Plants 0.6-1 m tall, glabrous. Stem erect, purple-red tinged, robust, ± curved in lower part. Leaves alternate; petiole long, 2-4 cm; leaf blade pale green abaxially, dark green adaxially, ovate, 10-15 × 3-5 cm, thinly membranous, lateral veins (8 or)9 or 10 pairs, base attenuate into petiole, margin crenate-serrulate, teeth mucronulate, apex acuminate. Inflorescences in upper leaf axils, 1-or 2-flowered; peduncles very short, 2-2.5 mm. Pedicels fibrous, 2-2.5 cm, bracteate at base; bracts 6-7 mm, setose. Flower purplish yellow, large, 3-4 cm, ca. 1.3 cm wide. Lateral sepals 4; outer 2: ovate-lanceolate, ca. 1.2 cm, inequilateral; inner 2: narrowly linear, ca. 8 mm, hyaline margined. Lower sepal saccate, 3-4 cm deep, gradually narrowed into an incurved spur ca. 2 cm; mouth oblique, tip acuminate. Upper petal orbicular, apex cuspidate, abaxial midvein thickened, cristate at middle; lateral united petals not clawed, ca. 3 cm, 2-lobed; basal lobes broadly orbicular; distal lobes dolabriform, longer, apex rounded; auricle inflexed, large. Filaments fibrous, ca. 4 mm; anthers ovoid, apex obtuse. Ovary linear. Capsule linear, 2.5-4 cm, longitudinally costate. Seeds many, orbicular, compressed, ca. 3 mm. Fl. Aug-Sep.
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