Description from Flora of China
Herbs or shrubs. Branches usually forked, hairy. Leaves opposite, petiolate, simple; leaf blade dentate or crenate, often rugose. Spikes terminal, usually long, lax; rachis often angular, excavated. Flowers ± immersed in excavated rachis when young. Calyx membranous, 4- or 5-dentate, becoming variously slit. Corolla slender, cylindric; tube often curved, ± villous in throat; lobes 5, spreading. Stamens 2, fertile, inserted on distal part of corolla tube; anthers divaricate. Ovary supported by a short ringlike disc, 2-locular; ovules 2 per locule. Stigma capitate. Capsules splitting into 2 1-seeded cocci.
About 65 species: tropical America, a few species introduced in the Old World; one species a naturalized weed in China.