(6) 
of Cyanophyceae, which, once become visible, can promptly give 
rise to a deep bluish-green colouring of the liquid. In the beginning 
these Cyanophyceae are seen to develop as free colonies at the sides 
of the flask, later there also appear floating films, which latter consist 
chiefly of Anabazna, while among the colonies growing on the glass- 
wall, not only the large flat colonies of Anabaena, but likewise the 
characteristic, but rarer bluish-grey slimy lumps of Nostoc paludosum 
are most striking. A third, very intensely coloured species, which 
is nearly as common, I determined as Nostoc sphaericum }). 
Motile Cyanophyceae, such as Oscillaria, do not result under these 
conditions, or only in much smaller numbers than those mentioned ; 
probably for them the proportion of organic substances in the said 
nutrient liquids is still too large and that of nitrogen compounds perhaps 
too slight. I have also found that Oscillaria is microaérophilous*) in 
the dark, so that, at the places fit for its development, at least 
temporary anaérobiosis should be possible, which is not the case 
in my experiment. 
Chlorophyceae, especially Chlorococcum and Chlorella are, as might 
be expected, not wholly absent in these cultures; but their number 
is so small that they are without any influence on their external 
character. This fact is the more remarkable because, if to the culture 
fluid is added 
0,02 pCt. NH* NO® 
already after a shorter time than the above mentioned, a dense 
film of Chlorophyceae, in which Chlorococcum infusionum is the 
principal species, grows rapidly on the surface. Only when the nitrogen- 
compounds added to it have been quite consumed, the green film 
grows darker, as then again flakes of Cyanophyceae, in particular of 
Anabaena, begin to form. 
The experiments have essentially the same course when the tap- 
water-phosphate flasks are not infected with garden-soil, but with a 
small flake taken from a previous culture of Cyanophyceae. Here I 
saw however, in some cases appear Anabaena only, which under 
these conditions of culture evidently supplanted the other Cyano- 
phyceae. 
If in my experiments I use Delft canal-water, instead of tap- 
water, and omit the infection with garden-soil, the process is some- 
1) Not all the species of Cyanophyceae obtained could be determined. Some of 
them I think have not been described. 
2) It is macroaérophilous in the light. 
