ABSORPTION OF NITROGEN FROM SOIL AND AIR 61 



the growth of the plants is somewhat retarded, but the inoculated 

 plants soon recover and begin to show more rapid progress in their 

 development as compared with control plants. Towards the end 

 of vegetation the bacteria contained in the cells considerably 

 decrease in number and are changed into an irregular form (Fig. 

 26, C, D, E), the so-called bacteroids. Apparently the greater 

 part of the bacteria die and are dissolved as a result of the effect 

 produced on them by the living protoplasm of the nodular cells. 

 The products of their disintegration are absorbed and assimilated 

 by the plant. The nodules having rotted and separated from the 



Red Clover 



Fig. 26. — Cross-section of a young nodule of vetch, showing infection thread of 



Bacillus radicicola. B, infection thread, having entered the root hair; C, D, E, 



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



roots, the surviving bacteria are liberated into the soil where they 

 continue to exist, though here they multiply more slowly than in 

 the nodules. A new seeding of a leguminous plant in the same soil 

 then may have its roots inoculated with the bacteria. 



The relationship existing between leguminous plants and nodu- 

 lar bacteria is usually regarded as one of symbiosis, namely, an 

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

 The bacteria receive carbohydrates from the plant and in turn 

 supply the plant with combined nitrogen. This relationship, like 

 other types of symbiosis, however, may be regarded as a sort of 

 balanced parasitism. First, the bacteria are the assaulting party, 

 behaving as parasites in respect to the plant and causing, as 



