Annual herb (in ours). Leaves alternate.
Modified sterile flowers 0. Inflorescence a spike-like thyrse, not white-woolly. Tepals 5, free, ± glabrous or pilose near the base only. Stamens 5. Stigmas 2-3. Capsule circumscissile. Seeds flattened, black, shiny.
Worldwide: 45 species in the tropics of both New and Old Worlds.