Plants sparsely to densely pubescent proximally, some-times glabrous distally. Stems (0.2-)0.5-3(-5) dm. Basal leaves: petiole 0.1-0.5 cm; blade oblanceolate, obovate, or oblong, 0.3-1.5(-2) cm × (1-)2-6(-8) mm, margins entire or dentate, apex acute to obtuse, surfaces uniformly pubescent, trichomes cruciform and 3-rayed, adaxially sometimes mixed with coarse, simple ones. Cauline leaves 1-6; blade similar and equaling or smaller than basal. Fruiting pedicels slender, (1.5-)2-4(-6) mm. Flowers: sepals 0.5-1 × 0.3-0.7 mm; petals oblong, 1.5-3 × 0.5-1 mm; filaments 0.4-0.6 mm; anthers (0.05-)0.1-0.2(-0.25) mm. Fruits usually orbicular to obovate, rarely broadly oblong, 1-locular, not twisted; valves thin, veins sometimes prominent, usually pubescent, rarely glabrous, trichomes simple, to 0.5 mm, hooked (sometimes restricted to margins, or mixed with smaller, sessile, cruciform or 3-rayed ones); septum absent; ovules 2(-4) per ovary; style 0.05-0.2 mm. Seeds 1, (0.8-)0.9-1.1(-1.2) × (0.5-)0.7-0.9(-1) mm. 2n = 26.
Flowering Feb-Jun. Open grassy slopes, grassy glades in woodlands, chaparral, sandy and gravelly flats, flood plains, rock outcrops, cliffs and ledges, on limestone, serpentine, sandstone, granitic and basaltic substrates; 0-1800 m; B.C.; Ariz., Calif., Idaho, Mont., Nev., Oreg., Wash.; Mexico (Baja California).
Records of Athysanus pusillus for Montana appear to be solely from Ravalli County, and for Nevada from Storey and Washoe counties.
FNA 2010, Kearney and Peebles 1969
Duration: Annual Nativity: Native Lifeform: Forb/Herb General: Small, herbaceous annual, erect or prostrate, 2-25 cm, pubescent to strigose, hairs simple or 2-5 branched, leafy, especially towards the base. Leaves: Basal leaves oblanceolate to oblong, to 10 mm, pubescent or strigose with coarse hairs, cauline leaves sessile, somewhat smaller than the basal leaves. Flowers: Pretty white flowers with minute oblong petals, borne in terminal racemes, racemes elongate, 1-sided, flowers bisexual with 4 sepals. Fruits: Flat, obovate to oblong capsule with white hairs forming a halo around the margins, hairs hooked, simple, or 3-rayed, surface pubescent with prominent veins, ovary with one locule. Ecology: Found on dry, sandy and gravelly soils, on serpentine, limestone, sandstone, granitic, and basaltic substrates in grasslands, floodplains, woodlands, and chaparral, from 0-6,000 ft (0-1829 m); flowers February-June. Distribution: Idaho to British Columbia, AZ and CA Notes: The hairs radiating from the margins of the flat pods are a good indicator for this species. Ethnobotany: Unknown Synonyms: Athysanus pusillus var. glabrior, Thysanocarpus oblongifolius, Thysanocarpus pusillus Editor: LCrumbacher, 2011 Etymology: Athysanus comes from the Greek a, "without," and thusanos, "fringe," and referring to the wingless fruit, while pusillus means small, weak or insignificant.