— 167 — 



m very young nor in very old specimens. But as I thought that 

 perhaps also a kind of seasonal variation [Zacharias 1903 pag. 595] 

 took place, as by the Ceratia and Daphnice or other plankton- 

 organisms it has occasioned me to examine during a longer time 

 the variations of Scenedesmus quadricauda, acutus and acuminatus 

 in a pond near Stavnsholt and of Pediastrum simplex in Bure Sø 

 (North Sealand). These species are found in plankton all the year 

 round. Then it turned out that in the Scenedesmus species bristles 

 are to be found all the year, but in the Pediastrum simplex the 

 bristles disappeared during the winter. 



In the following table I have indicated the dates when the 

 samples of Pediastrum simplex were taken, the temperature of the 

 water and in the last row if bristles were found (+) or not (—) 



- + 



16° G + 



15° „ + 



16° „ + 



14° „ + 



13° „ + 



7° „ slight traces of bristles 



8° „ 



3° „ 



4° „ -4- 



3° „ -5- 



4° , 4- 



0° „ -5- 



8° „ ~ 



9° , 



15° „ + 



17° „ + 



17° „ + 



In some of the bristle preparations, where Pediastrum simplex 

 is found completely without bristles, some specimens of Pediastrum 

 Boryanum are also found, and these were furnished with fully 

 developed bristles, so that it is obvious that the two species differ 

 in this respect. Moreover in a culture of Pediastrum duplex with 

 well developed tufts of bristles there were not a few specimens of 

 Scenedesmus quadricauda and acutus; but on these I have not been 

 able to find any bristles whatever in my numerous preparations. 



When the bristles are seen in dark-ground illumination, they 

 are like luminous straight lines bristling from the algæ coenobia. 

 In the staining preparations they are most frequently lying very irre- 



ns 



15 /8 

 25 /8 

 5 / 9 

 15 / 9 

 30 /9 

 15 /l0 

 31 /l0 

 l3 /ll 

 111 



27 /<> 

 19 /3 

 4 /4 

 17 /4 

 30 / 4 

 14 /5 



13 / 6 



1910 



1911 



