Plants 30-60 cm, slender, submerged or erect. Stem simple. Leaves narrowly linear, 4-8 mm broad, encircling the stem, triquetrous at the base, rarely floating, lower part of leaf with scarious margin. Flowers in unisexual heads, borne on axes, subtended by the leafy bracts. Female heads 3-5, the lower usually pedunculate. Male heads 3- 8, remote and distinct at anthesis. Male flowers with 3 or more stamens, free or connate at the base; anthers bilobed, basifixed. 1-2 mm long; filaments c.5 mm long. Female flowers sessile or subsessile; perianth seg-ments 3 or more, 2-3 x 1-1.5 mm. spathulate, translucent with erose tips: ovary mostly sessile or shortly stipitate, usually bilocular, sometimes with one locule: style simple; stigma simple and conical. Fruits fusiform, tapering below to an obconic base with 2-4 mm long stalk (formed by shrinkage of the fruit).