1919] 



ALLEN — AZOTOBACTER CHROOCOCCUM 5 



ent, since the optimum lies too high. They call attention to 

 the fact that H. Fischer suggested the role of humus in the 

 soil to be that of an oxygen carrier, and that Bonnema sug- 

 gested that Fe 2 03 is the real agent in nitrogen-fixing proc- 

 esses, since in contact with the air it transforms continually 

 small amounts of nitrogen into nitrite, and that this contin- 

 ually formed nitrite is then transformed by the microorgan- 

 isms into cell protein. On these points Remy and Rosing 

 state that they would soon offer experimental evidence, but to 

 our knowledge it has not appeared. 



Kaserer ('12) believes that the results obtained by Remy 

 and Rosing were due to the impurities in the iron compounds 

 used rather than to the iron itself, but Rosing ('12) does not 

 accept this explanation. 



Sohngen ('13) studied to some extent the conditions for 

 promoting the growth of Azotobacter, the nitrifying bacteria, 



and bacteria in general. As concerns Azotobacter he states 

 that the results of Krzemieniewski, Kaserer, and Remy and 

 Rosing were in general confirmed. In addition he found that 

 colloidal silicic acid when added to Beijerinck's medium pro- 

 duced an even more beneficial effect than the raw humus which 

 he used. With Beijerinck's solution alone he obtained 1.9 

 mgs. nitrogen fixed per gm. of mannite, whereas when 2 gms. 

 raw humus were added to 100 cc. of culture solution 6.7 mgs. 

 were fixed, and when 500 mgs. colloidal silicic acid in the sol 

 form were added 8.0 mgs. of nitrogen were fixed per gram of 

 mannite. All the results cited were obtained with a pure cul- 

 ture in a 32-day incubation period. 



Sohngen also obtained excellent results by inserting a strip 

 of filter paper or of cotton cloth in the culture medium. Azo- 

 tobacter grew almost exclusively in contact with the filter 

 paper or cloth at or just above the junction of the strips with 

 the nutrient solution. This indicates, according to him, that 

 microbial life in the soil takes place chiefly upon the colloids. 

 He believes that the beneficial action of the colloidal silica 

 and of the raw humus is due to the fact that these colloids 

 adsorb nitrogen and oxygen, and in this manner impart more 

 quickly the necessary elements, and better growth results. 



