and Laboratory Methods. 



1979 



arable depth before the young plant feels the necessity for more nitrogen. As 

 the rootlets extend, the nitrogen fixers penetrate into them, and, growing there, 

 form small nodules, which gradually increase in size as the bacteria multiply. 

 See photograph (Fig. 5) of nodules on roots of locust tree. 



Fig. 5. — Nodules on Roots of Locust Tree. 



At first the bacteria grow at the expense of the plant, but when once they 

 have made a good start they become able to make use of the free N of the air^ 

 forming certain nitrogenous products. Whether these products are in the form 

 of nitrates or some other compound, is not yet determined, but at any rate the 

 plant can utilize them, obtaining in this way nitrogen necessary for its growth. 



