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).
Morphology Potamogeton species range from large (stems of 6 m or more) to very small (less than 10 cm). Height is strongly influenced by environmental conditions, particularly water depth. All species are technically perennial, but some species disintegrate in autumn to a large number of asexually produced resting buds called turions, which serve both as a means of overwintering and dispersal. Turions may be borne on the rhizome, on the stem, or on stolons from the rhizome. Most species, however, persist by perennial creeping rhizomes. In some cases the turions are the only means to differentiate species. 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). Broad-leaved taxa are mainly tetraploid, with 2n = 52 (e.g. P. alpinus, P. praelongus), but a few species are diploid (e.g. P. coloratus or octaploid (2n = 104; e.g. P. illinoensis).
Taxonomy Potamogeton is a genus of freshwater aquatic plants in the Potamogetonaceae. Molecular analysis has identified Groenlandia as the sister group to Potamogeton, although the Angiosperm Phylogeny Group also lists Zannichellia, Althenia and Pseudoalthenia as closely related genera. Some researchers have used molecular analysis and pollen grain shape 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. The genus is generally divided into two groups: broad-leaved and linear-leaved. The broad-leaved group includes such species as P. natans, P. perfoliatus and P. alpinus. The linear-leaved group includes such species as P. rutilus, P. compressus and P. berchtoldii. Series Batrachoseris historically contained only one species, Potamogeton crispus, however more recent research has also added P. maackianus and P. robbinsii into this grouping. These general divisions have been supported by molecular analysis, except that P. crispus clusters with broad-leaved species in section Potamogeton. In a detailed review of the genus, Wiegleb and Kaplan recognised 69 species, but the variability of many species means that there is disagreement regarding the exact number of species. Currently, the number of accepted names is 94. Hybridisation provides an added complexity to the taxonomy.
Distribution Potamogeton species are found worldwide in many aquatic ecosystems. However, the greatest diversity of species occurs in the northern hemisphere, especially in North America, which is thought to be where the genus originated. Molecular evidence suggests that several independent colonizations of the southern hemisphere have occurred. However, due to their self-propagation from turions, Potamogeton populations show very low infrapopulation diversity, 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.
Hybrids Potamogeton species hybridise freely. At least 27 hybrids have been observed in the British Isles alone, and more than 50 worldwide, of which 36 have been confirmed using genetic techniques. The majority of these are sterile, but many are long-lived and may occur in the absence of one or even both parents. Most hybrids have been described between broad-leaved species, but this probably reflects the relative scarcity of characters among fine-leaved taxa, so that hybrids are much more difficult to identify. Use of genetic markers suggests that hybrid taxa are also reasonably frequent among fine-leaved species. At least one species, P. obtusifolius, is thought to have arisen via hybridisation.
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