1. Idahoa scapigera (W. J. Hooker) A. Nelson & J. F. Macbride, Bot. Gaz. 56: 474. 1913; Platyspermum scapigerum W. J. Hooker, Fl. Bor.-Amer. 1: 68, plate 18, fig. B. 1830. TYPE: United States, “moist rocks and in shallow soil at the Great Falls of the Columbia, 1825, D. Douglas s.n. (holotype, K!; isotypes, BM!, GH!).

Herbs, annual, glabrous throughout. Basal leaves rosulate; petiole (0.2 –)0.5 –2( –3) cm; leaf blade ovate, obovate or spatulate, (3 –)7 –20( –25) x 2 –8( –12) mm, margin entire to lyrate; cauline leaves absent. Fruiting pedicles (2 –)4 –15( –22) per plant, slender, straight, ascending or erect, (1.5 –)2.5 –11( –13) cm. Sepals purplish, oblong or ovate, 1.5 –1.8 x 0.5 –0.8 mm, margin not hyaline; petals white, oblanceolate, 2 –2.5 x 0.5 –0.8 mm, attenuate to base, obtuse at apex; filaments 1.4 –1.6 mm; anthers ovate, 0.15 –0.2 mm. Fruits orbicular or orbicular-ovate and only slightly longer than broad, (5 –)6 –9( –10) mm, strongly latiseptate; valves obscurely veined, rounded at both ends; gynophore obsolete or rarely to 1 mm; style subconical, 0.3 – 0.8 mm. Seeds brown, strongly compressed, orbicular, 3.5 –5 mm in diam., coarsely reticulate except for wing; wing 0.8 –1.2 mm wide. 2n = 16.

Flowering: Mar –May.

Habitat: sagebrush slopes and scabland, dry ground, rocky volcanic flats and outcrops, open moist slopes and banks, gravelly hills, meadows, rocky hillsides, seepy sunny slopes.

Elevation: 300 –1700 m.

Distribution: Canada (British Columbia), United States (California, Idaho, Montana, Nevada, Oregon, Washington).