Arabis williamsii Rollins, Syst. Bot. 6: 62, fig. 2. 1981

Perennials; short-lived; sexual; caudex not woody. Stems usually 1 per caudex branch, arising from center of rosette near ground surface, 0.7-2.2 dm, glabrous throughout. Basal leaves: blade oblanceolate, 1-2 mm wide, margins entire, ciliate near petiole base, trichomes (simple), to 0.4 mm, surfaces sparsely to densely pubescent, trichomes short-stalked, 4-8-rayed, 0.03-0.1 mm. Cauline leaves: 6-11, sometimes concealing stem proximally; blade auricles to 0.5 mm, surfaces of distalmost leaves glabrous. Racemes 5-17- flowered, unbranched. Fruiting pedicels erect, straight, 2-4(-7) mm, glabrous. Flowers erect at anthesis; sepals glabrous; petals white, 3-4 × 0.5-0.8 mm, glabrous; pollen ellipsoid. Fruits erect, usually appressed to rachis, not secund, straight, edges parallel, 2-3.5 cm × 2.2-2.5 mm; valves glabrous; ovules 60-96 per ovary; style 0.05-0.2 mm. Seeds sub-biseriate, 0.7-0.9 × 0.5-0.6 mm, not winged or minutely winged distally.

Flowering May-Jun. Gravelly soil in sagebrush-grassland communities; 2300-2800 m; Wyo.

Boechera williamsii is distinguished from all other sexual species of the genus by its combination of relatively short (3-4 mm), white petals, minute, 4-8-rayed trichomes on basal leaves, erect, appressed fruits, and relatively small, wingless seeds. It is known only from Fremont and Sublette counties in west-central Wyoming. The taxon traditionally treated as Arabis (Boechera) williamsii var. saximontana is here recognized as an apomictic species of hybrid origin (see M. D. Windham and I. A. Al-Shehbaz 2007b for detailed comparison).