Mostly aquatic herbs, with very slender tufted stems. Leaves opposite, entire, linear to obovate, estipulate. Flowers small, axillary, perfect or imperfect, with or without 2 saclike bracts. Perianth none. Stamen 1, the filament elongate; anthers cordate, 4-loculed, dehiscent by longitudinal slits. Pistillate flower a single 4-locular ovary of 2 carpels; styles filiform, distinct. Fruit nutlike, compressed, 4-lobed, separating into 4 1-seeded mericarps; seeds pendulous; endosperm present. One genus, with about 20 species, widely distributed. (Munz, Flora So. Calif. 321).