Perennial herb (occasionally annual or ± shrubby), flowering the first year, up to 60 cm. high (rarely to 3 m.). Stems slender, branching from near the base, erect or ± spreading, terete or flattened or ± ridged, with raised lines decurrent from the stipules and the leaf-bases, glabrous to pubescent or densely hirsute. Leaves not crowded, subsessile or narrowing to a short petiole; lamina (1) 2–6.5 x 0.2–2.2 cm., linear to elliptic or obovate, acute to obtuse or rounded, cuneate at the base, entire or ± crenate to serrate-dentate, membranous or ± coriaceous, glabrous to pubescent or densely hirsute, venation prominent or not; stipules c. 1.5–4 mm. long, subulate, glabrous or pubescent, terminating in a gland. Flowers solitary in the axils of foliage leaves. Pedicels 4–18 mm. long, slender, glabrous or ± pubescent, with two triangular or subulate bracteoles in the upper half. Sepals 3–4 mm. long; subequal, lanceolate, acute, ± prominently carinate, glabrous or ciliate-pubescent (especially the keel). Anterior petal (5) 10–15 (20) mm. long, including a short obtuse spur, unguiculate, the claw exceeding the calyx, the lamina varying in shape from suborbicular or obcordate to subquadrate and in colour from rose-pink to violet or bluish, glabrous or ciliate; lateral petals asymmetric, ± triangular-ascending, rounded and often darker in colour at the apex; dorsal petal symmetric, triangular-acuminate, often expanded and darker at the apex. Androecium shorter than the lateral petals, the 3 lateral stamens with short filaments, the 2 anterior ones with longer filaments bearing pilose spur-appendages; anthers oblong-elliptic, glabrous or pilose, with a rounded connective-appendage. Ovary ovoid-globose, glabrous; style about twice as long as the ovary, expanded above, ascending. Capsule 5–8 mm. long, 3-sided, pale yellow, glabrous, coriaceous, smooth or slightly reticulate. Seeds c. 1–5 mm. long, c. 9–12 per capsule, ovoid-ellipsoid, longitudinally ribbed, glabrous.
