Streptanthus micranthus A. Gray, Mem. Amer. Acad. Arts, n.s. 4: 7. 1849; Heterothrix micrantha (A. Gray) Rydberg; Pennellia robinsonii Rollins; Thelypodium longifolium (Bentham) S. Watson var. catalinense M. E. Jones; T. micranthum (A. Gray) S. Watson

Stems simple or several from base, 2-12.3 dm, pubescent proximally, trichomes predominantly dendritic, mixed with simple and long-stalked, 2-rayed ones, glabrous or sparsely pubescent distally, trichomes dendritic. Basal leaves caducous; petiole 0.5-4 cm; blade oblanceolate, (1-)3-5 cm × 6-34 mm, margins sinuate, lobed, dentate, or, rarely, subentire, surfaces pubescent, trichomes dendritic. Cauline leaves (distal) subsessile; blade oblanceolate, 3-10 cm × 1-20 mm, surfaces glabrous or glabrate. Racemes not secund, 1.5-6.7 dm in fruit. Fruiting pedicels slightly erect to divaricate-ascending, straight, (3-)3.5-8.5(-11) mm. Flowers: sepals purple or green, broadly oblong, 2.7-4 × 1.2-2 mm; petals purplish apically, spatulate to oblanceolate, 1.5-4(-5) × 0.5-2.2 mm; filaments 0.9-2.6 mm; anthers 1-1.2 mm. Fruits erect to ascending, straight, (1.5-)2.2-5.8 cm × 0.6-1.8 mm, terete; valves glabrous or sparsely pubescent; septum transparent; ovules 90-140 per ovary; style obsolete to 0.5 mm. Seeds biseriate or uniseriate, plump, not winged, oblong, 0.7-0.8 × 0.4-0.5 mm; seed coat mucilaginous when wetted; cotyledons incumbent.

Flowering Jun-Sep. Pine-oak forests, open grasslands, rolling andesitic loam soils, among volcanic rocks, igneous cliffs; 1100-3600 m; Ariz., N.Mex., Tex.; Mexico (Chihuahua, Coahuila, Distrito Federal, Durango, state of México, Michoacán, Querétaro, San Luis Potosí, Sonora).