126 



third, very intensely coloured species, which is nearly as common, I determined 

 as Nostoe sphaericum 1 ). 



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 micro- 

 aerophilous 2 ) in the dark, so that, at the places fit for its development, at least 

 temporary anaerobiosis should be possible, which is not the case in my experiment. 



Chlorophyceae, especially Chloroeoecum 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 4 NO 3 



already after a shorter time than the above mentioned, a dense film of Chlorophyceae, 

 in which Chloroeoecum 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 somewhat different. First a rich light- 

 brown culture of Diatomaceae takes rise, in which here and there colonies are seen of 

 Chlorophyceae belonging to the genera Raphidium, Seenedesmus, Chlorella and Chloro- 

 eoecum, but without their multiplying sufficiently to alter the brown colour of the 

 culture. After 8 to 9 weeks however, the colour at once grows darker by the then 

 occurring increase of the Cyanophyceae, which increase continues a long ti me 

 evidently as long as there is a sufficient quantity of kalium phosphate and other 

 mineral food. 



I think the result of this last experiment should be explained as follows. 

 Canal-water contains a greater amount of organic substances than the tap-water 

 cultures; as long as these substances are present the Diatoms are prevailing; they 

 use these substances for their carbon-nutrition, together with the carbonic acid 

 from the air, and at the same time assimilate the nitrogen-compounds. When these 

 are consumed the Cyanophyceae appear. 



That the Diatoms can in fact utilise a fairly high rate of organic substances, is 

 well known to the students of that group. The following experiment which, to my 

 knowledge, has not yet been described, proves that the Diatoms are the very 



*) Not all the species of Cyanophyceae obtained could be determined. Some of 

 them 1 think have not been described. 



*) It is macroaerophilous in the light. 



