Claspingleaf Pondweed (Potamogeton perfoliatus)

Description

Potamogeton perfoliatus (claspingleaf pondweed, perfoliate pondweed, redhead grass) is a perennial aquatic plant in the family Potamogetonaceae occurring in both standing and flowing freshwater habitats. It is widely distributed globally, occurring in all continents except South America and Antarctica. Perfoliate pondweed grows from a robust creeping perennial rhizome, intermittently producing round stems up to 3 m long. The submerged leaves are oval and translucent, with no stalk, 20-115 mm long and 7-42 mm wide, clasping the stem (perfoliate), a flat apex, and 5-12 veins on either side of the midrib.They vary considerably in colour and may be bright green, dark green, yellowish, olive or brownish. There are no floating leaves. The stipules are rather delicate and usually fall off quite soon after the new leaf has unfurled. The insignificant flowers are produced between June and September. Fruits are 4 mm across, olive green in colour.Two subspecies have been described. Subspecies perfoliatus occurs in the Old World. In North America, populations on the eastern seaboard and the Gulf States are considered to be subsp. bupleuroides.Since many pondweeds, including P. perfoliatus, vary naturally in response to growing conditions, it is possible that these variants are ecotypes rather than genuine subspecies. The related P. richardsonii was formerly considered a subspecies of P. perfoliatus, but is now classed as a separate species.Like most other broadleaved pondweeds, perfoliate pondweed is tetraploid, with 2n=52.Over much of its range, perfoliate pondweed is unlikely to be confused with any other pondweed species except perhaps P. praelongus. In North America it could be confused with P. richardsonii, but the latter has stipules that disintegrate to persistent fibres even on the lower part of the stem, whereas those of P. perfoliatus disintegrate entirely.However, perfoliate pondweed regularly hybridises with other Potamogeton species including P. crispus (P. - cooperi (Fryer) Fryer), P. gramineus (P. - nitens Weber), P.lucens (P. - salicifolius Wolfg.), P. alpinus (P. - prussicus Hagstr.), P. berchtoldii (P. - mysticus Morong), P. wrightii (P. - anguillanus Koidz.), P. maackianus (P. - leptocephalus Koidz.), P. alpinus (P. - prussicus Hagstr.), P. nodosus (P. - assidens Z. Kaplan, Zalewska-Ga-osz et M. Ronikier), P. richardsonii (P. - absconditus Z. Kaplan, Fehrer & Hellq.), P. epihydrus (P. - versicolor Z. Kaplan, Hellq. and Fehrer) and P. praelongus (P. - cognatus Asch. & Graebn.). The first three of these are quite common. A triple hybrid, P. gramineus - lucens - perfoliatus (P. - torssandrii (Tiselius) D-rfler), is also known.Many of these hybrids are perennial and long-lived, sometimes occurring in the absence of one or even both parents. Therefore, care should be taken with identification of unusual specimens.