Herbs, annual, 0.3-1.3 m tall. Stems slender. Stipules lanceolate or ovate-lanceolate, 3-4 mm, densely pubescent; petiole 2-5.5 cm; stipels extremely small, usually deciduous, narrowly triangular; petiolules usually 2-3 mm; leaflets papery or almost membranous; terminal leaflet rhomboid-ovate or oblate-ovate, 2.5-5.5 × 2-5 cm, both surfaces sparsely yellowish brown villous, base rounded, broadly cuneate or nearly truncate, apex obtuse or sometimes with mucro; lateral leaflets usually obliquely ovate or ovate-rhombic, dimorphic. Upper inflorescences axillary, shortly racemose; bracts almost membranous, ovate to elliptic, 3-5 mm. Cleistogamous flowers often present. Normal flowers: pedicel 1-2 mm. Calyx tubular, 5-7 mm, 5-lobed; lobes unequal, sparsely pubescent, apex acuminate. Corolla purplish or white; petals subequal; standard obovate, 1-1.7 cm; wings oblong, with long stipes and acute auricles at base; keels similar to wings. Ovary hairy. Cleistogamous flowers underground, sessile; style curved. Legumes dimorphic: those of normal flowers oblong or obovate-oblong, 2-3.5 × ca. 0.6 cm, compressed, 2-5-seeded; legumes of cleistogamous flowers elliptic or suborbicular, 1-4-seeded. Seeds reniform-orbicular, 2-4 × 2-3 mm; hilum short. Fl. and fr. Aug-Nov. 2n = 40.
Woods and shaded places in lowland all over Japan. Mountain slope, road side, field, grassland at elevations of 300-3,000 metres.
Can be grown by seedlings. Seeds needs soaking.
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