Potamogeton
Potamogeton, commonly pondweed, is a genus of aquatic, mostly freshwater, plants of the family Potamogetonaceae. Plants are mostly perennial and typically produce rhizomes which are the common over-wintering form. Many species also produced specialised overwintering buds called Turions which may be borne on the rhizome, on the stem or on stolons from the rhizome. The leaves are usually opposite except in the flowering region of the stem. This 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 the leaves are typically thin and translucent. Some species, especially in ponds and very slow moving waters, have floating leaves which tend to be more leathery.
Diagnostic of most species of Potamogeton is the presence of a delicate membranous sheathing scale 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 flowers, which are often overlooked are composed of 4 rounded segments borne in a spike.
Potamogeton species are found throughout the world where there is standing or running water. There are estimated to be around 90 species but hybridisation provides an added complexity to the taxonomy
Not all plants called pondweed are in the genus Potamogeton. "Canadian pondweed", Elodea canadensis, which shares some characteristics, is , for example not a Potamogeton.
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Floating leaves of Potamogeton natans
