Notes on the Potamogetones of the Zuidlaren Lake and its adjacent Waters

More than half a century ago Vuyck (1895, p. 630) urged the need of a renewed investigation of the occurrence and distribution of Potamogetones in the northern provinces of the Netherlands. Judging from the localities recorded by Kloos (1936), it is obvious that this recommendation is still actual, especially so for the province of Groningen. Whilst Potamogetones occur there in almost every water, on the maps of Kloos (l.c.) only a small number of squares is marked. Therefore I collected in the summer of 1950 the Potamogetones of the Zuidlaren Lake and some adjacent waters, situated about 10 km S of the city of Groningen. Potamogetones of this greatest oligohaline lake of the Netherlands were mentioned earlier by Havinga (1919, p. 14; this paper contains ample general information on the Lake) and by Clason (1928, II, p. 42). However, some species found by Havinga I could not find again, also some species I found in 1928 were not collected in 1950 and with some other species happened just the reverse. Kloos (l.c.) does not record any species from the Lake; however, on his maps the square comprising the lake-area is crossed for P. natans L., P. lucens L. and P. oblongus Viv. (= P. polygonifolius Pourr.). In the following notes I stick to the nomenclature and the descriptions of Ascherson and Graebner (1913); numbers refer to table I.