Glabrous, aquatic herbs with fibrous roots, over wintering by winter buds or rhizomes. Leaves 2-ranked, alternate or opposite, alike or dimorphic (submerged leaves sessile or petiolate, usually translucent; floating leaves petiolate, usually opaque, lamina dilated). Stipular sheaths free, axillary, or adnate to the leaf base. Inflorescence an axillary or terminal, peduncled, ovoid or cylindrical spike, lax or with flowers irregularly whorled, submerged or emergent, enclosed by stipular sheaths when young. Flowers ebracteate, sessile, tetramerous, small, bisexual, hypogynous. Perianth 4-segmented (sometimes the segments have been treated as outgrowth from the connectives of the anthers). Stamens 4, basally adnate to the perianth segments; anthers 2-locular, extrorse, sessile. Carpels 4 (rarely 3-1), apocarpous, 1-celled with one campylotropous ovule; stigma sessile or subsessile. Fruitlet wall opening with a lid of the dorsal area. Seeds lacking endosperm; hypocotyl massive; seed coat free from fruitlet wall; embryo uniform or spiral.
A family of 2 closely related genera and over 90 species, mostly of fresh water habitat, chiefly in temperate regions and cooler climates; represented in Pakistan by a single genus.
Formerly included under Najadaceae (Benth. & Hook. f., Gen. Pl. 3: 1018. 1876; Hook. f., Fl. Brit. Ind. 6:562.1893), which has now been split into a number of smaller families.
Acknowledgements: We are grateful to the United States Department of Agriculture for financing this research under P.L. 480. Thanks are also due to Mr. J.E. Dandy (BM) and Mr. B.L. Burtt (Edinburgh) for helpful suggestions.