60 THE PRINCIPLES OF AGRICULTURE. 



off. Out of the soil and the atmosphere 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 found in all plants, and 

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

 gen, nitrogen, sulphur, phosphorus, potassium, calcium, 

 magnesium, and iron. Besides these, there are generally 



found sodium, silicon, 

 chlorine, and traces 

 of some other sub- 

 stances. 



Food from the At- 

 mosphere. The food 

 Avhich plants obtain 

 from the atmosphere 

 is mostly carbon. The 

 leaves absorb carbonic 

 acid gas (C0 2 ), and 

 separate the carbon 

 from the oxygen, re- 

 The under side of a Leaf . taining the former 



(Magnified, showing Cells and Mouths, or Stoinata.) n 



and returning the lat- 

 ter to the atmosphere. This carbon, combining with 

 hydrogen and oxygen in the plant, is converted into 

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

 structure of the plant. 



This process of absorbing and decomposing carbonic 

 acid gas takes place only in sunlight. In some unknown 

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



