Flora of New Zealand Series

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.