Plants annual, to 60 cm tall. Stem slender, simple, rarely shortly branched. Leaves alternate; petiole 1-1.5 cm; leaf blade linear-oblong or linear-lanceolate, rarely linear, 7-13 × 0.8-1.8 cm, membranous, often with 1 pair of purple stipitate glands at basal margin, glabrous, lateral veins 9-11 pairs, base gradually attenuate into petiole, margin sharply serrate, apex acute or long acuminate. Inflorescences axillary, 2-flowered; peduncles short, 2-3 mm. Pedicels slender, with 2 subulate-linear bracts at base; bracts persistent, broadly oblong, apex aristate. Flowers pale purple, large, to 4.5 cm deep. Lateral sepals 4; outer 2: subglobose, curved, 3-veined, apex aristate-acuminate; inner 2: linear, 1-veined, aristate. Lower sepal saccate, narrowed into a hooked, incurved spur. Upper petal orbicular, apex cornute-rostellate, abaxial midvein swollen at middle; lateral united petals ± clawed, 2-lobed; basal lobes ovate-oblong, apex rounded or emarginate, fibrously mucronulate; distal lobes broadly oblong, emarginate, mucronulate; auricle inflexed, small. Anthers obtuse. Capsule linear. Fl. Jun-Jul.
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