Limnoplankton 443 



species of these genera and almost invariably sterile. In the smaller alpine 

 lakes several slender species of Mougeotia are often abundant and they may 

 form no small part of the phytoplankton 1 . Curious coiled Mougeotia- filaments 

 sometimes occur (W. & G. S. W., '09 A ; '09 B). It would appear that this 

 coiling is a limnetic character developed to augment the floating capacity 

 of the filament and is strictly comparable with that which occurs in certain 

 species of Anabtena, Lyngbya arid Melosira. 



********* 



In making a close study of the phytoplankton of lakes, difficulties of 

 comparison are everywhere met with, even when dealing with lakes in the 

 same area. The constituents of the phytoplankton are not the same in all, 

 and species which occur abundantly in one lake may not occur in any of the 

 others. These differences in constituents are partly territorial and partly 

 local, and are in some measure due to the rigorous conditions which govern 

 the distribution of so many aquatic Algae. A territorial distinction occurs 

 in those lakes situated in drainage-basins in which the rocks are older than 

 the Carboniferous. Local differences between the lakes of one area situated 

 in similar basins, when they occur, are often the result of contamination of 

 the water (W. & G. S. W., 12). 



The varying nature of the plankton of different lakes is to be correlated 

 with the fact that the various groups of Alga? require different physiological 

 conditions for rapid multiplication. For instance, the factors which favour 

 the prolific growth of desmids in the plankton 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 favour- 

 able for a similar increase in another. A careful study of the constituents of 

 the phytoplankton in relation to the lake-basins brings with it the conviction 

 that the factor of greatest importance in both the qualitative and quantitative 

 distribution of plankton is the amount and nature of the dissolved salts present 

 in the water. The highest percentage of dissolved salts is found in those 

 lakes which are contaminated from adjacent farms, villages and towns, and 

 such lakes contain a greater quantitative bulk of plankton. 



Slightly contaminated lakes contain a greater number of diatoms than 



1 In the alpine lakes of the Pike's Peak Region, Colorado, Shantz ('07) states that species of 

 Spirogyra and (Edogonium form a large part of the summer plankton. Fragmentary filaments 

 of various species of (Edogonium are also very frequent in the summer plankton of the British 

 lakes. The species are mostly of moderate size, with filaments 2035 M in diameter. Dakin & 

 Latarche ('13) make an extraordinary statement that these (Edogonium-records may have been 

 confused with Tribonema, which is almost identical with (Edogonium ' ! There is, of course, no 

 close resemblance between (Edogonium and Tribonema ; even dead empty cells could not be 

 confused. Moreover, the (Edogonium-fi\a,ments which find their way into the plankton and there 

 live for some time as adventitious constituents, are of much greater diameter than any known 

 species of Tribonema. 



