1908.] The British Freshwater Phytoplankton. 199 



aquatics thrive, and furnish all the requirements for the prolific growth of 

 Desmids. 



This seems a natural explanation of the occurrence of a rich Desmid- 

 flora, and one which is accepted by Wesenberg-Lund* as the main cause of 

 the phenomenon. 



A detailed study of the distribution of Desmids has shown us, however, 

 that the mere presence of suitable habitats is insufficient to account for the 

 great richness of the Desmidiacese in certain areas of these Islands. Among 

 the mountains of the Pennine Chain are some of the finest peat-bogs in 

 the British Islands — in all outward appearances ideal spots for the 

 occurrence of a rich Desmid-flora. Such are Cocket Moss, Austwick Moss, 

 and others less well known. Again, on Thursley Common in Surrey, and 

 in the New Forest, are bogs which are unsurpassed in Britain as habitats for 

 the Desmidiaceae. In all these localities the bogs are deep and dangerous, 

 fed mostly by bottom springs, and furnish an ideal home for quantities of 

 submerged Sphagnum and Utricularia minor. Desmids occur in countless 

 millions among the larger aquatics ; in fact, collections, made in these 

 localities would be regarded as " rich " or " very rich " by those who had no 

 experience of the western areas. Utricularia minor, which harbours some 

 of the best of British Desmids among its leaves, flowers profusely in the 

 localities mentioned, and no finer specimens can be obtained even in Mayo 

 and Galway. Yet the Desmid-flora which occurs among the Utricularia in 

 the above-mentioned localities is not to be compared with that which 

 occurs in precisely the same environment in the western British areas, and, 

 moreover, it contains none of the real British rarities. 



How is it that an ideal locality such as Cocket Moss, which would be 

 described as " rich in Desmids," contains practically none of those species 

 which are both dominant and characteristic of the western areas ? The 

 conditions are almost identical with those obtaining in the western bogs, and 

 the rainfall is from 50 to 60 inches. 



From a consideration of the above remarks it is obvious that some factor, 

 other than mere abundance of rainfall and presence of ideal habitats, has a 

 profound influence on the distribution of Desmids. This at once causes us 

 to enquire into the statement that we have previously made,t viz., that the 

 rich Desmid-areas correspond geographically with the Precambrian and Older 

 Palaeozoic outcrops (together with the intrusive Igneous material). (Consult 

 figs. 5 and 6.) 



* Wesenberg-Lund, ' Plankton Investigations of the Danish Lakes,' Copenhagen, 1908 

 p. 281. 



t Vide p. 197 of the present paper. 



P 2 



