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have been the first organisms which peopled the earth, as no other living beings 

 are known which, like the Cyanophyceae, are able to build up their organic constituents 

 from carbonic acid and atmospheric nitrogen. 



Once asquainted with the culture conditions of the Cyanophyceae I could 

 easily obtain pure cultures on a solid medium. I therefore used as well silica as 

 agar which by long washing with tap-water had been freed from the soluble organic 

 substances, but saturated with the constituents of the tap-water. Plates of this 

 agar, to which nothing else had been added but 0.02 pCt. K 2 H P O 4 , and on 

 which tap-water cultures of Anabaena had been sown out, were placed in the light of a 

 window on the north, and after 10 to 14 days already produced extensive Anabaena- 

 colonies free from bacteria. If the plates are not thoroughly washed Anabaena 

 does not grow at all on them. 



With plates prepared of silica instead of agar I obtained the same results. 



The washing of the plates is effected by placing them, after solidification in the 

 glass-box, into a large beaker with water, which is continually renewed during a 

 few days by a current from the tap. 



Then kalium phosphate is introduced into the plates by pouring over them a 

 solution of this salt in distilled or tap-water, which solution is renewed a few times. 

 Finally the superfluous water adhering to the plates is removed by heating the 

 glass-box for a short time over a Bunsen-flame. 



Oscillaria and allied species do not grow on the thus prepared media, they 

 even die on it already after some days. Mr. A. van Del den, however, has succeeded 

 in my laboratory to obtain a pure culture on a solid medium of such a motile 

 form related to Oscillaria. 



This culture necessitated two other precautions. First the organic substance 

 had to be removed from the agar more completely than is wanted for Anabaena, 

 and therefore it proved necessary to wash with a current of distilled water. Second, 

 the addition of a little of a nitrogen compound, e.g. a trace of ammonium-nitrate 

 proved necessary, or at least favorable. On such agar the growth of the organism 

 remains however very scanty, and, as besides many species of chlorophyceae can 

 develop under these circumstances, we leave herewith the group of oligonitrophili, 

 whose specific faculty consists in their being able to live on the nitrogen from the air, 

 in opposition to the Diatoms and the Chlorophyceae. Hence this faculty seems also 

 peculiar to a part only of the Cyanophyceae. 



The question put at the head of this paper should thus be answered as follows. 



In culture liquids, containing besides the mineral constituents of the food, a 

 slight quantity of garden-soil, but to which no other nitrogen-compounds have been 

 added, develop, under the influence of the light and the carbonic acid from the air, 

 various species of Cyanophyceae, chiefly belonging to the genera Nostoc and Anabaena, 

 Germs of these are very numerous in garden-soil. The presence of nitrogen-compounds 

 prevents the development of the Cyanophyceaf, but furthers that of certain Chloro- 

 phyceae and Diatomaceae. 



