144 SOILS AND MANUEES 



In both cases the plants continue to grow, and may, in 

 time, bear seed. If they are now removed from the sand 

 and examined it will be seen the roots of the plants grown 

 in sand impregnated with soil bacteria are covered with 



FIG. 18. (1) No Combined Nitrogen ; (2) Inoculated 

 with Crushed Nodule of a Bean. 



nodules, but none will be found on the roots of those grown 

 in sterilised soil. Thus it is shown that the presence of soil 

 bacteria causes the development of nodules on the roots 

 and makes the plants independent of compounds of nitro- 

 gen in the soil. In the absence of such bacteria no nodules 

 are formed, and leguminous plants must, like others, 



