Plant: perennial from rhizomes, dioecious; stem < 2 m, glaucous or gray-green Leaves: cauline; submersed leaves 1.5-3.5 cm, whorled, 3-6 per node, with thread-like pinnate divisions; midrib ± flat, wider than divisions, divisions < 7 mm, 20-30 per leaf; emergent leaves simple, opposite, leaf- or bract-like, entire to pinnately divided INFLORESCENCE: spike-like, terminal, or flowers clustered, axillary, ± emergent Flowers: unisexual; lower pistillate; middle sometimes bisexual; upper staminate; calyx lobes 4; petals generally 4; Staminate flower: stamens 8; ovary vestigial; Pistillate flower: stamens 0; ovary 4-chambered, stigma plumose Fruit: Fruit: segments 4, nut-like; Seed 1 per chamber Misc: Ponds, ditches, streams, lakes; < 500 m.