7-22(-26) per areole; seeds orange or yellow when fresh, becoming reddish brown	> 13
13	Fruits brilliant scarlet, carmine, or crimson; radial spines often more than 12 per areole: (8-)10-22(-26)	Mammillaria heyderi
13	Fruits relatively pale purplish pink, whitish, or pale green; radial spines (5-)6-12 per areole	> 14
14	Outer tepals entire; inner tepals white to pale pink often striped with pink or lavender; spines 6-10 per areole; central spines 0-1 per areole; fruits purplish pink; Texas, New Mexico	Mammillaria meiacantha
14	Outer tepals minutely fimbriate (8× lens); inner tepals yellow-green or cream with yellow-green midstripes; spines 11-16 per areole; central spines 1-4 per areole; fruit whitish, pale green, or pink; Arizona, Mexico	Mammillaria macdougalii