65. Mentzelia affinis Greene, Pittonia. 2: 103. 1890.
Plants candelabra-form, 5–40(–50) cm. Basal leaves persisting; petiole present or absent; blade linear-lanceolate to linear, margins usually deeply to moderately lobed, sinuses extending 1/4+ to midvein, rarely entire. Cauline leaves: petiole absent; blade ovate-lanceolate to lanceolate, to 17 cm, margins usually dentate or entire, rarely deeply lobed. Bracts green, ovate to lanceolate, 2.7–6.6 × 0.9–2.1 mm, width 1/5–1/3 length, not concealing capsule, margins 3-lobed or entire. Flowers: sepals 1–7 mm; petals yellow to orange proximally, yellow distally, 4–12 mm, apex acute; stamens 20+, 3–6.5 mm, filaments monomorphic, filiform, unlobed; styles 3–6.5 mm. Capsules narrowly cylindric, 7–32 × 1–3 mm, axillary curved to 90° at maturity, often prominently longitudinally ribbed. Seeds 10–20, in 1 row distal to mid fruit, tan, dark-mottled or not, triangular prisms, surface ±smooth under 10× magnification; recurved flap over hilum absent; seed coat cell outer periclinal wall flat. 2n = 18.
Flowering Mar–May. Sandy, rocky, or gray-white silty soils, grasslands, creosote-bush scrub, Joshua-tree or saguaro woodlands; 0–1200 m; Ariz., Calif.; Mexico (Sonora).
Herbarium specimens of Mentzelia affinis are often difficult to distinguish from those of M. dispersa despite distinct evolutionary histories (J. M. Brokaw and L. Hufford 2010). Several characters, including flower size, leaf margins, and capsule surfaces, differ substantially between these species, but habitat is the most dependable diagnostic character. Verified populations of M. affinis have not been found above 1200 meters in desert habitats, and grassland populations are usually restricted to much lower elevations. Sympatric populations of M. affinis and M. dispersa have not been found, and, in areas of range overlap in southern California, M. dispersa has not been found below 1200 meters or in desert vegetation.