Arabis lyallii S. Watson, Proc. Amer. Acad. Arts 11: 122. 1876; A. armerifolia Greene; A. densa Greene; A. drummondii A. Gray var. alpina S. Watson; A. drummondii var. lyallii (S. Watson) Jepson; A. multiceps Greene; A. murrayi G. A. Mulligan

Perennials; long-lived; (ces-pitose); sexual or apomictic; caudex woody. Stems usually 1 per caudex branch, arising from center of rosette near ground surface, 0.3-1.5(-2) dm, glabrous throughout. Basal leaves: blade linear-oblanceolate, 1-5(-8) mm wide, margins entire, usually ciliate at least proximally, trichomes (simple and/or short-stalked, 2- or 3-rayed), 0.1-0.3 mm, surfaces (sometimes the entire leaf) usually glabrous or (rarely, the youngest leaves of sterile shoots) pubescent, trichomes 4-6-rayed, 0.05-0.1 mm. Cauline leaves: 1-5, usually not concealing stem; blade auricles 0.5-1.5 mm or, rarely, absent, surfaces glabrous. Racemes 2-10(-15)-flowered, unbranched. Fruiting pedicels erect, straight, 3-8(-15) mm, glabrous. Flowers erect at anthesis; sepals glabrous; petals lavender to purplish, 6-8.5 × 1.5-3 mm, glabrous; pollen ellipsoid or spheroid. Fruits erect, appressed to rachis, not secund, straight, edges parallel, 3-5.6 cm × 1.5-2.5 mm; valves glabrous; ovules 34-64 per ovary; style 0.1-0.7 mm. Seeds biseriate or sub-biseriate, 1.5-2 × 1-1.5 mm; wing continuous, 0.3-0.5 mm wide.

Flowering Jun-Aug. Cliffs, talus slopes, gravelly soil in alpine and subalpine habitats; 1400-3700 m; B.C., Yukon; Calif., Idaho, Mont., Nev., Oreg., Utah, Wash.

The taxon traditionally treated as Arabis (Boechera) lyallii var. nubigena is here segregated as B. paupercula (see M. D. Windham and I. A. Al-Shehbaz 2006 for detailed comparison). Completely glabrous individuals of B. lyallii are sometimes confused with B. davidsonii, but they are easily distinguished by the absence of persistent leaf bases on caudex branches, erect and appressed (versus ascending) fruits, and biseriate to sub-biseriate (versus uniseriate) seeds. Both sexual and apomictic collections are known; further study is needed to determine whether they truly are conspecific.