Potamogeton pusillus L. en P. berchtoldi Fbr. in Nederland

Some Netherlands botanists already had recognized Potamogeton berchtoldi Fieber as an infraspecific taxon, however as a species it was first mentioned by Clason (1957). Plasticity and nomenclature of the Pusilli generally, and of P. berchtoldi Fbr. and P. pusillus L. particularly, are discussed. The taxonomy of the subsection Pusilli Graebner and of the two indigenous species is dealt with in connection with plants found in the Netherlands. The author is of opinion that P. trichoides Cham, et Sch. should be regarded as representing a series Monogyni (Hagstr.) Clason, which would have to find a place next to the series Connati Hagstr. (P. friesii Rupr. and P. pusillus L.) and the series Convoluti Hagstr. (P. obtusifolius M. et K., P. berchtoldi Fbr. and P. rulilus Wolfg., which has not yet been found in the Netherlands). The problem of the hybrids of the Pusilli is discussed. In the Netherlands occurs P. berchtoldi var. acuminatus Fbr.; var. berchtoldi has not yet been found with certainty; var. lacunatus (Hagstr.) Fernald and var. tenuissimus (M. et K.) Fernald are reduced to formae. The distribution of the two species is only incompletely known. Provisionally it can be said that it looks as if P. pusillus L. prefers somewhat saline, alkaline waters outside the higher pleistocene regions of the Netherlands. P. berchtoldi Fbr. f. lacunatus (Hagstr.) Clason appears to occur by preference in more or less running waters. The abundant lacupae of this form, wherewith a greater buoyancy is reached, may perhaps be considered a reaction on the tension exercised by streaming water.