The Development of Vegetation in the English LoJces. 267 



General Conclusion. — A conclusion of even more significance is now 

 permissible. The fact that these lakes lie in glacial rock-basins entitles us to 

 assume that at the close of the glacial period their condition was more 

 uniformly rocky than it is to-day. Lakes lying on the hardest rocks have 

 changed least, they are still rocky and relatively irriraitive. Lakes lying on 

 softer strata are now more silted and therefore more highly evolved. It is thus 

 possible to consider the group of lakes as a series illustrating the stages in 

 the post-glacial development of a typical rock basin lake, a conception of great 

 value, since it enables us to study the stages in development of a post-glacial 

 fauna and flora. 



On. this assumption, Wastwater and Ennerdale are most primitive, and 

 probably are not greatly altered from their immediate post-glacial condition. 

 Windermere, Ullswater, and particularly Esthwaite, are most highly evolved, 

 the remaining lakes exhibiting various intermediate stages, of which Derwent- 

 water is perhaps the most valuable. In adopting this view, it must be 

 admitted that the topography of the extremely evolved lake basins — notably 

 of Esthwaite — was probably never so primitive as that of Wastwater. This, 

 however, affects the degree of evolution only ; it does not affect its direction, 

 which, in lakes lying, among similar rocks, and relatively uniform glacial 

 deposits, must have proceeded along the same main Hnes. 



We can now apply this theory of lake development to the distribution of 

 the aquatic vegetation. 



Vegetation. 



The vegetation of a lake area falls conveniently into two main groups : — 

 (1) attached plants, or benthos ; (2) free-floating plants, m plankton. 



Benthos. — The distribution of the characteristic deep-water plants in these 

 lakes is shown in Table lY, the numbers given representing percentages of 

 the total deep-water flora. For convenience, the species are divided into 

 three groups : — 



(A) Primitive plants : Isoetes and Characece. 



(B) Silt-requiring plants : Juncus, Ccdlitriche, and Potamogeton. 



(C) Plants of organic soils : Myriophyllum, etc. 



Elodea can be placed in both (B) and (C). 



As most of the soundings on which these figures are based were made in 

 bays, the proportions given for plants in group B are probably too high, but 

 the results are comparable, since the same error exists in each tabulation. 



An analysis of the habitat conditions under which these plants Kve shows 

 that their distribution depends upon the characters of the substratum rather 



