MECHANISM OF FIXATION 307 



that amids were first formed and that these migrated to the nodules, 

 reacted with glucose and produced protein which served as the 

 nutrient medium for the bacteria. In this connection he advanced 

 the idea that the bacteria produced an enzyme which enabled the 

 plant to fix the nitrogen. This theory, however, was shown to be 

 untenable by Whiting who grew soybeans and cowpeas under careful 

 control conditions. One lot received a definite proportion of oxygen, 

 and carbon dioxid, a second oxygen and carbon dioxid, while a third 

 received ordinary air. He found that these plants utilize atmos- 

 pheric nitrogen through their roots and not through their leaves. 



Nobbe and Hiltner (1893) considered the root tubercles to be the 

 parts of the leguminous plants where the free nitrogen is assimilated 

 and that the direct agents of the assimilation are the bacteroids and 

 not the bacteria themselves. As to the metabolism of the nitrogen 

 by these bacteroids the ideas at present are very indefinite. Loew 

 and Aso (1908) suggested that ammonium nitrite was the first 

 compound produced, the nitrous acid being readily reduced- to 

 ammonia. 



Gautier and Drouin considered that the nitrogen is oxidized to 

 nitrous and nitric acids, whereas Winogradsky has advanced the idea 

 that the free nitrogen in the plasma of the organism may unite with 

 nascent hydrogen and form ammonia which by oxidation would 

 become assimilable. 



Gerlach and Vogel concluded that there is a direct union of free 

 nitrogen with some organic compound inside the bacterial cell. 

 Heinze thinks it probable that nitrogen is at once brought into 

 combination with a carbohydrate (glycogen) and suggests that a 

 salt of carbonic acid may be formed first, or that carbonic acid may 

 be produced from cyanamid. All of these theories, however, are 

 purely speculative as there is little experimental evidence on the 

 subject. 



It is in keeping with our knowledge of bacteria to assume that 

 the changes are catalyzed by enzymes produced by the bacteria, 

 and Hiltner reported the findings of a substance which is produced 

 by the legume bacteria which can dissolve the cell wall and root 

 hairs. Yet Beijerinck claims that no enzyme has been found which 

 attacks starch, cellulose, or saccharose. No true proteolytic enzyme 

 has been reported, but Benjamin has reported the presence of urease 

 in the nodules of various legumes. This enzyme is, however, found 

 quite generally in plants and may have come from the host and not 

 the bacteria. Fred, although unable to detect a proteolytic enzyme, 

 has obtained evidence of the presence of oxidases in the slime of 

 various legume bacteria. 



There are two main suppositions regarding the assimilation of the 

 nitrogen by the plant as follows : (1) That the bacteroids are bodily 

 absorbed by the plant fluids; and (2) that the bacteroids, by some 



