ED, 
which nothing else had been added but 0.02 pCt. K? HP 04, 
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. ff 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 soli- 
dification 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 DELDEN, 
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 oligoni- 
trophili, 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 toa part only 
of the Cyanophyceae. 
The question put at the head of this paper should thus be an- 
swered 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 Cyanophyceae, 
but furthers that of certain Chlorophyceae and Diatomaceae. 
