2. Thlaspi arvense L. (field penny cress, stinkweed)

Pl. 327 i–k; Map 1395

Plants with a sometimes faint fetid odor when bruised or crushed, sometimes glaucous. Stems (9–)15–55(–80) cm long, glabrous. Leaves 0.5–7.0(–8.0) cm long, linear to ovate or broadly elliptic or oblong, the basal auricles pointed or rarely rounded, the margin often toothed, the basal leaves few or absent at flowering time. Sepals (1.5–)2.0–3.0(–3.3) mm long. Petals 3–4(–5) mm long. Styles 0.1–0.3 mm long. Fruits 8–20 mm long, broadly elliptic or obovate to nearly circular in outline, flattened, the margins broadly winged their entire length, the apical notch narrowly U-shaped and longer than deep. Seeds mostly 3–8 per locule, 1.6–2.0(–2.3) mm long, broadly ellipsoid to obovoid, the surface with a series of concentric, arched ribs, dark gray to black. 2n=14. April–June.

Introduced, common throughout Missouri (native of Europe, widely naturalized in North America). Moist, disturbed depressions of upland prairies and banks of streams; also crop fields, fallow fields, old fields, pastures, lawns, railroads, roadsides, and open, disturbed areas.

When not in fruit, this species can be difficult to distinguish from Microthlaspi perfoliatum. It differs in lacking glaucousness, in having slightly larger flowers, and in having usually somewhat more toothed middle and upper leaves. In Missouri, T. arvense is more variable morphologically than M. perfoliatum and can be far more robust than that species.