6. Senna Mill. (senna)

Plants annual or perennial herbs (trees and shrubs elsewhere), unarmed, with 1 to several stems, the roots often blackish; root nodules lacking. Leaves short- to more commonly relatively long-petiolate, the petiole or rachis base with a large, variously shaped gland. Stipules small and scalelike to nearly hairlike, often shed early. Leaf blades evenly 1 time pinnately compound. Leaflets few to numerous, opposite, variously shaped, often asymmetrical at the base (with one side angled and the other rounded or cordate), the margins entire. Inflorescences axillary racemes, sometimes appearing aggregated in terminal clusters, the flower stalks lacking bracts. Flowers perfect, slightly perigynous, slightly to moderately asymmetrical, the buds usually nodding. Hypanthium very short and more or less disc-shaped. Calyces of 5 free sepals. Corollas of 5 free petals; these dissimilar in shape and position and usually slightly dissimilar in size, abruptly tapered at the base, rounded at the tip, yellow to orangish yellow, drying white, often with dark veins. Stamens 10 but the upper 3 reduced to staminodes, the filaments short, the anthers attached at the base, graded in size, the upper 3 strongly reduced and infertile, the middle 4 intermediate and fertile, the lower 3 large and fertile, all somewhat curved, the fertile ones dehiscing by an apical pore. Styles curved. Fruits legumes, elongate, not twisted, flattened to circular or rectangular in cross-section, narrowed to a short, stalklike base, indehiscent or, if dehiscent, then the valves not separating elastically or coiling. Seeds variously shaped and colored, usually somewhat flattened, with an elliptic pleurogram, this sometimes conspicuously different in color from the remainder of the seed. About 240–260 species, nearly worldwide, most diverse in the New World tropics.

Senna is a large, widespread, and morphologically diverse genus that traditionally was included in a broadly defined Cassia L. along with Chamaecrista. The differences between these taxa were discussed by Irwin and Barneby (1981, 1982), whose overall classification presently is followed by most botanists. In contrast to Chamaecrista, the stipules of Senna are small and weakly developed, and the flowers lack small bracts on the stalks; the stamens of Senna have relatively large, curved anthers of various sizes in a more or less graded series.