60 



THE PRINCIPLES OF AGRICULTURE. 



o'T. Out of tlic soil and the atniosplicrc they obtain the 

 food which sustains their life and growth. 



The Food of Plants. — If we analyze the substances of 

 which a plant is composed, and ascertain the elements 

 which it contains, we shall know exactly what food it re- 

 quires, and what the soil and atmosphere must contain 

 in order to afford proper nourishment. 



The following elements are tound in all plants, and 

 are essential to their growth : carbon, hydrogen, oxy- 

 gen, 7iitroyen, sulphur, plionpltorus, ijotassium, calcium, 

 maynesium, and iron. Besides these, there are generally 



found sodium, silicon, 

 chlorine, and traces 

 of some other snb- 

 stances. 



Food from the At- 

 mosphere. — The food 

 wliicli plants obtain 

 from the atmosphere 

 is mostly carbon. The 

 leaves absorb carbonic 

 acid gas (COo), and 

 separate the carbon 

 from the oxygen, re- 

 taining the former 



{MiiQnified, shouing Cells ami Mouths, or Stoinula.) , , . .i i ■ 



and returnmg the hit- 

 ter to the atmosphere. This carbon, combining with 

 hydrogen and oxygen in the ])lant, is converted into 

 starch, sugar, cellulose, etc., and tluis enters into the 

 structure of the plant. 



This process of absorbing and decomposing carbonic 

 acid gas takes phuu; only in sunlight. In some unknown 

 way, the influence of the rays of light is required to 



The Under Side of a Leaf. 



