aquatic, rootless, perennial herbs. Stem slender, much-branched. Leaves
whorled, sessile, dichotomously dissected into slender segments, exstipulate.
Flowers small, axillary, unisexual, hypogynous, male and female flowers usually
on different nodes of same plant, male often above the female. Perianth
(sepals) 8-15 in a whorl, connate base. Stamens 10-20, free, spirally arranged
on flat receptacle, filaments and anthers not well differentiated. Carpel 1,
ovary 1-celled and 1-ovuled, style terminal, stigma decurrent. Fruit achene or
nut. Seeds thin testa.