in N. L. Britton et al., N. Amer. Fl. 25: 85. 1907.
Herbs, (5–)20–40(–50) cm, glabrous or glabrate; branches from distal nodes, alternate, virgate (proximal unbranched main axis long in comparison to distal portion). Leaves alternate; stipular glands usually present at all nodes; blade linear, 10–25(–30) × (0.5–)1–1.5 mm, base flat, not clasping, margins eglandular. Inflorescences: cymes monochasial, dense, sparingly branched, internodes sometimes all shortened, or long proximally, flowers clustered at inflorescence tips; bract margins eglandular. Pedicels 0.5–3 mm, scarcely longer in fruit, ascending, not bent at apex. Flowers: sepals erect, narrowly ovate, 3–4 mm, subequal, margins glandular-toothed, surfaces glabrous; petals widely spreading, yellow, often tinged with orange-red, obovate, (4–)6–7(–10) mm, apex obtuse, slightly erose; cup yellow, rim with petal attachments set in notches; stamens exserted; filaments (4–)6–8 mm; anthers yellow, dehisced anthers 1.5–2 mm; ovary chambers 6; styles 3, yellow, (3–)4–6(–8) mm, exserted. 2n = 36.
Phenology: Flowering May–Jun.
Habitat: Chaparral or grasslands, usually on serpentine soils, sometimes on nonserpentine soils.
Elevation: 30–700 m.
Hesperolinon breweri is found in the Vaca Mountains and vicinity in the southern Inner North Coast Ranges in Napa and Solano counties, and in the northeastern San Francisco Bay area on Mount Diablo in Contra Costa County. It is similar to H. californicum and can be distinguished by its bright yellow petals tinged with orange and yellow dehisced anthers, filaments, and cup, rather than petals white tinged with pink, and pink or white dehisced anthers. Hesperolinon breweri occurs within or at the edge of chaparral; H. californicum is found in open grassy areas adjacent to chaparral.