Centrosema virginianum (L.) Benth. (spurred butterfly pea)

Bradburya virginiana (L.) Kuntze

Map 1722

Plants perennial herbs, often reclining or climbing on other vegetation, with a branched rootstock. Stems 50–150 cm long, ascending to trailing, twining, unarmed, glabrous or sparsely pubescent with short, fine, straight hairs. Leaves alternate, pinnately trifoliate, the petiole 2–3 cm long, glabrous or short-hairy. Stipules 2–4 mm long, lanceolate, with prominent parallel venation; stipels 2–4 mm long, linear to hairlike, persistent. Leaflets 2.0–6.5 cm long, 10–25 cm wide, oblong-ovate to narrowly ovate, narrowly elliptic, or lanceolate, broadly angled or rounded at the base, angled to a bluntly or occasionally sharply pointed tip, the margins entire, the upper surface glabrous, the undersurface glabrous or finely pubescent with short, somewhat hooked hairs, pinnately veined, the undersurface with a noticeable network of veins. Terminal leaflet with the stalk 10–20 mm long, symmetric at the base; lateral leaflets essentially sessile above the pulvinus, often slightly asymmetric at the base. Inflorescences axillary, of solitary flowers or small clusters of 2(–4) flowers, the inflorescence stalk 0.8–1.0 cm long, glabrous or finely hairy, the bracts 4–6 mm long, ovate, shed early; bractlets 8–12 mm long, broadly lanceolate to ovate, closely subtending the flowers and often partially obscuring the calyx, persistent. Flowers all opening (cleistogamous flowers not produced), twisted at the base during development so that the top of the flower (banner) is oriented toward the bottom at maturity (resupinate). Calyces 10–16 mm long, 4-lobed, the tube more or less bell-shaped, slightly pouched on 1 side at the base, glabrous or minutely hairy, the lobes as long as or longer than the tube, somewhat unequal (the uppermost slightly larger than the others and sometimes with 2 narrow lobes apically), narrowly triangular, tapered to sharply pointed tips. Corollas papilionaceous, pinkish purple to lavender or pale to light purple (the banner with a white region toward the center, this often with irregular, darker purple lines), the banner 25–30 mm long, 25–35 mm wide, the expanded portion broadly obovate to nearly circular, with an inconspicuous spur at the base, shallowly notched at the tip, often with a shallow longitudinal keel, the margins flat to slightly curled backward, the wings 14–16 mm long, 4–5 mm wide, the expanded portion oblanceolate to oblong, somewhat cupped around the keel and fused to it toward the base, the keel 14–17 mm long, 7–8 mm wide, strongly curved downward, fused to the rounded or bluntly pointed tip. Stamens 10, all of similar length, 9 of the filaments fused and 1 free, the fused portion 14–16 mm long, the free portion 2–3 mm long, the anthers small, attached near the midpoint. Ovary 8–10 mm long, short-hairy, the style 6–10 mm long, finely hairy, the stigma appearing expanded and fringed. Fruits legumes, the portion above the sessile or minutely stalklike base but including the beak 7–13 cm long, 3–4 mm wide, linear, straight, tapered (often slightly asymmetrically so) at the tip to an elongate (20–40 mm), mostly straight beak, flattened, with a longitudinal ridge near each margin, leathery, glabrous at maturity, dehiscing by 2 valves, these green to tan at maturity, twisting spirally after dehiscence, with numerous seeds. Seeds 2.0–2.5 mm long, 1.5–2.0 mm wide, oblong to bluntly rectangular in outline, flattened, the surface dark brown, sometimes somewhat mottled, smooth, somewhat shiny, not sticky. 2n=18. July–August.

Uncommon, known thus far only from a single historical collection from Shannon County (southeastern U.S. west to Oklahoma and Texas; Mexico, Central America, South America). Habitat unknown.

The habit and flowers of Centrosema virginianum and Clitoria mariana are superficially very similar, but there are several subtle differences. Centrosema virginianum has thinner, more twining stems, leaflets with a prominent network of veins, a pair of large bractlets closely subtending each flower, a shorter calyx tube with relatively long, narrow lobes, a spurred banner petal, wings slightly shorter than to about equal in length to the keel, and a linear, prominently beaked legume. Centrosema virginianum is somewhat variable in leaflet shape and size, with some populations in other states having very narrow leaflets (Isely, 1998). The sole Missouri specimen originally was misdetermined as Clitoria mariana. It was redetermined by Paul Fantz of North Carolina State University and reported as part of a distributional summary of the group in temperate North America (Fantz, 2002a).