Phaseolus L.

Plants annual or perennial herbs, taprooted or the rootstock tuberous, sometimes climbing on other vegetation. Stems trailing or climbing, often twining, branched or unbranched, unarmed, glabrous or sparsely to densely pubescent with short, spreading to downward-curved or -angled hairs, some or all of the hairs usually minutely hooked at the tip. Leaves pinnately trifoliate, mostly long-petiolate, the petiole glabrous or hairy. Stipules 1.5–3.0 mm long, oblong-triangular to narrowly oblong-ovate, herbaceous, appearing basally attached, reflexed or spreading, persistent, strongly parallel-veined; stipels 0.8–2.5 mm long, persistent. Leaflets ovate to broadly ovate, the lateral ones usually asymmetrically so, the terminal leaflet sometimes somewhat rhombic, broadly rounded or broadly angled (obliquely so in lateral leaflets) at the base, tapered to a sharply pointed tip, unlobed (lobed elsewhere), the margins entire, the surfaces usually hairy. Inflorescences axillary, racemes or few-branched panicles, well-developed, with several to numerous flowers, and extending past the leaves or short, few-flowered, and appearing as clusters, the flower stalks with small bracts at the base, these 1–3 mm long, lanceolate to ovate or triangular, often with strong parallel venation, more or less persistent; each flower also closely subtended by 2 small bractlets, these 0.7–4.0 mm long, oblong to lanceolate or broadly ovate, persistent. Calyces with the tube broadly bell-shaped, more or less 2-lipped, the upper 2 lobes short and broad, fused with only a shallow notch at the tip, the lower 3 lobes triangular, unequal, the lowermost lobe longer than the others. Corollas papilionaceous, pink to purple, greenish white, or white (if colored, the petals often paler toward the base), the banner and wings cordate or abruptly tapered to an often relatively broad stalklike base, the banner with the expanded portion broadly ovate to nearly circular, usually shallowly notched at the tip, abruptly arched upward above the midpoint but generally appearing somewhat hooded over other petals, the wings oblong, usually longer than the banner and keel, the keel slender, spirally coiled. Stamens 10, 9 of the filaments fused to above the midpoint and 1 filament more or less free, the anthers small, attached near the base, all similar in size. Ovary sessile or nearly so, with a small nectary disc encircling the base, this sometimes somewhat lobed, the style coiled, often jointed toward the base, bearded on the upper side, the stigma more or less lateral or oblique, elongate. Fruits legumes, oblong or linear, flattened or not, straight or curved, at least along the lower margin, sessile or nearly so, asymmetrically short-tapered to a beak, the margins otherwise often parallel, glabrous or minutely hairy (at least when young), dehiscent by 2 valves, these papery to leathery in texture, becoming spirally twisted during dehiscence, with 2 to numerous seeds. Seeds kidney-shaped to broadly oblong in outline, flattened, the surface variously colored, often mottled, smooth. Sixty to 65 species, North America, Central America, South America, cultivated nearly worldwide.

Phaseolus once was thought to contain about 500 species, but it has been reorganized taxonomically, with many of the species transferred to other genera, such as Vigna (Lackey, 1981, 1983). The genus contains five main domesticated species of varying historical and modern economic importance as food plants: P. acutifolius A. Gray (tepary bean), P. coccineus L. (runner bean), P. lunatus L. (Lima bean), P. polyanthus Greenm. (year-bean), and P. vulgaris L. (common bean). Gathering of wild beans for food has been documented as far back as 10,000 years ago and actual domestication may have occurred as long as 7000 years ago (Kaplan and Kaplan, 1988). Bean domestication paralleled that of maize (Zea mays L., Poaceae). These two foods are complimentary in that individually their proteins are deficient in a few important amino acids, but in tandem the combination of amino acids in the 2 groups allows the human body to build complete proteins.