1. Potamogeton L. (pondweed)

Plants perennial, glabrous. Stems diffuse, highly branched, rooting at lower nodes. Leaves alternate, sometimes appearing opposite or tufted, filiform to ovate or elliptic, sometimes differentiated into floating and submerged leaf types. Stipules present or sometimes degraded at maturity, free and appearing axillary, sometimes partially or totally fused to the petiole forming an open sheath or sometimes free from the leaf base but forming a sheath that clasps the stem. Inflorescences axillary or terminal spikes, the peduncle not becoming coiled during maturation of the fruits. Flowers perfect, minute. Perianth of 4 tepals, these narrowed to a claw. Stamens 4, opposite the tepals and fused to the tepal bases, the filaments absent or nearly so. Gynoecium of 4 separate pistils, the ovaries superior, the style lacking or short, the stigma 1. Ovules 1 per ovary. Fruits achenes or drupelike, often somewhat flattened and sometimes beaked and/or keeled, the embryo usually somewhat cuved or coiled. About 90 species, worldwide.

The linear‑leaved species of Potamogeton are sometimes confused with those of Najas and Zannichellia. Leaves of both these other genera are mostly opposite or whorled, rather than alternate, and those of Najas further differ in having toothed margins (sometimes visible only under magnification).

The pondweeds are all aquatic, varying from submerged species to those with floating leaves. Plants of some taxa sometimes occur emergent on mudflats and lakeshores. The spikes of Potamogeton are usually held just above the water surface, or less commonly are submerged. The flowers are variously adapted to pollination by wind or by water. The fruits and foliage provide food for various kinds of wildlife, and the plants also provide cover for fish and other aquatic animals.