. FIXATION OF NITROGEN 129 



amined under the microscope, the plant cells will be found 

 filled with myriads of motile bacteria, which in the young 

 nodules are rod-shaped, lull in the older ones assume ab- 

 normal shapes known as bactiru'nh. In some not well un- 

 derstood manner, the bacteria are able to obtain their nitro- 

 gen from the air and to make it available to the plant. 

 The bacteria derive from the plant the fermentable mate- 

 rial necessary to secure the energy demanded for the fix- 

 ation of the nitrogen. The relation is thus a mutually help- 

 ful or a symbiotic one. 



The plant can thus obtain a sufficient supply of nitrogen 

 to make a good growth, but when growing under natural 

 conditions the plant derives a greater or less amount of its 

 nitrogen from the soil in the same way as do other plants. 

 No one can state the proportion of nitrogen taken from the 

 air or from the soil under any given set of conditions. All 

 that can be said is that the plant is unable to use the free 

 nitrogen of the air unless the nodules are present on the 

 roots. If the nodules are few, a small part of the nitrogen 

 may come from the air, while if the roots are well covered 

 with nodules, the plant will undoubtedly take the major 

 part of its nitrogen from the air. Every farmer should 

 make an effort to have all the legumes he may grow well 

 inoculated. 



Other conditions must be made as favorable for the le- 

 gume as possible. There should be an adequate supply of 

 potash and phosphorus in the soil, and the reaction should 

 be favorable for the particular legume. When these con- 

 ditions are met and the appropriate bacteria are present or 

 have been added, nodule development should be abundant. 

 It must be remembered that the only way in which the 

 legume can increase the fertility of the soil is with reference 

 to the single element nitrogen. A leguminous crop may be 

 grown on a field and be removed, and the soil remain as 



