SPECIES OF AZOTOBACTER 177 



solution. With 20 grams of glucose or mannite in a 

 litre of nutrient solution Freudenreich found that Azoto- 

 bacter chroococcum assimilated or " fixed " from 1 2 to 

 24 mg. of nitrogen in two or three weeks. When grown 

 on gypsum plates with less liquid present and abundant 

 air supply 160 mg. of nitrogen were assimiliated in the 

 same time. By cultivating it upon agar 'mannite- 

 phosphate plates A. Koch obtained an increase of 

 1 80 mg. of combined nitrogen per litre of medium. 



Gerlach and Vogel found that the amount of nitrogen 

 assimilated is influenced largely by the amount of glucose 

 present in the nutrient solution. From I to 12 grams 

 of sugar per litre proved beneficial, and the amount of 

 nitrogen assimilated increased proportionally ; i 5 grams 

 checked the action of the organisms to some extent. 

 The following are some of their results after five weeks' 

 growth : 



Milligrams of 

 Nitrogen fixed. 



With I gr. of grape sugar present . . 7.4 

 3 * / *3 



5 - 39-4 



10 . . 91.4 



12 . 127.9 



,,I5 * V 62.9 



They also showed that the power of nitrogen fixation 

 decreases with the age of the culture, as indicated below, 

 and is reduced by cultivating the organisms for several 

 generations in nutrient media : 



Milligrams of N. 

 in i litre. 



A culture 1 8 days old " fixed " . . 127.9 m 5 weeks' 

 no . . 54-9 



328 . . 23.4 



M 



