Twining annual herbs. Stems slender, terete, 1–2 m long, covered with 1–3 mm long bulbous based spreading or retrose brownish hairs, rooting at nodes and internodes of the stem in absence of support. Stipules sub-medifixed, elliptic-lanceolate, 5–7 mm long, base obtuse to rounded, 5–7-nerved, apex acute, densely pubescent. Leaves 3-foliolate; petioles 5–7 cm long, densely hairy with bulbous based retrose or spreading whitish hairs as stem. Leaflets membranous, entire; lateral leaflets ovate-rhomboid with obliquely rounded, obtuse or truncate at base, 3–4.5 x 2–3 cm, margins entire or sometimes shallowly lobed, acute or shortly acuminate at apex, sparsely hairy; rachis 2–3 mm long, covered with whitish brown, 0.5–1 mm long, spreading or retrose hairs; terminal leaflet slightly larger than the lateral ones, ovate, 3.5–6.5 x 1.7–3.5 cm, rounded at base, shortly acuminate or acute at apex, rachis 3–4 mm long, covered with whitish-brown, 0.5–1 mm long, spreading or retrose hairs; stipels two, linear, 1–2 mm long, sparsely hairy. Flowers of two kinds, aerial chasmogamic flowers and underground cleistogamic flowers. Chasmogamous flowers 2–6 in axillary or terminal, lax racemes, yellow, 4.5–6 x 7–9 mm; peduncle slender, 1.5–3 cm long, densely covered with retrose whitish-brown hairs as young branches; pedicels short, 2–2.5 mm long, densely covered with whitish-brown hairs; bracts linear-lanceolate, 3–4 mm long, herbaceous, densely covered with 1–2 mm long hairs; bracteoles inserted just above the bract, linear, 3–3.5 mm long, acute at apex, densely hairy as bract. Calyx campanulate, hairy; tube c. 3 mm long; teeth triangular, 1.2 x 1 mm, sparsely hairy along margins. Standard yellow, asymmetrical, broadly ovate, 4.5–6 x 7–9 mm, emarginate at apex, central protuberance (up to 1 mm long) inside; claw c. 3 mm long. Wing petals yellow at upper portion and whitish below, 5–6 x 2.5–3 mm, membranous; right wing half concealing the upper portion of keel petals; left wing spreading horizontally and supported by a pocket on left hand keel petal. Keel petals yellowish, 5–6 mm long, spirally incurved with horn-like 1.6–2 mm long pocket, obtuse at apex. Stamens 9+1, included; staminal tube 5–6 mm long; filaments of staminal tube c. 5 mm long; free filament c. 10 mm long; anthers basifixed, 0.2–0.3 mm long. Style filiform 7–9 mm long, bearded at apex, broadly ‘S’ shaped before stigma, shortly beaked beyond the stigma; the beak linear, 0.4–0.5 mm long; ovary linear, 4.5–5 x 1–1.4 mm, minutely hairy. Pods cylindrical, 3–6 x 0.3–0.4 cm, apex acute slightly curved, sparsely hairy. Seed 6–12, rectangular, 2.5–3 x 2–2.2 mm, dark brown, mottled with black patches; seed coat shiny; hilum protruded out and well developed, elliptic, 0.9–1 mm long, white. Germination hypogeal; the first and second leaves simple, petiolate, ovate, base rounded, apex acute, sparsely hairy. Cleistogamous flowers 2–4 on 2–5 cm long peduncles, white (albino), 4–4.5 x 2–2.5 mm, remain closed; pedicels 1–1.2 mm long, minutely hairy; bracts elliptic–lanceolate, 1–2 mm long, acute at apex, hairy along margins with bulbous based, 0.4–0.7 mm long hairs, 1-nerved; bracteoles linear, 0.7–1.2 mm long, covered with white spreading, 0.3–0.5 mm long hairs. Calyx campanulate, membranous, c. 2.5 mm long; teeth triangular, c. 0.7 mm long, glabrescent. Standard, wing and keel petals are similar to that of chasmogamous flowers except smaller in size. Stamens 9+1, filiform; filaments 3.2–3.5 mm long; anthers basifixed, yellowish, 0.2–0.25 mm long. Style filiform, 3.2–3.9 mm long, shortly beaked beyond the stigma; beak linear, 0.2–0.3 mm long. Pods cylindrical, 1.5–2.5 cm long, usually curved, white (albino), glabrescent, apex acuminate. Seeds 3–5, whitish brown, oblong or sub-cylindric, 2.5–3 x 2–2.2 mm; seed coat shiny; hilum poorly developed, not protruded out, linear, 1–1.1 mm long, yellowish-white. Germination hypogeal; the first and second leaves simple, petiolate, ovate, elliptic, base rounded, apex acute, sparsely hairy. (Figs 1, 2, 3, 4).

Vigna yadavii is morphologically close to Vigna dalzelliana (Kuntze) Verdc. but differs in its underground obligate cleistogamous flowers on positively geotropic branches, hairy calyx, small corolla, linear style beak and dimorphic seeds.

The species is named in honor of Prof. S.R. Yadav, Department of Botany, Shivaji University Kolhapur, India (MS), in recognition of his valuable contribution to taxonomy of flowering plants of Western Ghats of India.

India, Maharashtra, Nasik Dist., Kasara-Ghat near Igatpuri and Kalvan, Saptashrungi hills; Satara Dist., Thoseghar.

Vigna yadavii is a twining annual herb, which grows on hill slopes in grasses and herbs at about 1200 m elevation from above sea level in Western Ghats of India. The common associates of the species are Abelmoschus manihot (L.) Medik., Apluda mutica L., Carissa congesta Wt., Crotalaria pallida Ait., C. mysorensis Roth., C. leptostachya Benth., Cymbopogon martinii (Roxb.) Wats., Elephantopus scaber L., Eragrostis spp., Flemingia strobilifera (L.) R.Br. ex Ait., Hemidesmus indicus (L.) R.Br. ex Shult., Themeda spp. and Urena lobata L.

Vigna yadavii shows morphological similarities with Vigna dalzelliana (Kuntze) Verdc. but differs from the latter species by the characters given in Table 1. The presence of underground obligate cleistogamous flowers on positively geotropic branches is most useful distinguishing characteristics of V. yadavii. In addition to this, dimorphic seeds and differences in hilum, aril, style beak and corolla are also useful distinguishing characteristics of the new species. Vigna dalzelliana has a unique flattened style beak but that of V. yadavii is linear. The poorly developed aril of seeds of the cleistogamous flowers is also diagnostic feature of V. yadavii.

During rainy season (August–November), the species produces chasmogamous flowers on aerial branches and underground obligate cleistogamous flowers on positively geotropic branches. The cleistogamous flowers are much smaller than chasmogamous flowers and white-albino in color. They remain closed. The pods of cleistogamous flowers are colorless, short, curved and 3–5-seeded. There are no structural differences in chasmogamic and cleistogamic flowers except for the smaller size and white albino color of the latter.