NITROGEN-FIXING BACTERIA. 143 



a deoxidizing effect when grown in solutions containing nitrates, with 

 the production of nitrites and ammonia. This double action leads us 

 to inquire into the nature of the physiological processes taking place in 

 either case. His investigations in this field led Stoklasa to assert that 

 there is much in common between the two processes. He believes that 

 the deoxidation of nitrate is due to the action of the bacteria on water, 

 with the liberation of hydrogen and the formation of hydroxyl. Nascent 

 hydrogen is a powerful reducing agent, and would of itself withdraw 

 the oxygen from nitrates to form nitrites, while hydroxyl in contact 

 with ammonia will cause the formation of water and the liberation of 

 nitrogen. Part of this nitrogen is undoubtedly utilized by the bacteria, 

 and the rest is returned to the air. How the inert nitrogen molecule 

 is torn apart and the nascent nitrogen atoms thus formed utilized by the 

 bacteria for their growth is a question that is more difficult of solution. 

 We do know that the amount of nitrogen fixed by B. megaterium is 

 affected by the composition of the nutritive medium. The same is 

 true of denitrification. The molecular structure of some carbohydrates 

 or of organic acids and the arrangements of the atoms in the molecule 

 influence the activity of the bacterial cell and its life processes. I have 

 found, for instance, that the more complex citric acid molecule offers a 

 more favorable source of energy to a denitrifying organism that I have 

 isolated than do either succinic, tartaric or lactic acid. And the lab- 

 oratory work clearly indicates that the amount of organic substance in 

 the soil, as well as its nature, determines the course of development, and 

 the prevalence of the one or the other of the soil organisms. Certain 

 it is that where the fixation of nitrogen takes place in the soil, it 

 occurs only when its store of nitrogen is very meager. This is analo- 

 gous to the behavior of the legumes. It has been found that these 

 plants when growing in a soil rich in soluble nitrogen do not to any 

 considerable extent draw upon the atmosphere for that element. It is 

 only when the soil offers little or no nitrogen that the atmospheric 

 treasure house is unlocked for it. All experimental evidence thus far 

 accumulated indicates that there is a struggle between the plant and the 

 bacteria invading its roots, that the latter are so modified by the aggres- 

 sive activity of their host that they form a fine network of tissue in 

 which the atmospheric nitrogen is captured, as it were. That this as- 

 sumption is not entirely erroneous is shown by the fact that legumes, 

 inoculated with cultures of B. radicola that are particularly virulent, 

 although they form root tubercles, show nitrogen hunger when there is 

 none in the soil at their disposal, and the microscopic examination re- 

 veals bacteria that are not modified as is the case in vigorous plants. 

 In other words, the bacteria resist the encroachment of their host and 

 would not be compelled to furnish it with the nitrogen that it can 

 not get otherwise. In connection with this, the question naturally 



