600 SUMMARY OF CURRENT RESEARCHES RELATING TO 



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bine to some extent the characteristic features of the Central European 

 and Northern European lakes, but are, on the whole, more nearly akin to 

 the latter. In addition, they have peculiarities which mark them off 

 from either of these groups ; for instance, the relatively high tempera- 

 tures in winter. The phytoplankton is never of very great bulk, and it 

 is quite exceptional for it to colour the water to any appreciable extent. 



The Myxophyceae play quite a secondary part in the plankton of the 

 British lakes as compared with the Central European lakes. The 

 Flagellata are well represented by various Peridinieae. The Bacillariege 

 are abundant, but they rarely occur in such great quantities as in the 

 Central European lakes. The Chlorophyceas are well represented, more 

 especially by the Desmidiacere : indeed, the most interesting feature of 

 the British fresh-water phytoplankton is the dominance of Desmids. In 

 this point the plankton of the western British lake areas differs markedly 

 from all other European plankton. The authors discuss at some length 

 this phenomenon of the rich Desinid flora, and begin by giving a brief 

 outline of the general distribution of the Desniidiacere in the British 

 Islands, quite irrespective of the fresh- water plankton. They note the 

 much greater richness in the western areas of the country as compared 

 with the eastern, which are exceedingly poor. On passing from the newer 

 Tertiary formations to the older Palaeozoic and Precambrian formations, 

 the Desmid flora gradually increases in richness, attaining its maximum 

 diversity in certain of the Precambrian areas. Thus it is obvious, that 

 since the great majority of the British lakes are situated in the western 

 parts of our islands, and the western districts are the richest in Desmids, 

 the plankton of these lakes should naturally contain an abundance of 

 Desmids. So much is this the case, that the plankton may be correctly 

 described as a Desmid-plankton. 



As regards the relationship between the conditions of environment 

 and the richness of the Desmid flora, the authors find that : — 1. The 

 rich Desmid areas correspond very accurately with the areas of the old 

 geological formations ; they are mostly mountainous districts, with con- 

 siderable outcrops of igneous rocks. 2. These areas also correspond, 

 but with less accuracy, to the areas of greatest rainfall. The comparative 

 absence of Desmids from certain localities in the Pennine Chain and in 

 the New Forest, which would seem ideal habitats for those plants, is 

 explained by the fact that the richness of a Desmid flora bears a distinct 

 relationship to the antiquity of the geological formations of the area 

 under consideration ; and the richest floras are only found in those 

 areas which combine the most suitable habitats (boggy hillsides with an 

 abundant rainfall), with a drainage-water derived from geological forma- 

 tions older than the Carboniferous. 



The authors consider that the Desmids of the plankton have without 

 doubt originated from the Desmid communitv of the surrounding area. 

 They find, also, that neither plankton Desmids, nor those which occur 

 in other situations, undergo any seasonal form-variations. Finally, 

 they comment upon the cosmopolitanism of the fresh-water plankton 

 community, except as regards Desmids. For that group they find that 

 wherever there are lakes with a rich Desmid flora in the plankton, there 

 one also gets a more or less definitely localised plankton community. 



