Clypeola alyssoides Linnaeus, Sp. Pl. 2: 652. 1753; Alyssum calycinum Linnaeus

Annuals; canescent throughout, trichomes appressed, 6-10- rayed, mixed with simple and forked on pedicels and sepals. Stems simple or few to several from base, erect, ascending, or decumbent, (unbranched or branched distally), 0.5-3.5 (-5) dm. Cauline leaves subsessile or (proximal) shortly petiolate; blade usually narrowly oblanceolate to linear, sometimes spatulate or obovate, 3-4(-4.5) cm × (0.5-) 1-3.5(-5) mm, base attenuate or cuneate, apex obtuse or acute. Fruiting pedicels divaricate or ascending, straight, slender, 2-5(-6) mm, trichomes stellate, with fewer, simple and forked ones. Flowers: sepals (persistent) oblong, (1.5-)2-3 × 0.7-1.1 mm, pubescent as pedicels; petals (often persistent) white or pale yellow, usually linear to linear-oblanceolate, rarely obovate, 2-3(-4) × 0.3-0.7(-1) mm, apex emarginate, glabrous or sparsely stellate-pubescent abaxially; filaments (slender) not appendaged, toothed, or winged, 1-1.5 mm; anthers ovate, 0.15-0.2 mm. Fruits orbicular, (2-)3-4(-5) mm diam., apex emarginate or truncate; valves uniformly inflated at middle, strongly flattened at margins, sparsely stellate-pubescent; ovules 2 per ovary; style (slender), 0.3-0.6(-1) mm, basally stellate-pubescent or glabrous. Seeds oblong to ovoid, compressed, 1.1-2 × 0.7-1.1 mm, margins narrow, ca. 0.1 mm wide. 2n = 32.

Flowering May-Jul. Roadsides, railways, waste grounds, disturbed sites, grassy areas, fields, sagebrush flats, limestone ledges or bluffs; 0-1800 m; introduced; Alta., B.C., Man., Nfld. and Labr. (Nfld.), Ont., Que., Sask.; Alaska, Ariz., Calif., Colo., Conn., Idaho, Ill., Ind., Iowa, Kans., Maine, Md., Mass., Mich., Mont., N.J., N.Mex., N.Y., Ohio, Oreg., Pa., R.I., S.Dak., Utah, Vt., Va., Wash., W.Va., Wis., Wyo.; Europe; sw Asia; n Africa.