ASSIMILATION OF NITROGEN BY PLANTS 



233 



by the roots of other plants. It has been estabUshed by exact 

 experiments that in sowing leguminous plants with cereals, for 

 instance, a mixture of vetch with oats, the latter obtain con- 

 siderably more nitrogenous substances than when they are sown 

 separately. Toward the end of vegetation, the bacteria -con- 

 tained in the cells decrease in number and are changed into an 

 irregular form (Fig. 69, C, D, E), the so-called ''bacteroids." 

 Apparently, the greater part of the bacteria die and are digested 

 as a result of the effect produced on them by the living proto- 

 plasm of the nodular cells. The products of their disintegration 



•^ c 



Pm 





RtiClovtr E 



Fig. 69. — A cross section of a young nodule of vetch, showing infection thread 

 of Bacterium radicicola. B, infection thread, having entered the root hair; 

 C, D, E, bacteroids from different plants (after Smith, et al.). 



are absorbed and assimilated by the plant. The nodules having 

 rotted and separated from the roots, the surviving bacteria are 

 liberated into the soil where they continue to exist, though here 

 they multiply more slowly than in the nodules. With a new 

 seeding of a leguminous plant in the same soil, its roots are again 

 inoculated with the bacteria. 



The relationship existing between leguminous plants and 

 nodular bacteria is usually regarded as symbiosis, or the intimate 

 union of two organisms, both of which are benefited. The 

 bacteria receive carbohydrates from the plant. In turn, the 

 bacteria supply the plant with combined nitrogen, which they 

 elaborate from the inexhaustible source of this element, the 

 atmosphere. This relationship, like other types of symbiosis, 



