2. Arabidopsis arenosa (Linnaeus) Lawalrée, Bull. Soc. Roy. Bot. Belg. 42: 242. 1969.
Herbs, biennial or short-lived perennial with a caudex. Stems (5–)8–80(–100) cm, erect, simple or with few to many branches from the base, usually branched above, basally pilose to subhirsute with simple trichomes to 1 mm, apically glabrous. Basal leaves rosulate; petiole (0.3–)0.7–2(–3) cm, pilose; leaf blade obovate to oblanceolate, 1–4.5(–6.5) x 0.5–1.3(–1.7) cm, sparsely to densely pubescent with simple and short-stalked forked trichomes, margin pinnatifid to lyrate-pinnatifid and with 3–11 lateral lobes on each side, rarely dentate, apex obtuse to subacute; terminal lobe considerably larger or nearly as large as adjacent lateral lobes; cauline leaves 3–10, short petiolate or uppermost sessile; blade lanceolate to narrowly oblong, middle ones 1–4 (–5) x 0.3–1.5 cm, margin pinnatifid to dentate on lower leaves, dentate or entire on uppermost ones, apex acute to obtuse. Fruiting pedicels slender, divaricate, straight, 4–10(–15) mm. Sepals 2.2–3 mm, glabrous or distally sparsely pilose with simple trichomes, lateral pair saccate; petals white to lavender, obovate, 5–8 x 2.5–4 mm, apex obtuse or truncate; claw often apically with a pair of minute teeth; filaments white, 3–4 mm; ovules 28–74 per ovary. Fruits linear, distinctly compressed, smooth to slightly torulose, (1–)2–4(–5) cm x (0.8–)1–1.5 mm; valves with a distinct midvein; style to 0.5 mm; gynophore to 0.5 mm. Seeds oblong, compressed, light to reddish brown, (0.6–)0.8–1.2 mm, obscurely to prominently winged distally; cotyledons accumbent. 2n = 16, 32.
Habitats: forest margins, roadsides, railroad tracks, river or stream banks, trails, disturbed areas, slopes, fields, grassy areas.
Elevation: 0–2100 m.
Distribution: Europe: native in Austria, Belarus, E Belgium Bosnia Herzegovina, Bulgaria, Croatia, Czech Republic, NE France, Germany, Hungary, N Italy, Latvia, Lithuania, Macedonia, Poland, Romania, Slovakia, Slovenia, Switzerland, Ukraine, and Yugoslavia; naturalized in Belgium, Denmark, Estonia, Finland, Greenland, Holland, Norway, Russia and W Siberia, and Sweden.