140 



Bacteria in Relation to Country Life 



shade the soil create conditions as to moisture, tem- 

 perature and light that are different from those created 

 by cereal crops. The differences do not, by any ineans, 

 stop there. The crops take plant-food and moisture from 



Fig. 20. Colonies of soil bacteria. 



the soil and give back to it some of their substance, 

 something that passes out of the roots and into the soil. 

 Our knowledge of the amount and nature of the substan- 

 stances thus given up to the soil by the plants is meager. 

 It is not known as yet to what extent these secretions 



