Submerged aquatic herbs; linear or spathulate leaves; flowers unisexual; sepals 3; petals 3 or none; stamens 3 to many; carpels 2-15; ovary inferior, 1-celled or imperfectly several-celled; stigmas 3-6; fruit somewhat fleshy or capsular. 15 genera and about 100 spp., always aquatic.