1. Arabidopsis arenicola (Richardson ex W. J. Hooker) Al-Shehbaz, R. Elvin, D. Murray & S. I. Warwick, Can. J. Bot. 84: 279. 2006; Eutrema arenicola Richardson ex W. J. Hooker, Fl. Bor.-Amer. 1: 67, plate 24. 1833; Arabis arenicola (Richardson ex W. J. Hooker) Gelert, Bot. Tidsskr. 21: 289. 1898; Parrya arenicola (Richardson ex W. J. Hooker) J. D. Hooker, Trans. Linn. Soc. London 23: 285. 1861. TYPE: Canada, “deep sand upon shores of Arctic America between longitude 107º and 150º,” Richardson s.n. (holotype, not seen; isotype, CAN!).

Sisymbrium humifusum Vahl, Fl. Dan. t. 2297. 1840; Arabis humifusa (Vahl) S. Watson, Proc. Amer. Acad. Arts 25: 124. 1890. TYPE: plate 2297 above.

Arabis humifusa (Vahl) S. Watson var. pubescens S. Watson in Gray & Watson, Syn. Fl. N. Amer. 1: 160. 1895; A. arenicola var. pubescens (S. Watson) Gelert, Bot. Tidsskr. 21: 290. 1898. TYPE: Canada, Manitoba, York Factory, Hudson Bay, Bell s.n. (holotype, not seen; isotype, CAN).

Herbs, perennial with branched or rarely simple caudex covered with petiolar remains of previous years, glabrous throughout or sparsely to moderately pubescent with simple and short-stalked, forked trichomes to 0.5 mm. Stems 0.5–2(–3) dm, erect, ascending, or decumbent, simple or few branched above, often several caudex, glabrous or sparsely pubescent rarely above middle. Basal leaves rosulate, fleshy; petiole 0.5–2 cm, ciliate or glabrous; leaf blade oblanceolate to spatulate, 0.5–2 cm x 2–6 mm, pubescent or glabrous, margin entire or dentate, not ciliate, apex obtuse or acute; cauline leaves (1–)2–4(–6), sessile, oblong to oblanceolate, 0.7–1.8(–2.5) cm x 2–5 mm, glabrous or sparsely pubescent, base not auriculate, margin entire or rarely dentate, apex obtuse or acute. Racemes simple, dense; fruiting pedicels ascending to divaricate, 5–10(–12) mm, slender, glabrous. Sepals oblong or ovate, 1.7–3 x 1–1.5 mm, the lateral pair not saccate; petals white, oblanceolate or rarely spatulate, 4–5 x 1–2 mm, apex rounded; claw entire; filaments slender, 1.7–3 mm; anthers ovate, 0.4–0.5 mm; ovules 30–54 per ovary. Fruit linear, (0.8–)1–2(–2.8) cm x 1.5–2.2(–2.5) mm, suberect to divaricate, terete or only slightly flattened, straight; valves glabrous, smooth, with an obscure midvein; style 0.4–0.8 mm. Seeds reddish brown, narrowly oblong, 0.9–1.1 x 0.5–0.6 mm, uniseriate or sub-biseriate, pointed distally; cotyledons incumbent. 2n = 32.

Flowering: Jun–Aug.

Habitat: sandy beaches, gravel, flats of riverbeds.

Elevation: 0–1500 m.

Distribution: Canada (Newfoundland, Labrador, Manitoba, Nunavut, Ontarioa, Quebec, Saskatchewan), Greenland.