4. Lepidium densiflorum Schrad. (pepper grass, green-flowered pepper grass)

Pl. 323 e, f; Map 1363

Plants annual or biennial. Stems 10–50(–65) cm long, erect or ascending, these and rachis of inflorescence pubescent with minute, straight, spreading hairs with rounded tips (visible with magnification, appearing as papillae). Leaves (1.0–)2.5–8.0(–11.0) cm long, linear to oblanceolate or elliptic in outline, the bases not clasping, glabrous or less commonly minutely hairy on the undersurface, the lower and basal leaves often 1 time pinnately lobed, the margins entire to coarsely toothed, the upper leaves reduced, linear, the margins usually entire. Sepals 0.5–0.8(–1.0) mm long, linear to narrowly elliptic. Petals absent or less than 1 mm long, white. Stamens 2. Styles 0.1–0.2 mm long. Fruits (2.0–)2.5–3.0(–3.5) mm long, broadly obovate, widest above the middle, the tip shallowly notched and sometimes narrowly winged, strongly flattened, glabrous, the stalks pubescent on the upper surface. Seeds 1.1–1.4(–1.5) mm long, narrowly obovate to elliptic in outline, wingless or obscurely winged around most of the margin, the surface minutely roughened, light orange. 2n=32. April–November.

Scattered nearly throughout Missouri (Canada, U.S., widely introduced in Europe, Asia). Glades, bluff tops, prairies, rocky openings of dry upland forests, pastures, crop fields, fallow fields, old fields, roadsides, railroads, and open, disturbed areas.

Lepidium densiflorum and L. virginicum are closely related and can be difficult to distinguish from each other. Some botanists have resorted to obscure characters of the embryos to separate them. Some authors have suggested that part of the problem may be because of hybridization between them, but this has not been documented cytologically (Al-Shehbaz, 1986). A few historical specimens from Jackson County may represent such hybrids. Morphological variation within the species also may account for some of the confusion. Several varieties have been named that are said to differ in position and density of the hairs and in other minor, mostly quantitative characters of the fruits. The Missouri plants have always been placed into var. densiflorum, but the varieties seem unworthy of taxonomic recognition.