408 Algae. 



given ot' the periodicity observed in Lochs Earn and Lubnaig, 

 Perths hire. 



The general conclusions are not merely drawn from the data 

 given in this paper, but also from Observation^ on the plankton of 

 dozens of other lakes in all the lake-areas in the British Islands. 



The greatest amount of phytoplankton is found in the late 

 summer and autumn, during the autumnal decline in temperature. 



Although there are certain more or less well-marked phases in 

 the phytoplankton, it is difhcult to compare the phytoplankton of 

 one lake with that of another, as the annual phases of one probably 

 do not correspond with thosc of the other. Moreover, it is found 

 that even in lakes situated in the same area, few species are com- 

 mon to all of them, and the dominant species are for the most 

 part different. 



Almost all the Chlorophyceae attain their maximum vegetative 

 abundance during the autumnal decline in temperature, and the 

 Desmids are most abundant in September (more rarely in August). 



Diatoms only occur in large quantity in the contaminated lakes, 

 and the pennate diatoms are more numerous and more conspicuous 

 than the centric diatoms. Some species have been observed with a 

 double maximum, one in the spring and the other in the autumn. 

 Such are Asterionella gracillima, Cyclo lella compta, and Rhisosolenia 

 morsa; and the double maximum of the same species is regarded 

 as of considerable interest. 



Most of the Myxophyceae are warm-periods forms, occurring in 

 greatest abundance in the early part of the autumnal fall in tem- 

 perature. Species of Anabaena, Aphanisomenon , and Oscillatoria 

 are for the most part only of secondary importance in the British 

 lakes, but Coelosphaerium Kütsingianum and Gomphosphaeria Näge- 

 liana sometimes occur in such quantity as to become dominant. 



Ceratium hirundinella only occurs in certain lakes and is there 

 a summer form with a small maximum in August or September. 

 Peridinium Willei is the most abundant species of the genus in the 

 English and Welsh lakes, and is also a summer form. In the Peri- 

 diniae the greatest vegetative development tnay be attained by the 

 various species at different times of the year. 



The differences between the plankton-constituents of the lakes 

 are partly territorial and partly local. Territorial distinctions occur 

 in those lakes situated in drainage basins in which the rocks are 

 older than the Carboniferous and local differences between two lakes 

 in similar basins are frequently due to contamination of the water. 



A careful study of the constituents of the phytoplankton in 

 relation to the lake-basins has convinced the authors that the factor 

 of greatest importance in both the qualitative distribution of plankton 

 is the amount of dissolved salts present in the water. The highest 

 percentage of dissolved salts is found in those lakes which are 

 slightty contaminated from adjacent villages and farms, and in such 

 lakes there is a greater number of diatoms than in uncontaminated 

 lakes, and as a rule some of them are perennial constituents of the 

 plankton. The desmid-fiora of such lakes is usually poor. Unconta- 

 minated lakes, on the other hand, contain fewer diatoms whereas 

 the desmids are generally numerous, and there is often a rieh 

 desmid-plankton. Lakes which possess a mixed plankton are probably 

 an intermediate character with regard to the nature and amount 

 of the dissolved salts in the water. G. S. West. 



