13. Lepidium didymum Linnaeus, Syst. Nat. ed. 12. 2: 433. 1767; Mant Pl. 1: 92. 1767.
Carara didyma (Linnaeus) Britton; Coronopus didymus (Linnaeus) Smith; Senebiera didyma (Linnaeus) Persoon; S. incisa Willdenow; S. pinnatifida de Candolle
Annuals; (fetid); glabrous or pilose. Stems few to several from base, erect to ascending or decumbent, branched distally, 1-4.5(-7) dm. Basal leaves (soon withered); not rosulate; petiole 0.5-4(-6) cm; blade 1- or 2-pinnatisect, 1-6(-8) cm, margins (of lobes) entire or dentate (sometimes deeply lobed). Cauline leaves shortly petiolate to subsessile; blade similar to basal, smaller and less divided distally, lobes lanceolate to oblong or elliptic, 1.5-3.5(-4.5) cm × 5-12 mm, base not auriculate, margins (of lobes) entire, serrate, or incised. Racemes elongated in fruit; rachis glabrous or pubescent, trichomes straight, cylindrical. Fruiting pedicels divaricate to horizontal, straight slightly recurved, (terete), 1.4-2.5(-4) × 0.15-2 mm, glabrous or sparsely pubescent adaxially. Flowers: sepals (tardily deciduous), ovate, 0.5-0.7(-0.9) mm; petals white, elliptic to linear, 0.4-0.5 × ca. 0.1 mm, claw absent; stamens 2, median; filaments 0.3-0.6 mm; anthers 0.1-0.2 mm. Fruits schizocarpic, didymous, 1.3-1.7 × 2-2.5 mm, apically not winged, apical notch 0.2-0.4 mm deep; valves thick, rugose, strongly veined, glabrous; style absent or obsolete, included in apical notch. Seeds ovate, 1-1.2 × 0.7-0.8 mm. 2n = 32.
Flowering Mar-Jul. Roadsides, waste areas, lawns, pastures, fields, gardens, disturbed areas; 0-1000 m; introduced; B.C., N.B., Nfld. and Labr. (Nfld.), N.S., Que.; Ala., Ariz., Ark., Calif., Conn., Fla., Ga., La., Maine, Md., Mass., Miss., Mo., N.J., N.Y., N.C., Ohio, Oreg., Pa., S.C., Tenn., Tex., Va., W.Va., Wis.; South America; introduced also in Mexico (Sinaloa), Central America (Honduras), Europe, Asia, s Africa, Australia.