A reassessment of the hybrid Potamogeton X gessnacensis G. Fisch. (P. natans X P. polygonifolius, Potamogetonaceae) in Britain

Potamogeton x gessnacensis G. Fisch. (P. natans L. x P. polygonifolius Pourr.), hitherto known in Britain from Caemarvonshire (v.c. 49) and East Ross (v.c. 106), was discovered in 1996 in Shetland (v.c. 112). Cytological examination of plants from Caemarvonshire and Shetland shows that they are similar, with a chromosome number of 2n = c. 39. This is intermediate between P. natans (2n = 52) and P. polygonifolius (2n = 28), both counts previously reported from British materiaL Populations of P. x gessnacensis are variously intermediate in morphology between the putative parents, the Caemarvonshire plants being closer to P. natans than the others; they are highly sterile. The habitat of P. x gessnacensis at its British sites is described. P. natans has not been recorded from any of the sites, which suggests that it may have been eliminated by competition with the hybrid.