[Arabis exilis A. Nelson, moreArabis holboellii var. retrofracta (Graham) Rydb., Arabis holboellii var. secunda (Howell) Jeps., Arabis lignipes A. Nelson, Arabis retrofracta Graham, Arabis secunda T.J. Howell, Arabis tenuis Greene, Boechera exilis (A. Nelson) Dorn, Boechera holboellii var. secunda (Howell) Dorn]
Biennials or perennials; short-lived; sexual; caudex present or absent. Stems usually 1 per caudex branch, arising from center of rosette near ground surface, 1.5-7(-10.5) dm, densely pubescent proximally, trichomes short-stalked, 2-8-rayed, 0.1-0.2 mm, sparsely pubescent distally. Basal leaves: blade oblanceolate, 2-7 mm wide, margins entire or shallowly dentate, sometimes ciliate at petiole base, surfaces densely pubescent, trichomes short-stalked, 5-10-rayed, 0.05-0.2 mm. Cauline leaves: 15-40, concealing stem proximally; blade auricles 0.5-2.5 mm or, rarely, absent, surfaces of distalmost leaves pubescent. Racemes 15-80(-140)-flowered, usually unbranched. Fruiting pedicels reflexed, abruptly recurved at base, otherwise usually straight, 7-12 (-18) mm, pubescent, trichomes appressed, branched. Flowers pendent at anthesis; sepals pubescent; petals white to lavender, 4-8 × 0.8-2.2 mm, glabrous; pollen ellipsoid. Fruits strongly reflexed, usually appressed to rachis, sometimes somewhat secund, straight, edges parallel, 3.5-9 cm × 0.9-1.8 mm; valves glabrous or sparsely pubescent throughout; ovules 60-116 per ovary; style 0.5-2 mm. Seeds uniseriate, 1-1.4 × 0.9-1.2 mm; wing continuous, to 0.1 mm wide. 2n = 14. Flowering Apr-Aug. Rock outcrops, open hillsides, gravel bars and sandy banks in grassland, sagebrush, oak woodlands, and open conifer forests; 300-3300 m; Alta., B.C., Man., N.W.T., Ont., Que., Sask.; Alaska, Calif., Idaho, Mich., Mont., Nev., Oreg., Utah, Wash., Wyo. Though often treated as a variety of Arabis (Boechera) holboellii (e.g., R. C. Rollins 1993; G. A. Mulligan 1996; N. H. Holmgren 2005b), B. retrofracta is easily distinguished from that species by having narrower (0.9-1.8 versus 2-2.5 mm), mostly non-secund fruits that are almost always appressed to rachises. The two taxa have allopatric distributions, with B. retrofracta found on the North American continent (mostly west of the Great Plains) and B. holboellii apparently confined to Greenland. Boechera retrofracta has formed hybrids with at least 12 other species. Besides differing in macromorphological characters, all those hybrids are distinct from B. retrofracta in the strict sense in having wider (20-30 versus 13-16 µm), spheroid pollen grains with asymmetric colpi.
Arabis kochii Blankinship is an illegitimate name, sometimes found in synonymy with Boechera retrofracta.