14. Mammillaria macdougalii Rose in L. H. Bailey, Stand. Cycl. Hort. 4: 1982. 1916.
Mammillaria gummifera Engelmann var. macdougalii (Rose) L. D. Benson; M. heyderi Muehlenpfordt var. macdougalii (Rose) L. D. Benson
Plants unbranched, deeply seated in substrate. Roots short, obconic taproots; secondary roots diffuse. Stems usually flat-topped (in old age or under dense brush hemispheric), 4-20 × 9-25(-30) cm, firm; tubercles 10-17 × 6-8.5 mm; axils woolly, wool 5-8 mm, bristles absent; cortex and pith not mucilaginous; latex sticky white, abundant in healthy tissue throughout cortex of stem, tubercles, and sometimes flower receptacles. Spines 11-14(-16) per areole, white to straw yellow, glabrous; radial spines 10-12 per areole, whitish to pale yellowish tan , needlelike, 20 mm, stiff; central spines 1-2(-4) per areole, ± porrect or 1 ascending and 1 descending, straight or slightly curved, 10 mm; subcentral spines 0. Flowers 3-4 × 2.5-3.5 cm; outermost tepal margins densely minutely fimbriate (at 10×); inner tepals greenish white to greenish yellow, 14-17(-25) × 2-3 mm; stigma lobes yellow-green to pale green, 2-3 mm. Fruits whitish green proximally, pale green distally with pale rose-purple on sunlit portions, obovoid to clavate, 15-26 × 7.5-15 mm, juicy only in fruit walls; floral remnant persistent. Seeds reddish brown or orange, 1.1-1.3 × 0.9 × 0.8 mm, pitted; testa intermediate, anticlinal cell walls undulate, interstices much narrower than pit diam., pits deeply concave, elongate. 2n = 22.
Flowering Mar-May; fruiting Oct-Mar. Rocky slopes and ridges, ecotone between Sonoran desert scrub and chaparral, oak woodlands, grasslands; 1100-1800 m; Ariz.; Mexico (Sonora).
Mammillaria macdougalii is the characteristic flat-topped mammillaria of the mountains near Tucson and Nogales, Arizona. Records from farther east are all (insofar as can be determined) misidentifications of M. heyderi var. bullingtoniana.