Crotalaria L.

Plants annuals (perennial herbs and shrubs elsewhere), with taproots. Stems erect or strongly ascending, unbranched or branched, usually 2- to several-angled or -ridged, unarmed, glabrous or hairy, often becoming reddish purple or purple with age. Leaves alternate, all appearing simple (unifoliate; palmately trifoliate elsewhere), short-petiolate. Stipules present (sometimes absent at the lower nodes), conspicuous or inconspicuous, free from the stem or long-decurrent below the nodes as wings of green tissue; stipels absent. Leaflets variously shaped, the margins entire, the surfaces glabrous or hairy, pinnately veined. Inflorescences relatively open racemes or sometimes reduced to a solitary flower, terminal or attached opposite the upper leaves, the bracts inconspicuous or conspicuous, linear to lanceolate or elliptic-lanceolate, usually persistent, the bractlets mostly inconspicuous, variously shaped, persistent. Flowers with short to relatively long, slender stalks. Calyces 2-lipped, the tube obliquely bell-shaped to conic, shorter than the lips, glabrous or hairy, the upper lip with 2 lobes, these usually slightly broader than the 3 lobes of the lower lip, which is concave and somewhat cupped around the wings and keel. Corollas papilionaceous, yellow (in our species), sometimes lined or tinged with red, sometimes fading to white, the banner larger than the wings and keel, the expanded portion obovate to nearly circular, bent upward or backward, short-tapered to a short, stalklike base, rounded or shallowly notched at the tip, with a shallow longitudinal groove, the wings positioned around the keel, the expanded portion asymmetrically and broadly oblong to rhombic-oblong, the stalklike base attached along the lower margin, the keel shorter than the wings (longer elsewhere), fused their entire length, usually finely hairy along the suture, strongly curved upward (curved downward elsewhere), twisted together at tip. Stamens 10, monadelphous, the filament tube split along the upper side, dimorphic, 5 stamens with longer filaments and larger, linear anthers attached near the base alternating with 5 on shorter filaments and smaller, broadly ovate anthers attached near the midpoint. Ovary short-stalked to nearly sessile, the style strongly bent at the base and strongly curved, hairy, the stigma terminal, minute, with a dense tuft of short, bristly hairs. Fruits legumes, oblong-cylindric or occasionally oblong-ellipsoid, tapered to a short-stalked base, tapered abruptly to a short, hairlike beak, inflated, the valves dry and papery to leathery and becoming black at maturity, glabrous, dehiscent longitudinally along the sutures, with 7 to numerous seeds. Seeds obliquely heart-shaped to asymmetrically broadly kidney-shaped, somewhat flattened, the surface brown to black, smooth, shiny. About 510 species, nearly worldwide, most diverse in tropical regions.

The Missouri species of Crotalaria are recognized by their unifoliate leaves, yellow papilionaceous flowers, and greatly inflated legumes that become black at maturity. The vernacular name rattle box refers to the tendency of the seeds to become detached while still in the fruit prior to its dehiscence, resulting in a rattling sound when the fruits are disturbed or shaken. Species of Crotalaria are used for green manure and as shade or cover for other crops. Some Old World species were introduced into the United States in the 1930s for soil cover, and are now firmly established and sometimes weedy in other states (Senn, 1939; Windler, 1974). Crotalaria is not used as forage because it contains hepatotoxic pyrrolizidine alkaloids similar to those found in some members of the Asteraceae tribe Senecioneae (Burrows and Tyrl, 2001). All parts of the plant are toxic, and care must be taken to avoid contamination of animal feed with the leaves or seeds.