submerged or floating in fresh or brackish water, with stem filiform and
branched. Leaves linear or setaceous, alternate or opposite, base sheathing.
Inflorescence a terminal spike. Flowers bisexual, first enclosed by sheathing
base on a short peduncle, finally straight or spirally twisted. Bracts and
perianth absent. Stamens 2, opposite each other, filaments short, broad,
anthers extrorse. Carpels 4-5, free, stipitate, unilocular, with solitary,
pendulous ovule, stigmas peltate or umbonate. Fruits of 4-5, stipitate, ovoid,
obtuse or beaked achenes, stalks spirally twisted. Seeds with straight embryo,