PLANT MANUFACTURE OF FOOD 141 



capitalized many of these food-manufacturing processes for his 

 own use, having discovered that the carbohydrate and protein 

 synthesizing abilities of green plants are most essential for his own 

 existence. But the dependence of man goes beyond the plant 

 kingdom. No plant can produce material substance out of noth- 

 ing. It is dependent upon continuous supplies of the raw materials 

 coming from the inorganic world. Nature or man must keep the 

 balance of reserve inorganic compounds such that plants will 

 be guaranteed the materials out of which to elaborate the neces- 

 sary organic compounds. Continual supplies of carbon dioxide 

 and water are taken care of by nature; from them the plants can 

 produce a continual supply of carbohydrates and fats. But for 

 proteins and allied products, upon which protoplasm formation 

 and thus life itself depends, the plants are dependent upon the 

 existence of certain elements in the soil. If year after year, crops 

 grown in a certain region are shipped elsewhere, valuable ele- 

 ments are being continually withdrawn; eventually the soil 

 fertility will reach such a negligible point that crops can not 

 profitably be grown in the region. Thus man's dependence upon 

 plants as foods goes still farther to a dependence upon certain 

 substances which must be present in the soil to allow food manu- 

 facture by plants; soil fertility must be kept up to a certain 

 definite point by the addition of fertilizers. And a knowledge of 

 just what these fertilizers must contain in order to guarantee 

 certain crops becomes an important and practical aspect of plant 

 biology. 



