1] UPON THE RATE OF GROWTH 309 



all organisms doing so, the objection, that is, on the ground of 

 the inertness of nitrogen, is disposed of. The question is now 

 merely one of fact. Do the algae, the higher plants, and the 

 animals make nitrogenous compounds out of free atmospheric 

 nitrogen? 



Of these groups, the algae first claim our attention because 

 of the inherent probability that they will act more like bacteria 

 than any other group, since they pass over into the bacteria 

 through such connecting forms as the Oscillariae and Nostocs. 

 SCHLOSING and LAURENT ('92), FRANK ('93), KOCH and 

 KOSSOWITSCH ('93), experimented with various species of Nos- 

 toc, Oscillaria, Lyngbya, Tetraspora, Protococcus, Pleurococ- 

 cus, Cystococcus, Ulothrix, etc., and found that, when supplied 

 with a non-nitrogenous food, these plants produced nitrogenous 

 compounds in the substratum, evidently gaining their nitrogen 

 from the previously purified air. 



Doubt exists, however, as to whether the free nitrogen is 

 taken in directly by the algae or only after having been assimi- 

 lated by bacteria associated with the algae and by them made 

 into nitrogenous compounds. For the latter alternative speak 

 the experiments of KOSSOWITSCH ('94) and MOLISCH ('95). 

 KOSSOWITSCH, who with KOCH had previously found that algae 

 gain nitrogen from the air when reared in impure cultures, 

 now took special pains to get algal cultures free from bacteria. 

 To this end he reared algae on potassium silicate permeated by 

 a nutritive solution. The pure cultures thus gained were then 

 grown in a sterilized flask to which air, freed from ammonia, 

 was admitted. The nutritive solution was made of salts free 

 from nitrogen but containing the other essential elements. 

 The results of this experiment were striking. The pure cult- 

 ures of algae grew for a time, but then ceased. New non- 

 nitrogenous food did not revive them, but the addition of 

 nitrates caused rapid growth. 



Other evidence was gained from analyses. When the cult- 

 ures of algae were pure there was no increase in the amount of 

 nitrogen in the dry matter of the algae. But when bacteria 

 were mingled with the algae, the quantity of nitrogen was 

 increased. This is shown in the following typical analy- 

 sis: 



