442 JOSEPH STRUTHERS 



under the action of chlorophyl and sunlight, absorb carbon 

 dioxide from the air and decompose it into carbon and oxygen, 

 utiHzing the former to build up the cellular structure and re- 

 turning the oxygen to the air. The chief elements assimilated 

 by plants, and consequently removed from the soil when the 

 plants are cut down and taken away, are nitrogen, phosphoric 

 acid and potash. These compounds play a very important 

 part in the functions of vegetation and are the most liable to 

 be exhausted from the soil. 



