J. Arnold Arbor. 21: 71. 1940
Plants perennial, with tubers. Stems erect, to 50 cm. Leaves: petiole mostly 3–20 mm, less than 3 mm only on smallest, distalmost leaves; blade ovate to hastate, basally lobed or unlobed, to 4 × 3 cm, base usually acute to obtuse, sometimes attenuate or hastate, margins usually serrate, sometimes crenate, apex acute. Pedicels (fruiting) (0–)2 × 2 mm. Flowers: petals orange, 5–11.5 × 3–5 mm, apex acute to rounded, hairy abaxially on distal 1/2; stamens 15–45, 4–8 mm, filaments usually monomorphic, filiform, rarely heteromorphic, outermost slightly spatulate, inner filiform; style 3–7.5 mm. Capsules subcylindric to clavate, 6–10 × 1.5–2.3 mm, base tapering gradually, capsule and pedicel not well-differentiated, walls thin, brittle. Seeds 2–3 per capsule, oblong, with transverse folds. 2n = 22.
Phenology: Flowering May–Nov.
Habitat: Steep limestone cliffs, gravelly slopes of gypseous clayey soils, desert scrub and Larrea communities.
Elevation: 900–2000 m.
Tex., Mexico (Chihuahua, Coahuila, Durango, Nuevo León).
Mentzelia pachyrhiza, native to the Chihuahuan Desert, reaches the flora area only in southern Brewster and Presidio counties. In southwestern Texas and northeastern Chihuahua, where M. pachyrhiza and M. oligosperma have overlapping ranges, H. J. Thompson and A. M. Powell (1981) reported that the former was found at elevations usually below 1100 meters in the Larrea zone, below the elevation of junipers and M. oligosperma. Thompson and Powell allied M. pachyrhiza with M. oligosperma and the South American M. grisebachii Urban & Gilg, now treated as a synonym of M. parvifolia Urban & Gilg ex Kurtz (M. Weigend 2007b). This placement is consistent with phylogenetic studies (J. Grissom and L. Hufford, unpubl.), which show that M. pachyrhiza is the sister species of M. parvifolia; they together are the sister of M. oligosperma and the Mexican M. pattersonii B. L. Turner. I. M. Johnston’s (1940) assertion that the large tuber of M. pachyrhiza is unique in sect. Mentzelia is incorrect. Tubers, often carrot-shaped, are common among the perennial species of the section.