116 TRANSFORMATION OF NITROGEN BY SOIL MICROBES 



outward into the nodule and favors active division of the plant 

 cells and growth in the terminal region. The plant cells at the 

 base of the young nodule swell and become vacuolated and occu- 

 pied with bacteria freed from the infecting strand (see 2, Fig. 53). 

 These bacteria appear as thin rod forms. As the nodule ages, the 

 bacterial cells in these older plant cells become swollen with proto- 

 plasmic constituents concentrated unevenly; these are known as 

 the bacteroids (see 3, Fig. 53). 



Unhanded 

 Non ■ Motile Rods 



Motile Rods 



Swarmers 



Coccoid Forms 



Pre • Swarmers 

 Fig. 54. — The life cycle of Bacillus radicicola (after Thornton and Gangulee). 



The changes in appearance of the bacterial cells appear to be 

 closely related to the food supply in the nodule. Where food 

 materials, presumably carbohydrates, are supplied in abundance, 

 the bacteria retain the appearance of rod-shaped cells which take 

 stains uniformly. As the food supply decreases, in the older 

 regions of the nodule, the bacteria become banded rods and finally 

 bacteroids. These morphologic changes are characteristic of 

 bacteria passing from stages of youth to old age and senescence 

 (see Fig. 54). Thus, if a culture of a legume organism is intro- 

 duced into a certain culture medium, it soon appears in the " pre- 

 swarmer " or non-motile coccoid stage. The cells increase in size 



