6 Introduction 



Algae also occur in warm streams, and the vegetation of hot-springs 

 consists exclusively of Algae. They can exist in hot water and hot 

 vapour up to a temperature of 94'5 C. (200 F.) 1 . It is worthy of 

 note that the Algae which occur at very high altitudes, and which 

 therefore exist at relatively low temperatures, and those which 

 inhabit the hottest springs, are, with few exceptions, species of 

 Myxophyceae and Bacillarieae. 



Some Algae become encrusted with carbonate of lime or with 

 silica, and play no small part in the formation of the deposits 

 which are generally found in the neighbourhood of hot-springs. 



The comparative richness of any district in freshwater Algae 

 depends very largely on its physical geography and on the geo- 

 logical formations. Mountainous tracts are more prolific than flat 

 districts, even though many of the larger Algae are absent from 

 them. Most of the larger filamentous Algae and an abundance of 

 the commoner unicellular forms are found in low-lying quiet 

 waters, but in mountainous areas the filamentous forms are chiefly 

 representatives of the Myxophyceae and Conjugatae, the presence 

 of numerous species of the genus Mougeotia being a marked feature 

 of such districts, and the unicellular forms are greatly increased 

 by the addition of numerous Desmids. If the mountains consist 

 of the Older Palaeozoic rocks, of Pre-Cambrian rocks, or of rocks of 

 Igneous origin, there is a surprising numerical increase, not merely 

 of species but also of individuals ; and in comparison, a mountainous 

 district of carboniferous limestone or other formation is distinctly 

 poor. Thus, the English Lake District, Wales, and certain parts 

 of Scotland and Ireland yield a much greater variety of Algae than 

 any other parts of the British Isles. The poorest area of all is the 

 fen district in the east of England. 



The most prolific localities in the British Islands, and perhaps 

 in the whole of Europe, for freshwater Algae are the small tarns 

 and peat-bogs which lie in the hollows of the Lewisian gneiss of 

 north-west Scotland. The plankton of the larger lakes of this 

 area is also much richer in the Desmidiaceae than any which has 

 been described from elsewhere. 



Most of the unicellular Algae and some of the filamentous ones, 

 unless specially protected as in many Desmids, are readily taken 



1 W. H. Brewer in Amer. Journ. Science, ser. 2, xli. These Algae were unicells, 

 filamentous Algae. having been observed up to a temperature of 85 C. (185F.); 

 G. S. West in Journ. Bot. 1902, p. 241. 



