— 202 — 



2. The cause why Pediastrum clathratum does not show tufts 

 of bristles during the winter in the lake of Bure Sø. 



In 1912 I mentioned that I had not been able to find any 

 trace of bristles in Pediastrum clathratum during the cold season, 

 in spite of the occurrence of numerous coenobia of this species 

 in the plancton. On the contrary, I succeeded in showing brist- 

 les in Pediastrum Boryanum (Turp.) Men. I was then unable to 

 account for this fact. Further, I mentioned that in a plancton 

 sample the tufts of bristles are never found in all the specimens 

 of Pediastrum, viz. neither in the very young nor in the very 

 old coenobia. 



I have now attempted by means of the literature to find out 

 the distribution of Pediastrum clathratum 1 ), and, as far as I can 

 see, it is a species that thrives best in warm water. It therefore 

 grows best in the tropical lakes and streams and in the shallow 

 lakes of the Balticum where the water during the summer is 

 much heated, but not, for instance, in the generally deep and 

 cold lakes of Norway. During the winter this species may be able 

 to keep alive but not to multiply particularly. Hence I imagine 

 that, when the temperature has fallen to a certain point, Pedi- 



*) I give here a list of the localities where Pediastrum clathratum has 

 been found. In some cases it is not possible to decide whether we have to 

 do with a Pediastrum clathratum or other forms of the old collective species 

 Pediastrum simplex Meyen. 



The list is hardly complete. 

 Europe: Denmark, Bure Sø (J. B. P.), Himmelbjergsøerne (Wesenberg- 

 Lund) Haderslev Dam (Heering). 

 Norway -i- 



Sweden, some lakes near Uppsala (Borge), Scania (Lemmermann). 

 Belgium, Bruxelles (de Wildeman). 

 British Islands (West). 

 Germany, many localities. 

 Switzerland. (Chodat). 

 Hungary, (Istwanffi). 

 Africa: The Victoria and the Njassa sea (Ostenfeld), the Nile (Lemmer- 

 mann). 



Asia: The Menam, Jangtsekiang (Lemmermann), the Aral sea (Osten- 

 feld). 



America: Paraguay (Lemmermann), Wisconsin (G. M. Smith). 

 Australia: -H 



Wille (1879) states that he found Pediastrum simplex Meyen at Novaja 

 Semlja, but it has probably not been Pediastrum clathratum. 



