Description from Flora of China
Herbs, annual or perennial, in marshy areas or aquatic, often aromatic when bruised, with transparent dots. Stems erect, prostrate, or creeping, simple or much branched. Submerged leaves in aquatic species whorled, pinnately parted to capillary-multiparted, glabrous. Aerial leaves opposite to whorled, sessile or petiolate; leaf blade lanceolate or pinnately divided, glandular punctate, margin entire to serrate, if leaf undivided then veins pinnate or parallel. Flowers axillary and solitary or in axillary or terminal spikes or racemes, sessile or pedicellate. Bracteoles 2 or absent. Calyx tubular; lobes subequal or upper lobe larger than others. Corolla tubular or funnelform, 2-lipped; lower lip 3-lobed; upper lip entire or 2-lobed. Stamens 4, included, didynamous; anther locules stipitate. Ovary glabrous. Stigma 2-lamellate. Capsule included within calyx, septicidal. Seeds minute, numerous.
About 40 species: tropical to subtropical Africa, Asia, Australia, and Pacific Islands; ten species in China.