6. Arabidopsis lyrata (Linnaeus) O’Kane & Al-Shehbaz, Novon 7: 325. 1997.
Herbs, biennial or perennial with a caudex and stolons. Stems (5–)10–30(–45) cm, erect or decumbent, 1 to numerous from base, usually branched and straight or flexuous above, basally pilose or subhirsute with simple and forked trichomes, apically glabrous. Basal leaves rosulate; petiole 0.4–2(–6) cm; leaf blade oblanceolate or ovate, (0.5–)1–3(–8) x (0.3–)0.5–1.5 cm, adaxially with a mixture of simple and stalked, 1-forked trichomes, rarely glabrous, margin entire, dentate, lyrate, or pinnatifid with 1–3 lateral lobes on each side, apex obtuse; cauline leaves shortly petiolate and uppermost often sessile; blade of middle ones oblanceolate, (0.5–)1–3.5(–5.5) x (0.1–)0.2–0.7(–1.3) cm, entire, repand, or obscurely toothed, rarely lower ones lobed, gradually reduced in size upward. Fruiting pedicels ascending to divaricate, straight, (0.4–)0.7–1.2(–1.5) cm. Sepals 2–3 mm, glabrous or densely pubescent, lateral pair saccate; petals white, rarely lavender or purplish, spatulate or obovate, 4–8(–9) x 1.5–3(–3.5) mm; claw not toothed; filaments white, 2–3 mm; ovules 20–36 per ovary. Fruits linear, flattened, torulose, (1–)2–4.5(–6) cm x 0.5–1.5 mm; valves with a distinct midvein extending full length; gynophore to 0.5 mm; style obsolete or to 1 mm. Seeds oblong, flattened, light brown, 0.8–1.2 mm, wingless, sometimes distally margined; cotyledons accumbent. 2n = 16, 32.
Habitat: cliffs, calcareous ledges and bluffs, rock crevices, ledges, thickets, sandy areas in open woods, sand dunes, river banks, flooded areas, sandstone hills, serpentine rocks and barrens, shale, talus, tundra, roadsides, gravelly slopes, forests, volcanic ash.
Elevation: 0–3500 m.
Distribution: Austria, Canda (Alberta, British Columbia, Manitoba, Northwest Territoris, Ontario, Saskatchewan, Yukon), Czech Republic, England, Germany, Hungary, Iceland, Ireland, N. Italy, Norway, NE European Russia, Scotland, Sweden, Ukraine, United States (Alaska, Connecticut, Delaware, District of Columbia, Illinois, Indiana, Iowa, Maryland, Massachusetts, Michigan, Minnesota, Mississippi, Missouri, New Jersey, New York, North Carolina, Ohio, Pennsylvania, Tennessee, Vermont, Virginia, West Virginia, Wisconsin).