STUDIES ON AZOTOBACTEE CHROOCOCCUM BEIJ. 361 



The tabulated results require no further comment than has 

 already been made. The nitric nitrogen is all, or nearly all, 

 found in the form of organic or organized nitrogen. 



The fact has thus been estabhshed that the nitrate nitrogen is 

 organized by Azotobacter in its process of growth but no insight 

 has been gained as to the modus operandi of this attack. A 

 review of the reported data on nitrogen relations seems to point 

 to the fact that the organism assimilates the nitrate in its early 

 stages thus causing a loss of nitrogen from the solution, but 

 later when the source of combined nitrogen is exhausted or 

 nearly exhausted, a second physiological phase sets in, in which 

 the cells assimilate atmospheric nitrogen, replacing thereby the 

 losses which the solution underwent in its early stages. That 

 this seems to be what actually takes place in the cultures of 

 Azotobacter is evidenced by the accompanying experiments 51 

 and 55. 



Experiment 51. Fifty cubic centimeters of a Vogel solution 

 to which 0.126 per cent of Ca(N03)2 + 4H2O had been added 

 were used; after inoculation and incubation at 25°C. for varjdng 

 lengths of time the cultures were analyzed with the results 

 given in table 22. 



Before discussing the results presented in the above table, a 

 new series of experiments aiming at the same end will be related. 

 The earhest incubation period of five days seemed to be too 

 long to allow a close study of the early assimilation of the nitrate 

 to be made. Shorter incubation periods were therefore observed 

 to obtain the required data. 



Experiment 55. Fifty cubic centimeters of the same solution 

 as was used in the previous experiment were pipetted into sterile 

 petri dishes of 20 cm. diameters, containing 0.5 gram of CaCOs. 

 After inoculation and incubation for varying periods of time 

 the cultures were analyzed with the results set forth in table 23. 



Although the actual amount of nitrogen unaccounted for is 

 in many cases small, yet indications are that the modus operandi 

 of the nitrate attack by Azotobacter chroococcum may be stated 

 to be as follows; the organism utilizes the combined nitrogen 

 (in the form of nitrates) as soon as placed in contact with it and 



