Madroño 4: 205, plate 31, figs. 1–10. 1938.
Annuals; (somewhat glaucous), hirsute. Stems unbranched or branched, (often bristly proximally), 0.2–0.9 dm. Basal leaves (soon withered); not rosulate; shortly petiolate; blade oblong-orbicular to obovate, 0.7–1.5 cm, margins dentate. Cauline leaves: blade broadly ovate to obovate, 0.8–1.7 cm × 4–13 mm (smaller distally), base amplexicaul, margins dentate. Racemes ebracteate, (with a terminal cluster of sterile flowers). Fruiting pedicels divaricate, 2–3 mm. Flowers: calyx narrowly campanulate; sepals green to purplish or (sterile flowers) lilac-purple, (narrowly ovate, sterile flowers linear-lanceolate), 3–5 mm, (8–13 mm in sterile flowers), keeled basally (not keeled in sterile flowers; sparsely hirsute in fertile flowers, glabrous in sterile flowers); petals purple (with darker purple veins), 8–11 mm, blade 4–6 × 2.5–3.5 mm, margins not crisped, (flaring), claw 4–5 mm, narrower than blade; stamens in 3 unequal pairs, (purple); filaments: abaxial pair (connate), 4–5 mm, lateral pair 2–3 mm, adaxial pair (connate nearly to apex), 5–6.5 mm; anthers: abaxial and lateral pairs fertile, 1.4–1.8 mm, adaxial pair sterile, 0.4–0.8 mm; gynophore 0.1–0.3 mm. Fruits divaricate, smooth, curved upward, slightly flattened, 1.3–2.5 cm × 2.5–3.5 mm; valves each with prominent midvein, (hispid, trichomes setiform, 0.5–0.8 mm); replum straight; ovules 46–60 per ovary; style 0.2–0.5 mm; stigma slightly 2-lobed. Seeds ovoid (plump), 1.2–1.5 × 0.6–0.8 mm; wing absent. 2n = 28.
Phenology: Flowering Apr–May.
Habitat: Gravelly sedimentary scree and lag-barrens in chaparral-oak woodlands
Elevation: 500-900 m
Of conservation concern.
Streptanthus callistus is known from the Mount Hamilton Range of Santa Clara County and is considered to be the most endangered species in the genus.