232 PLANT PHYSIOLOGY 



to be filled with bacteria, it was assumed that they assimilate 

 atmospheric nitrogen, while the plant in turn supplies these 

 bacteria with the nonnitrogenous nutritive materials and then 

 utilizes compounds resulting from their activity. This suppo- 

 sition 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 

 nitrate of soda. 



These tubercles found on the roots of leguminous plants are 

 tumors formed as a result of an excessive growth of the paren- 

 chyma cells of the cortex (Fig. 69, A). They consist of large 

 cells filled with bacteria. These bacteria enter the root through 

 the root hairs (Fig. 69, B), whence they penetrate in threadlike 

 formation, consisting of innumerable bacteria joined together, 

 into the parenchyma of the root, producing a localized perfora- 

 tion 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 bacteria nourish themselves at the expense of the 

 host plants. They are parasites. Hence, during the period 

 subsequent to inoculation, the growth of the plant is somewhat 

 retarded, but the inoculated plants soon recover and begin to 

 show more rapid progress in their development as compared with 

 the control plants. This occurs owing to the fact that with 

 rapid growth of the tissues of the nodule and the increased inflow 

 of nonnitrogenous substances from the leaves, the synthetic 

 activity of the nodule bacteria increases, and they start to elabo- 

 rate nitrogenous substances in such quantities that they suffice 

 not only for the needs of the bacteria, but also for providing 

 abundantly for the host plant. All the tissues of the leguminous 

 plants are therefore rich in nitrogen, and this explains the high 

 nutritive value of the seeds of peas, beans, lentils, and soybeans, 

 as well as the hay of clover, alfalfa, and other leguminous plants. 

 A part of the nitrogenous substances elaborated by the nodule 

 bacteria diffuses from the roots into the soil and may be utiUzed 



