Plants annual, 30-60 cm tall. Stem erect in upper part, prostrate basally, slender, glabrous; nodes slightly swollen, with adventitious roots. Leaves opposite, sessile or subsessile; leaf blade gray-green abaxially, green adaxially, linear or linear-lanceolate, rarely obovate, 2-10 × 0.5-1 cm, rigidly papery, with stipular basal glands, abaxially glabrous, adaxially scabrous, lateral veins 5-7 pairs, inconspicuous, base subcordate, truncate, or rounded, margin remotely spiniform-serrate, apex acute or slightly obtuse. Inflorescences 1-flowered, or flowers 2- or 3-fascicled in leaf axils; peduncles absent. Pedicels 2-4.4 cm, slender, often scabrous hairy on one side, bracteate at base; bracts linear. Flowers purple-red or white, large. Lateral sepals 2, linear, ca. 1 × 0.1 cm, apex acute. Lower sepal funnelform, ca. 1.5 cm deep, striate, gradually narrowed into an incurved or involute, slender spur. Upper petal orbicular, ca. 1 cm in diam., apex emarginate, mucronulate, abaxial midvein narrowly carinate; lateral united petals not clawed, 1.4-1.5 cm, 2-lobed; basal lobes suborbicular, small; distal lobes broadly obovate to dolabriform (axe-shaped), apex rounded-obtuse; auricle narrow. Stamens 5; filaments linear, compressed; anthers ovoid-orbicular, apex obtuse. Ovary fusiform. Capsule elliptic, turgid at middle, glabrous. Seeds black, globose, ca. 2 mm in diam., shiny. Fl. Jun-Aug, fr. Sep-Nov. 2n = 16.