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TEXTBOOK OF PLANT PHYSIOLOGY 



then utilizes the results of their activity. This supposition was 

 confirmed by direct experimental inoculation of sand cultures with 

 small quantities of soil in which leguminous plants had been 

 grown. Control plants, which had not been inoculated, did not 

 form nodules on their roots and showed no evidence of assimilation 

 of atmospheric nitrogen. Numerous nodules appeared on the 

 roots of plants in the inoculated vessels and the amount of nitrogen 

 contained in the yield was much greater than that originally 

 introduced into the soil in the form of saltpeter, NaNOs. 



Fig. 25. — Nodules containing nitrogen-fixing bacteria on the roots of leguminous 

 plants: A, red clover; B, sweet pea; C, soybean {after Transeau). 



These tubercles found on the roots of leguminous plants are 

 tumors formed as a result of an excessive growth of the parenchyma 

 cells of the cortex (Fig. 26). They consist of large cells densely filled 

 with bacteria. These bacteria enter the root through the root 

 hairs (Fig. 2QB), whence they penetrate in threadlike formation, 

 consisting of innumerable bacteria joined together by mucilage, 

 into the parenchyma of the root, producing a localized perforation 

 of the cell walls and stimulating the surrounding tissues to rapid 

 growth. The cells of the tumor become crowded with bacteria 

 but remain alive and preserve their large nuclei. At first the bac- 

 teria nourish themselves at the expense of the host plant. They 

 are parasites. Hence during the period subsequent to inoculation, 



