306 SYMBIOTIC NITROGEN FIXATION 



3. The organisms enter the plant and produce nodules, but no 

 fixation of nitrogen occurs. 



4. The organisms enter, produce nodules, and nitrogen is fixed 

 and assimilated by the plant. 



5. The organisms are so efficient in comparison with the plant 

 that the latter is injured. 



6. The organisms are parasitic and the plant is actually killed. 

 Certain products which are produced by the invading organism in 



connection with the host have been taken as evidence of the parasitic 

 nature of the bacteria, whereas others consider the nodule which 

 forms on the legume root a result of irritation due to a parasite. 

 Grieg-Smith, however, considers the formation of root tubercles not 

 as a result of irritating parasitic action but rather as the consequence 

 of the production of nutrients at that place resulting in better 

 nourishment and growth of the cells than in other parts of the roots. 



Fuhrmann considers that the fixation of atmospheric nitrogen 

 by the root-tubercle organisms begins when the bacteroids have 

 reached a stage when they are colored brown-red by addition of 

 tincture of iodine. This occurs only when the organisms are feeding 

 almost exclusively upon carbohydrates and the available nitrogen 

 compounds have been almost completely exhausted. Many 

 workers prefer to call the relationship up until this stage a true 

 parasitic and later a true mutual symbiosis. 



By careful staining Fred was able to demonstrate the entering of 

 the bacteria through the root hairs, immediately after which a 

 tubercle started to form. A series of sections showed that mitosis 

 goes on in the nodules much the same as it does in diseased tissue of 

 animals. The mitotic figures are larger, very irregular, and not well 

 marked and have an uneven number of chromosomes. In the 

 normal roots the mitotic figures are about one-sixth as large, very 

 clear, and the chromosomes in numerous pairs. This he considers 

 bears out the theory that the legume bacteria are symbiotic parasites 

 of the plant. 



If we accept Whiting's definition of mutual symbiosis "as the 

 contiguous association of two or more morphologically distinct 

 organisms not of the same kind, resulting in an acquisition of 

 assimilated food substances which implies that the organisms con- 

 cerned have the power of independent existence, but that both are 

 benefited by the close association," we must conclude that all the 

 evidence bears out the idea that the relationship existing between 

 Ps. radwicola and legumes is one of mutual symbiosis. 



Mechanism of Fixation (Metabolism). For a long time it was 

 believed that the nitrogen fixed by legume bacteria and assimilated 

 by the plant was obtained through the leaves. The organisms on 

 the roots were considered to in some way stimulate the plant so that 

 it possessed the power to assimilate nitrogen. Stoklasa considered 



