Shrubs or herbs, rarely trees or climbers, usually armed. Stipules not spinescent, usually 2 per pinna, small, caducous. Leaves bipinnate, often sensitive. Heads globose or spikes cylindric, solitary or fasciculate, axillary, pedunculate. Flowers small, bisexual or polygamous, sessile, usually 4-merous. Calyx connate, valvate, campanulate, toothed. Petals connate at base. Stamens 4 or 8, free, exserted; anthers without glands. Ovary many ovuled; style filiform. Legume oblong or linear, usually plano-compressed, mem­branous or leathery, composed of 1-seeded segments, separating from persistent margins. Seeds elliptic or orbicular, flat.

About 500 species: mostly in tropical America; three species (all introduced) in China.