120 POPULAR EEEOKS. 



■withdrawn from the air and made available for 

 plant food. These lowest forms of vegetable life 

 possess the power which the higher plants do not 

 have of absorbing nitrogen from the air. The 

 humus or vegetable matter in the soil permits the 

 growth of the bacteria, and these as they perish 

 leave their accumulated nitrogen for the use of the 

 higher plants. 



