PERIODICITY OF THE PHYTOPLANKTON OF SOME BRITISH LAKES. 429 
Lakes which possess a mixed plankton of Diatoms and Desmids are 
probably of an intermediate character with regard to the nature and 
amount of the dissolved salts in the water. The Myxophycer are to a 
great extent absent from the lakes with very pure water, but an examination 
of the occurrence and distribution of the plankton-species of Blue-green 
Algz indicates that the factors which control their relative abundanee | 
are somewhat different from those which govern the prolific occurrence of 
Diatoms. 
There is also а considerable reduction in the amount of Asterionella and 
of the star-dispositions of Tabellaria, or even an entire absence of them 
from lakes with pure water. These two genera of Diatoms are absent from 
the large African lakes, most probably owing to too high a temperature of 
the water *, but their absence from certain British lakes appears to be 
directiy concerned with purity of the water. Wastwater furnishes a good 
example of a lake from which these star-dispositions of the frustules of 
Diatoms are absent. 
The comparative and periodic study of the plankton of the British lakes 
has afforded evidence in the elucidation of the fact which we first definitely 
mentioned in 1905 f, and subsequently expounded at greater length 1, 
namely, that “the rich Desmid-areas correspond with the Pre ambrian and 
Older Paleozoic outcrops (together with the intrusive Igneous material).” 
It is the occurrence of a rich Desmid-plankton only in those lakes which 
are least contaminated. and have the purest water, which has led us to the 
conclusion that the principal factor controlling the abundance of Desmids is 
the nature and amount of the dissolved salts in the water. Hence it 
becomes obvious why the rich Desmid-areas should be on the old formations, 
as for the most part the drainage-water of such areas contains much less 
in the way of dissolved mineral salts than that of areas of the newer 
formations. This is due in a great measure to the relative hardness of 
these rocks, which in itself is due to their antiquity, and also partially to the 
comparative absence of lime, a substance which is distinctly unfavourable to 
the growth of the majority of Desmids. 
It must, however, be emphasized that not all the lakes of one of these 
rich Desmid-areas contain a rich Desmid-plankton. Some are too much 
contaminated by the sewage from farms and villages, and others which may 
have no such contamination, but yet have no Desmid-plankton, probably 
receive drainage-water which percolates through certain strata from which 
too much mineral matter is removed in solution. 
Thus the comparative richness of the Desmid-flora of the various parts of 
ж G. S, West, in Journ. Linn. Soc., Bot, xxxviii. 1907, p. 84. 
+ W. & С. S, West, in Trans, Roy. Soc. Edin. xli. part 3, 1905, p. 515. 
{ W. & G. S. West, in Proc. Roy. Soc. В, vol. Ixxxi. 1909, pp. 195-201. 
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