12 THE SCHOOL BOOK OF FORESTRY 



The cells of the plant that are rich in chlorophyll 

 have the power to convert carbonic-acid gas into 

 carbon and oxygen. These cells combine the car- 

 bon and the soil water into chemical mixtures 

 which are partially digested when they reach the 

 crown of the tree. The water, containing salts, 

 which is gathered by the roots is brought up to 

 the leaves. Here it combines with the carbonic- 

 acid gas taken from the air. Under the action 

 of chlorophyll and sunlight these substances are 

 split up, the carbon, oxygen and hydrogen being 

 combined into plant food. It is either used im- 

 mediately or stored away for future emergency. 

 Trees breathe somewhat like human beings. 

 They take in oxygen and give off carbonic-acid 

 gas. The air enters the tree through the leaves 

 and sm^ll openings in the bark, which are easily 

 seen in such trees as the cherry and birch. Trees 

 breathe constantly, but they digest and assimilate 

 food only during the day and in the presence of 

 light. In the process of digestion and assim- 

 ilation they give off oxygen in abundance, but 

 they retain most of the carbonic acid gas, which 

 is a plant food, and whatever part of it is not 

 used immediately is stored up by the tree and 

 used for its growth and development. Trees also 



