Variableleaf pondweed

Description

Potamogeton gramineus is a species of aquatic plant known by the common name various-leaved pondweed, variableleaf pondweed, grass-leaved pondweed or grassy pondweed, native to the northern hemisphere where it grows in shallow, clean water. This is an aquatic perennial growing from a creeping rhizome that anchors in wet substrate. It produces thin, cylindrical, heavily branching stems usually less than a metre in length. The submerged leaves are sessile, relatively narrow, typically 40–90 mm long and 5–12 mm wide along the main stem but smaller on the side branches. They are translucent and pale green with a white midrib, and finely toothed. Floating leaves are more oval in shape, 20–70 mm long by 7–34 mm wide, and borne on long petioles. The inflorescence is a short spike of many flowers arising from the water on a stout peduncle. This species readily hybridizes with many other species of Potamogeton. Hybrids have been recorded with P. perfoliatus (P. nitens Weber), P. lucens (P. angustifolius J.Presl.), P. natans (P. sparganiifolius Laest. ex Fr.), P. coloratus (P. billupsii Fryer), P. maackianus (P. biwaensis Miki), P. illinoensis (P. spathuliformis (J.W.Robbins) Morong), P. richardsonii (P. hagstromii A.Benn.), P. alpinus (P. nericius Hagstr.), P. nodosus (P. lanceolatifolius (Tiselius) C.D.Preston), P. oakesianus (P. mirabilis Z.Kaplan, Hellq. & Fehrer) and P. polygonifolius. In Britain, P. nitens and P. angustifolius are quite common. Recent molecular analysis has shown that Swedish collections of the putative hybrid P. gramineus P. polygonifolius are in fact P. gramineus P. nodosus. A triple hybrid, P. gramineus × lucens × perfoliatus (P. × torssandrii (Tiselius) Dörfler), is also known. As well as hybridising frequently, various-leaved pondweed is an extremely variable plant and care should be taken with its identification. Potamogeton gramineus is native to much of the Northern Hemisphere, with a Holarctic distribution. It occurs in northern Europe (Britain, Ireland, Iceland, Scandinavia, Germany, Switzerland, France, Poland, Russia, the Baltic States), Greenland, North America (USA except for the southeast, Canada). There are outlying populations in southern Europe (Iberia, Corsica, Italy), Caucasus, Pyrenees and western USA.