1908. J The British Freshivater Phytoplankton. 



197 



the dominance of Desmids in the plankton of the British lakes, a fairly 

 complete and necessary knowledge of the general distribution of Desmids 

 in the bogs, pools, etc., throughout the whole of the British Islands. The 

 first point of importance is that the great majority of the British lakes 

 (those constituting the western and north-western lake-areas) are all 

 situated in the richest Desmid-areas in these Islands, or for that matter 

 in Europe. It is, therefore, not in the least surprising that the plankton 

 of these lakes should on the whole contain an abundance of Desmids. 

 That the Desmids of the plankton should differ considerably from those 

 of the bogs of the drainage areas — a matter discussed in a later part of 

 this paper— does not affect the main question, viz., that the phytoplankton of 

 these lakes possesses in many instances such an abundance of Desmids that it can 

 be correctly described as a Desmid-plankton. 



Those facts which explain the abundance of Desmids in the bogs and 

 bog-pools, among the mosses of the dripping rocks, and among the leaves of 

 the submerged plants of the lake-margins, will likewise furnish the 

 explanation of the abundance of Desmids in the plankton, as the plankton- 

 Desmids have certainly originated from bog and swamp species, and others 

 are being constantly recruited from the same sources. In endeavouring to 

 discover the relationship between the conditions of environment and the 

 richness of the Desmid-flora, two facts stand out very clearly : — 



1. The rich Desmid-areas correspond very accurately with the areas of the 

 old geological formations. They are mostly mountainous districts, with 

 considerable outcrops of Igneous rocks. 



2. These areas also correspond, but with less accuracy, to the areas of 

 greatest rainfall.* 



It is now necessary to enquire more closely into the relationships between 

 the geological nature of the drainage-area, the rainfall, and the richness of 

 the Desmid-flora. 



We will first consider the rainfall of the areas in question. This is 

 relatively heavy, varying from about 45 to upwards of 100 inches, and is 

 due to two causes : first, to the fact that these areas are almost all near the 

 west coast, being districts in which large mountains are situated in close 

 proximity to the sea ; and secondly, to the prevailing westerly and south- 

 westerly winds. Such conditions naturally result in wet, mossy hill-sides, 

 with numerous bogs. There is consequently much peaty water, rich in 

 humic and other organic acids, in which submerged plants, such as 

 Utricularia minor, Sphagnum cuspidatum, S. subsecundum, and other 



* Mr. James Murray (in 'Roy. Phys. Soc. Edin. Proc.,' vol. 16, 1905, p. 58) also points 

 out that Sir John Murray had indicated this fact to him. 



VOL. LXXXI. — B. P 



