— 203 — 



astrum clathratum stops the formation of new coenobia, and as 

 the old colonies are gradually deprived of the tufts of bristles 

 either in striking against other organisms in the water or in an- 

 other way, new bristles are not to be seen in the plancton till 

 the next spring when the reproduction again has been resumed 

 for some time. Thus there is no seasonal variation in sense Wesen- 

 berg-Lund's (1908) in Pediastrum clathratum. 



Pediastrum Boryanum, on the contrary, is a marked cos- 

 mopolitan and occurs in arctic as well as in tropical and antarctic 

 countries. This species is mentioned in almost every flora of fresh- 

 water algæ from any locality of the earth. Thus it is rather in- 

 different to temperature and is evidently capable of intense re- 

 production at a pretty low temperature. This probably accounts 

 for the fact that some specimens of this species are furnished 

 with bristles in Bure Sø at a moment when Pediastrum clathra- 

 tum is quite destitute of these formations. 



I found Pediastrum Boryanum with highly developed tufts 

 of bristles in a lake in Iceland (Reykjavik Aug. 1914). Some small bits 

 of the plancton were desiccated on cover glasses, but they were 

 not stained, according to the method of Löffler, till after my 

 return (in about three months), and then with very fine results. 

 This may be of interest in that it proves the possibility of making 

 such coloured preparations of plancton samples collected in 

 countries where an examination of the samples on the spot is 

 impossible. 



Copenhagen. The botanical Museum of the University, Jan. 1921. 



Litterature : 



1. Boye Petersen: On tufts of bristles in Pediastrum and Scenedesmus. 



Bot. Tidsskrift B. 31, 1912. 



2. Lemmermann: Beiträge zur Kenntnis der Planktonalgen XXVI — ■ 



XXX. Arch. f. Hydrobiologie u. Planktonk. V. 1910. 



3. Pascher: Süsswasserflora Heft 5, Chlorophyceæ II, bearbeitet von 



Lemmermann, Brunnthaler und Pascher. Jena 1915. 



4. Wesenberg-Lund: Plancton Investigations of the Danish Lakes. Co- 



penhagen 1908. 



5. Zacharias: Ueber einige interessante Funde im Plankton sächsischer 



Fischteiche. Biol. Centralblatt Bd. 18 1898. 



