80 ELEMENTS OF GENERAL SCIENCE 



from carbon dioxide and water, but the plant cannot secure 

 its nitrogen from the air. The plant uses nitrogen from such 

 compounds as ammonia and nitrates. These are present in 

 good soils and are often added to poor soils as fertilizer. 

 The process by which the plant builds protein cannot be 

 overestimated. Animals are as unable to make protein from 

 simple substances as they are to make carbohydrates. 



