16 Algae. 



combine 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 relati- 

 vely high winter temperatures. Very many of these lakes never 

 freeze, and most of the others only rarely become covered with 

 ice, and for comparatively brief periods. The summer temperatures 

 are also comparatively low. In all the four British lake-areas the 

 water is soft, with only small quantities of dissolved lime. 



The phytoplankton is never of very great bulk, and it is quite 

 exceptional for it to coiour the water to any appreciable extent. 



The periodicity is very variable, being conspicuous in some 

 lakes, in others not very well marked. 



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 Ba- 

 cülarieae are abundant, but they rarely occur in such great quanti- 

 ties as in the Central European lakes. The Chlorophyceae are well 

 represented, more especially by the Desmidiaceae; indeed the most 

 interesting feature of the British fresh-water phytoplankton is the 

 dominance of Desmids. In this point the plankton of the wes- 

 tern British lake-areas differs markedly from all other European 

 plankton. 



The authors discuss at some length this phenomenon of the 

 rieh Desmid-flora, and they begin by giving a brief outline of the 

 general distribution of the Desmidiaceae 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, attain- 

 ing its maximum diversity in certain of the Precambrian areas. The 

 majority of the British lakes are situated in the western parts of 

 our Islands and since those areas are richest in Desmids, it is evi- 

 dent that the plankton of these lakes should naturally contain an 

 abundance of Desmids. So much is this the case, that the plankton 

 may correctly be described as a Desmid-plankton. As regards the 

 relationship between the conditions of environment and the richness 

 of the Desmid-fiora, the authors find that: 1. The rieh Desmid-areas 

 correspond very accurately with the areas of the old geological for- 

 mations. They are mostly mountainous districts, with considerable 

 outcrops of igneous rocks. 2. These areas also correspond, but with 

 less aecuraey, 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 distinet relationship to the antiquity of the geological for- 

 mations 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 formations older than the Carboniferous. 

 Thus the Desmids of the British freshwater plytoplankton are due 

 largely, and the western types entirely, to the Situation of the lakes 

 in the rieh Desmid-areas of the old formations. The antiquity of the 

 geological formations is not a special factor in the oecurrence in 

 the numerous plankton Desmids, but in the oecurence of Des- 



