Potamogeton x schreberi G. Fisch. (P. natans L. x P. nodosus Poir.) in Dorset, new to the British Isles

Potamogeton x schreberi G. Fisch. (P. natans L. x P. nodosus Poir., Potamogetonaceae) was discovered in 1992 in the River Stour, Dorset, the first record from the British Isles. Both P. natans and P. nodosus are recorded from the R. Stour, although neither has been found growing with P. x schreberi. The Dorset plant is described and compared with material from Germany and Switzerland. P. x schreberi differs from P. natans in having laminar submersed leaves and from P. nodosus in having phyllodes near the base of the stem and narrower submersed leaves which are entire rather than minutely denticulate . The differences between this hybrid and P. x fluitans Roth (P. natans x P. lucens L.) and P. x sparganiifolius Laest. ex Fr. (P. natans x P. gramineus L.) are discussed. P. x schreberi appears to be sterile, but is apparently able to persist and spread by vegetative reproduction .