[Chap. XIX 



SUBSTANCES MADE FROM FOOD 



177 



We have a sufficient background now to see that the food of green 

 plants and man are the same; that food consists of organic substances 

 that may be used as a source of chemically bound energy and as material 

 that may be directly transformed into protoplasm, cell walls, or other 

 essential constituents of cells. Accumulated insoluble food must of 

 course first be digested in either case. 



Water and inorganic salts (fertilizers) which are essential to plants 

 in many ways are not a source of energy to green plants. They may 

 become incorporated in cell structures more or less directly or through 

 their use as raw materials in food manufacture. Nitrates and sulfates, 

 for example, are utilized in the manufacture of amino acids and proteins 

 which in turn are assimilated in the making of protoplasm. Attempts to 

 include carbon dioxide, oxygen, water, and inorganic salts under the 

 term food result only in rendering the term meaningless and useless. 

 Some of the ways in which inorganic salts may affect plants have already 

 been noted; others will become evident later. 



Composition of a plant. All reports of the chemical composition of 

 plants and of protoplasm have at least two defects: (1) no distinction 

 is made between accumulated food and assimilated materials, and ( 2 ) 

 inorganic salts that have accumulated in crystals or are merely in solu- 

 tion in the water in the plant are not distinguished from those that 

 have become an integral part of the cell structures. 



With these defects in mind, the elemental composition of a corn plant 

 that has attained full growth is presented. 



Corn Plant 



Water 



79.7% 



Organic matter 19.5% 

 Mineral elements 0.8% 



Dry Matter 

 20.3% 



Organic matter as compounds: 



Carbohydrates and fiber 17.2% 

 Fats 0.5% 



Proteins 1 . 8% 



