Potamogeton is a genus of aquatic, mostly freshwater, plants of the family Potamogetonaceae. Most are known by the common name pondweed, although many unrelated plants may be called pondweed, such as Canadian pondweed (Elodea canadensis). The genus name means "river neighbor", originating from the Greek potamos (river) and geiton (neighbor).
The leaves are alternate, which contrasts with the closely related genus Groenlandia, where the leaves are opposite or whorled. In many species, all the leaves are submerged, and in these cases, they are typically thin and translucent. Some species, especially in ponds and very slow-moving waters, have floating leaves which tend to be opaque with a leathery texture. Leaf shape has been found to be highly plastic, with variability due to changes in light, water chemistry, planting depth, sediment conditions, temperature, photo period, waves, and seasonality. All Potamogeton have a delicate membranous sheathing scale, the stipule, at the leaf axil. This may be wholly attached, partly attached, or free of the leaf, and it may have inrolled margins or appear as a tube. The morphology of the stipule is an important character for species identification. The stems have small scales.
The flowers, which are often overlooked, are greenish-brown and are composed of four rounded segments borne in a spike. They are 2–4 merous, with superior ovaries and anthers that turn outward. The fruits are spheroidal and green to brown, usually 1–3 mm in diameter, with a noticeable 'beak'. Their pollen is inaperturate, monad, apolar and spherical.
Most fine-leaved pondweeds are diploid, with 2n = 26 (such as P. pusillus or P. trichoides) or less commonly 28 (P. compressus, P. acutifolius). although the Angiosperm Phylogeny Group also lists Zannichellia, Althenia and Pseudoalthenia as closely related genera. Some researchers have used molecular analysis to argue for the placement of Stuckenia at the genus level, but others have argued there is not enough difference to justify the change, and have kept Stuckenia as a subgenus of Potamogeton. especially when living in deep, shaded or turbulent water where colonies do not expend energy on flowers. This has made it very difficult to use genetic data to find patterns of geographical differentiation.
In 1916, Johan Hagström published Critical researches on the Potamogetons from detailed measurements of dozens of vegetative, anatomical and reproductive characteristics such as fruit beak, peduncle rigidity and leaf apex across hundreds of specimens. It was a pioneering work that preceded the development of cladistics by more than 30 years, and his clustering of groups into broad-leaf and linear-leaf is still considered the framework for all subsequent classification systems of Potamogeton. Hagström's work was developed further by Merritt Fernald (1873-1950) who created a detailed phylogeny for the linear-leaved species, and by Eugene Ogden who did so for the broad-leaved species.
New species continue to be described from less well-studied areas such as Asia and Africa, and it is possible that molecular analysis may reveal hitherto unknown cryptic species.
List of Potamogeton species
The following list is based on the latest listing of valid Potamogeton taxa held on The Plant List.