Description from Flora of China
Herbs, dwarf, aquatic or terrestrial, cespitose, creeping, or floating. Stolons rooting from nodes or acaulescent. Leaves fascicled, opposite, or alternate on elongated branches, long petiolate. Flowers axillary, minute, short pedicellate. Bracteoles absent. Calyx campanulate, 5-lobed. Corolla radiate-campanulate, regular; tube short; lobes 5, subequal. Stamens 4, equal, inserted near middle of corolla tube; filaments filiform; anthers confluent, 1-loculed. Ovary basally 2-loculed. Style short; stigma capitate. Capsule indistinctly dehiscent. Seeds numerous, ovoid, small; seed coat rugose.
About seven species: worldwide; one species in China.