428 МВ. W. WEST AND PROF. G. S. WEST ON THE 
These differences in the constituents are partly territorial and partly local, 
and are in part due to the rigorous conditions which govern the distribution 
of so many of the purely aquatic Algæ. We have already shown by a 
carefully collated mass of evidence that a territorial distinction occurs in 
those lakes situated in drainage-basins in which the rocks are older than the 
Carboniferous. Some of these lakes possess a Desmid-flora in the plankton 
such as does not oceur in any lakes which are situated in basins of newer 
formations. Local differences between two lakes in similar basins are 
frequently due to contamination of the water, some groups of Alg:e 
thriving under such circumstances whereas others are greatly reduced or 
eliminated. 
The varying nature of the plankton of different lakes is to be correlated 
with the fact that the various groups of Algz require different physiological 
conditions for rapid multiplication. For instance, the factors which favour 
the prolifie growth of Desmids are not those which enable an equally rapid 
increase in the majority of Diatoms; and likewise those factors which 
favour the great multiplication of one species of Diatom are not favourable 
for a similar increase in another. 
A careful study of the constituents of the phytoplankton in relation to the 
lake-basins has convinced us that the factor of greatest importance in both 
the qualitative and quantitative distribution of plankton is the amount of 
dissolved sults present in the water. The highest percentage of dissolved 
salts is found in those lakes which are slightly contaminated from adjacent 
villages and farms, and we have before remarked upon the greater 
quantitative bulk of the plankton of such lakes *. 
Slightly contaminated lakes contain a greater. number of Diatoms than 
uncontaminated lakes, and as a rule some of them are perennial con- 
stituents of the plankton, А few species frequently have enormous maxima, 
particularly Asterionella gracillima, Tabellaria fenestrata var. asterionelloides, 
and Melosira granulata, such maxima being probably due to the abundance 
of nitrates. The Desmida-flora of these lakes is usually poor, and few species 
ever occur in quantity. 
On the other hand, wacontaminated lakes (or lakes with very pure water) 
contain fewer Diatoms, and such as do occur in the plankton rarely attain 
even a small maximum. The Desmids are generally numerous and there 
is often a rich Desmid-plankton. The lakes of the Carnarvonshire mountains 
are excellent examples, being amongst the least contaminated of all the 
British lakes. Hence in these lakes there are relatively few Diatoms (only 
11:1 per cent, out of a total of 162 species) and many Desmids (62:4 per 
cent.), and some of them possess a very rich Desmid-plankton. 
* W.& G: S. West, in Proc, Roy. Soc. В, vol. Ixxxi, 1909, р. 175; ‘Naturalist, Sept. 1909, 
р. 380, 
