Erect, branching, annual herb to 0.6 m. tall; stems striate, glabrous or very sparsely glandular-puberulous when very young. Leaves 3-foliolate; leaflets 1–6.5 x 0.05–2.5 cm., linear, narrowly lanceolate or narrowly ovate, acuminate at the apex, cuneate at the base, sparsely and minutely asperulous at least on the margins; petiole up to 6.5 cm. long, glabrous. Inflorescence of moderately dense terminal racemes, rapidly elongating in fruit; bracts c. 1 mm. long, caducous, subulate; pedicels up to 4 mm. long, glandular-puberulous, slender. Sepals purplish at the tips, up to 5 mm. long, linear-subulate, unequal, glabrous or sparsely setulose on the keel and margins. Petals mauve, the two upper ones with a bright yellow spot surrounded by a short, dark purple line, 8–14 x 1.5–2.5 mm.; lamina narrowly elliptic-oblong, acute or rather blunt, basal claw almost as long as the lamina. Stamens 6–12, all fertile though some have smaller anthers; filaments slender, rather unequal, the longer at length slightly exceeding the petals; anther-thecae up to 2 mm. long, linear-oblong. Ovary linear, glabrous, on a gynophore c. 1.5 mm. long; style 0.5 mm. long; stigma capitate. Capsule 4–9 x 0.15–0.2 cm., on a glabrous gynophore up to 1.3 cm. long; valves linear, tapering to both ends, glabrous, with about 3 longitudinal nerves; persistent style 2–4 mm. long. Seeds c. 1.5 mm. in diam., brown, transversely ridged.