Description from Flora of China
Herbs, annual or perennial, aquatic, amphibious, or terrestrial, often anthocyanic with age. Stems glabrous, simple or branched, commonly 4-angled or 4-winged. Leaves decussate or whorled, sessile or subsessile; bracts of inflorescences like foliage leaves or smaller and different in shape. Flowers actinomorphic, monomorphic [or dimorphic], (3 or)4(-6)-merous, solitary, sessile or shortly pedicellate, in axils of bracts on main stem, on spikelike lateral branchlets, or in terminal spikes. Floral tube campanulate or urceolate, generally globose in fruit; bracteoles 2, at base of floral tube; sepals 3-6, ca. 1/3 length of floral tube or less, deltate; epicalyx alternating with sepals or absent. Petals 3-6, pink-purple to whitish. Stamens 1-6, opposite the sepals. Ovary 2-4-loculed; style long or short; stigma capitate, rarely more massive, discoid. Capsule finely transversely striate (10 × magnification), hyaline, septicidally dehiscent, 2-4-valved. Seeds numerous, brown or reddish brown, ovoid to ellipsoidal, concave-convex, less than 1 mm.
An additional species, Rotala hippuris Makino (Bot. Mag. (Tokyo) 12: 81. 1898), a popular aquarium plant considered endemic to Japan, is found in ornamental ponds in Taiwan. It was reported (Liu et al., Man. Taiwan Vasc. Pl. 3: 241. 2000) from ponds at Taoyuan, Taiwan. Whether it is native, naturalized, or cultivated there was not stated.
See also Cook, A revision of the genus Rotala (Lythraceae) (Boissiera 29: 1-156. 1979).
About 46 species: tropical and temperate regions of the world; ten species (one endemic, one naturalized) in China.
(Authors: Qin Haining (覃海宁); Shirley Graham)