Arabis glabra, tower mustard

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Biennial or short-lived perennial, 3.5-12 (15) dm tall (measurement includes the erect siliques), the first-year's growth a rosette, with a solitary stem in subsequent years; herbage hirsute below with spreading, simple or 2- or more-branched, appressed hairs, often glabrous above, the leaves glaucous; stems stout, erect, simple, rarely branched above; basal leaves oblanceolate, sometimes broadly so, tapering to a winged petiole, rounded to obtuse apically, dentate or sometimes entire, usually withered by anthesis; cauline leaves many, often overlapping, 5-14 cm long, 13-25 (40) mm wide, oblong-lanceolate, sessile, auriculate or sagittate, acute, obtuse, or rounded apically; racemeglabrous throughout; sepals 2.4-4.5 mm long, oblong, obtuse to rounded, pale yellow to purplish; petals 4-7 mm long, long-clawed, oblanceolate or narrowly spatulate, cream to yellowish, rarely pink; fruiting pedicels appressed-ascending, 5-12 (14) mm long; siliques strictly erect and closely appressed to the stem, straight, 6-9 cm long, 1-1.3 mm wide, the valves 1-nerved the full length or to about midlength; stigma much enlarged, conspicuously bilobed, sessile or on a short, stout style; seeds more or less in 2 rows in each locule, 1-1.3 mm long, polyhedral with 5-8 faces, orangish, narrowly winged at the apex or wingless, the cotyledons acumbent; 2n = 12, 16, 32.

Cronquist, A., Holmgren, N.H., and Holmgren, P.K. (2005) "Intermountain Flora" pg. 352, vol. 2 part B, The New York Botanical Garden New York.