Strophostyles helvola (L.) Elliott (wild bean, amberique bean)

S. helvola var. missouriensis (S. Watson) Britton

S. missouriensis (S. Watson) Small

Phaseolus helvolus L.

Pl. 407 e–g; Map 1804

Plants usually annual. Stems 70–300 cm long. Leaves with the petiole 2–5 cm long, hairy. Leaflets rounded or angled at the base, angled or tapered to a bluntly or sharply pointed tip, often with a minute extension of the midvein at the very tip, the upper surface glabrous or sparsely appressed-hairy, the undersurface sparsely to moderately appressed-hairy; the terminal leaflet with a stalk 6–15 mm long, the blade 1.8–6.5 cm long, 1–5 cm wide, ovate-triangular or narrowly ovate-triangular to oblong-triangular or pear-shaped, unlobed or with 1 or more commonly a pair of blunt basal lobes; the lateral leaflets sessile or with a stalk to 3 mm long, the blade 1.5–6.5 cm long, 0.6–4.0 cm wide, oblong-triangular or lanceolate to ovate or asymmetrically ovate, asymmetrically rhombic, or occasionally pear-shaped, unlobed or with 1 or less commonly a pair of blunt basal lobes. Inflorescences appearing as headlike clusters of 3–5 flowers, the inflorescence stalk 5–20 cm long, the flower stalks 1–2 mm long, the pair of bractlets closely subtending each flower 3.0–4.5 mm long, as long as or longer than the calyx tube. Calyces with the tube 1.5–3.0 mm long, sparsely hairy, the lobes 1.5–3.0 mm long. Corollas pinkish purple to pink, fading to greenish yellow, the banner 6.5–12.0 mm long, the wings 7–9 mm long, the keel 8–12 mm long, the beaklike apical portion relatively slender and usually sharply curved or twisted to the side. Fruits 3–9 cm long, 4–10 mm wide, sparsely hairy to nearly glabrous at maturity, 4–10 seeded. Seeds 5–8 mm long, the surface sometimes appearing waxy, brown, sometimes faintly mottled, pubescent with white-woolly patches. 2n=22. June–October.

Scattered nearly throughout the state (eastern U.S. west to North Dakota and New Mexico; Canada). Bottomland forests, mesic upland forests, banks of streams, rivers, and spring branches, margins of ponds and lakes, bottomland and upland prairies, savannas, sand prairies, and edges of glades; also pastures, old fields, edges of crop fields, ditches, levees, railroads, roadsides, and open disturbed areas.

Strophostyles helvola is distinctive within the genus in its lobed leaflets, relatively large flowers, and brownish seeds with white-woolly patches. It should be noted that on some plants only a few of the leaves have lobed leaflets, so users should carefully examine specimens before scoring this character. Steyermark (1963) separated the var. missouriensis, which he characterized as a less common variant within the Missouri range of the species differing in its larger, mostly unlobed leaflets with blunter tips and its slightly larger fruits and seeds. Riley-Hulting et al. (2004) did not consider this variant worthy of formal taxonomic recognition.

The species epithet has been spelled S. helvulus in some of the botanical literature, in keeping with the spelling that Linnaeus originally used for his Phaseolus helvulus, the basis of the present species. However, many later botanists considered this a correctable spelling error (the derivation of the epithet is from the Latin helvolus, meaning pale brownish yellow), thus S. helvolus is also well represented in floristic works. In order to stabilize both the spelling and application of the name, Verdcourt (1997) formally proposed conservation of the spelling S. helvola, and his proposal was approved at the subsequent International Botanical Congress.