Aquatic perennial herbs of brackish or fresh water. Leaves simple, usually alternate, often dimorphic, submersed ones narrower and more delicate than floating ones; in axil, a delicate membranous stipule, either free or adnate below. Inflorescence a many-flowered spike; peduncle axillary with 0, 1 or 2 bracts at base. Flowers small, green, bisexual, actinomorphic, without bracteoles, segments 4, free, valvate, concave. Stamens 4, on claw of segments; anthers 2-celled, apparently sessile, extrorse. Carpels 4, sessile, free, superior; stigma sessile or nearly so; ovule solitary, strongly curved. Achenes drupe-like when fresh; endocarp stony, splitting by separation of vertically-oriented operculum on outer face. Two genera - Potamogeton, cosmopolitan, of c. 100 spp.; Groenlandia of W. Europe, S. W. Asia, N. Africa, monotypic.

Infl. a bractless, ∞-fld spike; peduncle axillary and sheathed at its base. Fl. small, green, bisexual, actinomorphic. Per. represented by 4 free, valvate, concave segs adnate to the connectives of 4 stamens and alternating with 4 carpels. Anthers 2-celled, apparently sessile, extrorse. Carpels sessile, free, superior; stigma sessile or nearly so; ovule solitary, strongly curved. Achenes drupe-like when fresh; endocarp stony, splitting by separation of vertically oriented operculum on outer face. Aquatic herbs of brackish or fresh water or of marshy places. Lvs simple, us. alt., often dimorphic, submerged ones narrower and more delicate than floating ones. Stipule a delicate membrane, either free throughout or adnate to lf-base except for a free ligule, in each case either closed forming a tube round stem or open with margins overlapping on side of stem remote from lf. Two genera, one confined to N. Hemisphere.

Monoec. or dioec. Infl. of 1 to few minute unisexual fls enclosed within sheathing lf-bases. Per. 0 or represented by 3 membr. scales. Stamen solitary or 2–3; anthers 1–2-celled, dehiscing longitudinally; pollen globular or threadlike. Carpels 1–9, free, sessile or shortly stipitate; style simple or 2–4-lobed, stigmas capitate to funnelform; ovule solitary, pend., strongly curved. Fr. beaked with persistent style-base, indehiscent. Seed without endosperm. Aquatic, rhizomatous, perennial, glab. herbs of fresh, brackish or salt water, us. submerged. Lvs simple, narrowly linear, almost filiform, alt., but sts crowded as though whorled; stipules sheathing or open, either free throughout or adnate to lf-base except for free ligule or auricles. Genera 6, one widespread in temperate waters.