Description from Flora of China
Mucuna iriomotensis Ohwi.
Climbing woody vines. Young stems glabrous. Leaves up to 22 cm; petiole 3-20 cm, with sparse short spreading soft pale hairs; stipels slender, 2-3 mm; leaflets membranous to papery with short fine spreading hairs on both sides, lateral veins 4-6, lower ones rather straight; terminal leaflet rhombic-elliptic or rhombic-ovate, 8-13 × 6-8 cm, base cuneate but at extreme base obtuse-rounded, apex obtuse with abrupt acumen to 1 cm; lateral leaflets up to 12 cm, width ratio of abaxial to adaxial halves 2:1. Inflorescence 10-12 cm, pendulous with 5 or more nodes throughout most of length; pedicels 6-10 cm, with adpressed light brown short hairs; bracts broadly ovate, boat-shaped, 12-14 × 7-9 mm, bracteoles narrowly ovate, all caducous before flowering. Calyx tube ca. 8 × 13 mm, upper lip well marked, equaling lateral and lower lobes, all 10-12 mm. Corolla deep purple; standard 3-3.5 cm, ca. 2/3 keel length; wings ca. 5 × 2 cm; keel ca. 5 cm. Legume asymmetric, elliptic-ovate to oblong in outline, (4-)6-9 × 4-5 cm, 5-10 mm thick, leathery, surface with abundant very short spreading pale hairs and brown irritant bristles, apex rounded or acute, both margins with pair of sinuate wings 6-10 mm wide, both surfaces of legume with 9-12 obliquely transverse slightly undulating but well-spaced shallow lamellae of irregular height up to 5(-10) mm high and mostly interrupted along midline, some extending onto marginal wings of fruit. Seeds 1 or 2, brownish black, flattened-discoid, ca. 2 cm in diam.; hilum ca. 3/4 of seed circumference.
Mucuna membranacea is distinctive in the rhombic-ovate leaflets with rather straight veins, the short purple flowers with wide wings equaling keel, and the short broad legume with narrow marginal wings and simple lamellae interrupted along the midline of the legume.
Records of Mucuna nigricans Steudel from China (Taiwan) and Japan (e.g., Fl. Taiwan 3: 343. 1977) were based on misidentifications of this species.
Forests near rivers or seacoast; sea level to near sea level. Taiwan [Japan].