2 PRINCIPLES OF FARM PRACTICE 



from a compound always present in the air, known as carbon 

 dioxide, and the hydrogen from water. The starch-making 

 process goes on in the green part of the plant, mostly in the 

 leaves. But the work of starch-making from carbon dioxide 

 and water requires power, or energy. This power comes 

 from sunlight. Here we have an explanation of the fact that 

 green plants thrive only in the presence of sunlight. 



With starch-like substances thus formed as a basis, other 

 food materials are made. For example, an important food 

 known as protein is formed by using the elements composing 

 the starch-like compounds made by the leaves, and nitrogen 

 which is brought to the plant in substances dissolved in soil 

 water. Besides nitrogen, a number of other chemical ele- 

 ments obtained from materials dissolved in soil water seem 

 to be necessary for food-making in the plant. The most im- 

 portant of these, from an agricultural standpoint, are phos- 

 phorus and potassium. 



Why plants need water. That plants need water is shown 

 by the fact that they soon die when deprived of water. A 

 leading authority on plant life says that the greatest thing 

 influencing plant growth is water. It is used by the plant in 

 several ways. Its part in supplying the raw material for 

 starch-making and in bringing to the plant several elements 

 from the soil, as nitrogen, phosphorus, and potassium, has 

 been mentioned. In addition, water helps to make the plant 

 rigid; it composes most of the sap, transfers food materials 

 from place to place in the plant, and assists in the control of 

 temperature. . 



Water passes through healthy, vigorous plants in a con- 

 stant stream, entering through the roots and escaping in the 

 form of watery vapor through the leaves. A large amount of 

 water is thus used by a plant during the growing season. It 



