Description from Flora of China
Herbs or subshrubs, annual or perennial. Branches 4-veined. Leaves opposite, sessile, dentate, lobed, or pinnatifid. Spikes usually terminal, simple or branched; bracts small. Flowers alternate, nearly zygomorphic. Calyx membranous, 5-dentate. Corolla with a distinct tube; lobes 5, unequal, spreading. Stamens 4, fertile, didynamous, inserted at middle of corolla tube, included; anthers ovate. Ovary 4-locular; ovules erect, 1 per locule. Capsules dry, included in calyx, dehiscing into 4 1-seeded pyrenes.
About 250 species: chiefly in tropical America, one species in China.
Additional species grown as ornamentals.