Fls us. stalked and clustered at base of lvs, mostly minute, sts cleistogamous. Calyx subcampanulate, us. 5-toothed, sts 2-lipped. Corolla us. > calyx, tube crowned by spreading limb of 5 ± equal white lobes, upper surface papillose. Stamens us. 4, didynamous. Ovary ± globose, 2-celled at base. Stigma capitate, sts 2-lobed. Capsule minute, septidically dehiscent. Small annual to perennial aquatic herbs of mud or damp ground. Lvs ± clustered in rosettes on slender rhizomes, cylindric or differentiated into petiole and lamina, very susceptible to habitat conditions. Very widespread genus of some 11 spp.