1908.] The British Freshwater Phytoplanhton. 



205 



great development of spines. The species of Closterium found in the British 

 lake-plankton are mostly adventitious constituents, and are never abundant. 



Nearly all the plankton-Desniids are summer and autumn constituents r 

 and the majority of them attain their maximum abundance in September and 

 October, during the slight fall after the maximum summer temperature. 



Neither plankton-Desmids nor those which occur in other situations undergo 

 any seasonal form-variations. This we have conclusively proA^ed by the 

 examination of large numbers of periodic collections of these plants. This 

 is merely what one would expect, as environmental form -changes in Desmids 

 occupy long periods of time. As regards the plankton, the variations in the 

 conditions of buoyancy during the year in the surface waters of a lake are 

 not so great as the environmental differences between the habitats in which 

 the same species of Desmid will thrive. 



Large numbers of both the plankton and bog species survive the winter 

 in the vegetative condition, and the formation of zygospores appears to be 

 very rare. 



In Diatoms it is known that the seasonal form-variation, when it occurs, 

 is in the colony and not in the individual, but colonial Desmids are much 

 fewer and much less abundant than colonial Diatoms. 



Lastly, we would comment upon the cosmopolitanism of the frcshvmter 

 planktoii-coiamunity. This is generally true except for the Desmids. 

 Wesenberg-Lund* states that the numerous plankton researches in the 

 Central European lakes have been unable to demonstrate any special, 

 geographically localised plankton-communities. He remarkst that the 

 " freshwater plankton-communities, in contrast to all other communities on 

 land or water, everywhere contain the same types, nearly everywhere the 

 same species." As regards the Desmid-flora, however, these statements do 

 not hold good. Wherever there are lakes with a rich Desmid-flora in the 

 plankton, there one also gets a more or less definitely localised plankton- 

 community. It has been statedj that the Desmidiacete show more decided 

 geographical peculiarities than any other group of Freshwater Algte, 

 notwithstanding the fact that a large number of them are cosmopolitan and 

 ubiquitous all the world over. These geographical peculiarities occur 



one of anchoring the individual to its environment, and the other to serve as a protection 

 against Desmid-eating animals. Whether the more elongated spines of the plankton- 

 species likewise serve a similar function of protection against the depredations of the 

 plankton Rotifers and Entomostraca is a point which requires further investigation. 



* Wesenberg-Lund, loc. cit., 1908, p. 293. 



t Loc. cit., p. 313. 



} G. S. West, in 'Linn. Soc. Bot. Journ.,' vol. 38, 1907, p. 82 ; W. and G. S. West, in 

 ' Ann. Eoy. Bot. Gard., Calcutta,' vol. 6, part 2, 1908, p. 176. 



