10 FOREST TREE DISEASES. 



upward to the leaves or needles. Enormous quantities 

 of water are given off by the foliage in transpiration. 

 The mineral salts accumulate and are utilized for vari- 

 ous purposes. 



The green leaves have three distinct functions 

 transpiration of water and its regulation, breathing 

 (respiration), and assimilation of carbon dioxide. 



Transpiration of water takes place through innu- 

 merable small openings in the leaves, which regulate to 

 a certain degree the quantity of water thus given off. 



Every plant breathes (respires) exactly as every ani- 

 mal does; that is, oxygen is taken up and carbon di- 

 oxide, the product of respiration, is discarded. This 

 process goes on day and night, exactly as in the case of 

 animals. 



Carbon dioxide (a gas consisting of carbon and oxy- 

 gen) from the atmosphere is assimilated by green leaves 

 under the influence of sunlight. The carbon from the 

 carbon dioxide, together with the water and mineral 

 salts brought up from the roots, is used in the forma- 

 tion of chemical substances of highly complex nature 

 (starch and sugar) ; the oxygen from the same source is 

 exhaled. In the leaves also takes place the assimilation 

 of nitrogenous salts and formation of albumen. 



Starch, sugar, and albumens furnish food for the 

 growing cells of the cambium and the root tips, being 

 transported from the leaves, where they are formed, 

 downward through the living bark. The surplus above 

 the amount consumed is stored. 



All these functions must be perfectly balanced. The 

 development of the root system, for instance, which sup- 



