Aquatic herbs, monoecious or dioecious, submerged in fresh, brackish or salt waters. Rhizome slender, creeping. Leaves alternate, opposite or more or less whorled, linear, base sheathing; sheaths mostly ligulate at the apex; flower¬ing leaves sometimes reduced to sheaths. Flowers solitary or in axillary cymes. Perianth absent, cupular or of 3 free small scales. Male flowers with 1-3 stamens; pollen globose. Female flowers with 1-9 free carpels; carpel unilocular, with solitary pendulous ovule; style short or long, simple or 2-4 lobed, capitate, peltate, spathulate or ligulate. Fruit sessile or stipitate, indehiscent. Seed pendulous, without endosperm.