18 THE NATURE AND WOBK OF PLANTS 



16. Substances serving as food. — A fairly perfect 

 food for the plants in the water ciilture experi- 

 ment described in § 13 may be made by adding to 

 each quart of rain water put into the jars ten grains 

 each of common salt, plaster of Paris, Epsom salts, 

 calcium phosphate, and a few drops of iron chloride. 

 It will be noted that common salt is a compound of 

 sodium and chlorine. The sodium does not actually 

 enter into the body of the plant, yet its presence in 

 the food is quite useful. 



17. Co7npounds in the plants. — The different sub- 

 stances taken up by the plant consist of twelve 

 or thirteen elements, and these are united again in 

 such manner as to form many hundreds of different 

 compounds. The formation of these compounds is 

 generally for some specific purpose. Thus sugar is 

 built up from the carbon, hydrogen, and oxygen taken 

 from the food substances, and it is used in the con- 

 struction of living matter, kept as a reserve material, 

 or it may be sent from one part of the body of the 

 plant to another. Acids are formed for many pur- 

 poses, celhdose for cell walls and coverings for the 

 protoplasm, proteins to build up the living substance 

 itself, which is constantly wearing out. 



