HOW FOOD IS USED BY PLANTS 385 



The whole matter may be represented as like the slope of 

 a hill. We begin at the bottom with carbon dioxide, water, 

 nitrates, etc., and by means of photosynthesis, protein manu- 

 facture, and assimilation we ascend to the highest point 

 protoplasm ; the descent on the other side is represented by 

 respiration, which reduces the protoplasm and compounds 

 closely associated with it to carbon dioxide, water, nitrogen 

 compounds, and other decomposition products. 



