Plants annual, 60-90 cm tall. Stem robust, branched, conspicuously striate when dry. Leaves alternate, petiolate; leaf blade elliptic or elliptic-ovate, rarely ovate-lanceolate, 3-8 × 1.5-2.5(-3) cm, membranous, with 2 or 3 pairs of stipitate basal glands, abaxially sparsely pilose on veins, lateral veins 8 or 9 pairs, base broadly cuneate or subrounded, margin coarsely crenate, apex long acuminate. Inflorescences in upper leaf axils, 1-flowered; peduncles (0.8-)1-1.5 cm. Pedicels bracteate above middle; bracts persistent, oblong-ovate, 5-6 mm, abaxially pilose on veins, 5-7-veined. Flowers yellow, large, to 4.5 cm deep. Lateral sepals 2, orbicular or slightly cordate, margin irregularly fimbriate-lacerate, 7-9-veined; midvein and other veins abaxially thickened, narrowly carinate, irregularly laciniate. Lower sepal striate, deeply saccate, to 3 cm deep, abruptly narrowed into a hooked, incurved spur; spur short, 5-6 mm, 2-lobed. Upper petal orbicular, 1.3-1.4 cm in diam., abaxial midvein broadly cristate, crista 2-lobed; basal lobe large, rounded; distal lobes small, undulate, apex mucronulate; lateral united petals ± clawed, 2-2.2 cm, 2-lobed; basal lobes orbicular; distal lobes oblong-dolabriform, ca. 2 × as long as basal lobes; auricle rounded. Filaments 3-5 mm; anthers suborbicular, apex obtuse. Ovary fusiform. Capsule linear, ca. 2.5 cm, papillose. Seeds 7 or 8, brown, subglobose, compressed, ca. 2 mm, glabrous, ± smooth. Fl. Aug-Sep.
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