2a. Arabidopsis arenosa (Linnaeus) Lawalrée subsp. arenosa
Biennials or perennials; (caudex short); glabrous or pubescent, trichomes simple with or stellate or forked ones. Stems often several from base, erect or decumbent, usually branched distally, (0.5-)1.5-6(-8) dm, pubescent basally with simple, 1-forked, or 3-rayed stellate trichomes, sparsely pubescent or glabrous apically. Basal leaves: petiole (0.6-)1.5-3.5(-5) cm; blade oblanceolate, (0.6-)2-6(-9) cm × (3-)10-20(-30) mm, margins pinnatisect or pinnatipartite, with 3-10 lobes on each side, apex obtuse to acute, surfaces sparsely to densely pubescent. Cauline leaves shortly petiolate or sessile; blade oblanceolate, (0.4-)1.5-5(-6.5) cm × (2-) 8-20(-30) mm (gradually smaller distally), margins usually pinnatifid to coarsely toothed, rarely subentire. Fruiting pedicels divaricate or ascending, (4-)5-12(-15) mm. Flowers: sepals (1.7-)2-3 mm, lateral pair subsaccate basally, (glabrous or pubescent); petals white or lavender, spatulate or obovate, (5-)6-8(-9) × (2.5-)3-5 mm, (claw to 2 mm, with a tooth on each side); filaments 2.5-4.5 mm. Fruits torulose, flattened, (1.5-)2-4(-4.8) cm × 0.8-1.2 mm; valves each with distinct midvein; style (0.2-)0.5-0.9(-1.3) mm. Seeds light brown, (flattened), ovoid, 0.8-1 mm; cotyledons accumbent. 2n = 16, 32.
Flowering Jul. Above fjords; 0 m; Greenland; Europe.
This is the first record of subsp. arenosa from North America. It is based on M. P. Porsild s.n. (MO) collected from western Greenland, 28 July 1935. It is not known if the occurrence in Greenland represents an introduction or the westernmost extent of its native range.