58. Boechera macounii (S. Watson) Windham & Al-Shehbaz, Harvard Pap. Bot. 11: 267. 2007; Arabis macounii S. Watson, Proc. Amer. Acad. Arts 26: 124. 1891; A. microphylla Nuttall var. macounii (S. Watson) Rollins, Rhodra 43: 428. 1941; Boechera microphylla (Nutt.) Dorn var. macounii (S. Watson) Dorn, Vasc. Pl. Wyoming, ed. 3, 376. 2001. TYPE: Canada, British Columbia: Revelstoke, 13 May 1890, John Macoun s.n. (holotype, GH!; isotypes, CAN!, CAS!, K!, MO!, NY!, US!).
Arabis densicaulis A. Nelson, Bot. Gaz. 30: 190. 1900. TYPE: United States, Wyoming, Park Co., Yellowstone National Park, Undine Falls, 6 Jul 1899, Aven Nelson & Elias Nelson 5680 (holotype, RM!; isotypes, E!, K!, MO!, NDG!, NMC!, NY!, RM!, US!).
Plants mostly long-lived perennials, often with woody caudices, lacking crowded, persistent leaf bases; apomictic, with spheroid pollen. Stems 1 or several per caudex branch, arising near ground surface from margin or center of basal rosettes, 2.0–3.6 dm, lower parts with abundant simple and short-stalked, 2-rayed trichomes 0.1–0.8 mm, upper parts glabrous. Leaves at stem bases oblanceolate to linear-oblanceolate, 1–3(–5) mm wide, entire or minutely toothed, ciliate proximally with simple trichomes to 0.7 mm, blade surfaces densely pubescent with short-stalked, 3–6-rayed trichomes 0.07–0.30 mm; cauline leaves (5–)9–16, often concealing stem proximally, the uppermost glabrous, with auricles 0.5–1.5 mm. Inflorescences mostly unbranched, 10–33-flowered; fruiting pedicels 5–13(–17) mm, divaricate-ascending to horizontal, straight, with few appressed, branched trichomes or rarely glabrous. Flowers divaricate-ascending at anthesis; sepals pubescent; petals lavender to purple, 4–5 ´ 0.5–0.8 mm, glabrous; ovules 56–78 per fruit. Fruits 3.5–6.5 cm ´ 0.8–1.2 mm, divaricate-ascending to descending, not appressed to rachises, not secund, usually curved, with parallel edges, glabrous; style 0.1–0.4 mm. Seeds uniseriate, 1.0–1.2 ´ 0.7–1.0 mm; wing often continuous, 0.05—0.15 mm wide.
Habitat: rocky hillsides in open pinewoods.
Elevation: 2200–2900 m.
Distribution: Canada (British Columbia), United States (Idaho, Montana, Wyoming).