Correspondence Courses in Agriculture. 1 



the length of the branch and the height of the tree. The growth 

 of the roots in length and thickness is somewhat similar to that of 

 the branches. 



Life Processes of the Tree. A tree, being a living organism, must 

 carry on certain life processes, much as with an animal. It must 

 breathe, absorb and digest food and transport the nourishment to 

 the parts requiring building up. 



Breathing, or Respiration. Breathing is carried on through min- 

 ute openings (stomata) in the leaves. The tree breathes in oxygen 

 and breathes out carbolic acid gas, the same as an animal. This 

 oxygen is used in carrying on the life processes of the tree. To 

 some extent breathing takes place through small openings in the 

 bark of the stem (lenticles), the oxygen being conducted to the 

 inner portions of the stem through the medullary (pith) rays. 



Absorption. Raw materials (for the manufacture of food) are 

 absorbed by the root-hairs and through the leaves. In the former 

 case the raw materials consist of mineral (or soil) salts in solu- 

 tion; in the latter, of carbolic acid gas. The mineral salts in solu- 

 tion are carried from the roots, through the stem and branches to 

 the leaves where they are combined with the carbolic acid gas of the 

 air to form food (sugar). When wood is burned, the mineral salts 

 remain as ash; the portion of the material from the air (carbon) 

 goes off as gas (carbolic acid gas) and disappears. The mineral 

 salts occur (usually) in very weak solution. That enough may be 

 brought up to supply the requirements of the tree, a far greater 

 amount of water must be absorbed than is actually needed. The 

 excess of water passes out through the stomata of the leaves as 

 water vapor (transpiration). This action serves to cool the leaves 

 and the tree, as does perspiration in animals. Transpiration takes 

 place in winter as well as summer and is often the cause of "winter 

 killing" when excessive. 



Digestion. The raw material (carbolic acid gas) absorbed through 

 the stomata of the leaves, forms the bulk of the food manufactured. 

 By the action of sunlight on the chlorophyl (green matter of the 

 leaf), the carbolic acid is decomposed into carbon and oxygen 

 (photosynthesis) ; the carbon being retained, the oxygen returned 

 to the air. The carbon is combined with hydrogen and oxygen of 

 the water and the mineral salts to form sugar, a good material 

 capable of utilization by the tree. In other words, the raw mate- 

 rials absorbed in the root-hairs and the leaves must be digested 

 in the leaves before they become available as food. 



Circulation. After the food material is thus prepared in the 

 leaves, it is carried down through the inner-bark of the branches, 

 stem and roots and through the medullary rays to the tissues re- 

 quiring nutrition and building up. Some of the food material is 

 used at once in forming the annual ring, flowers, fruit and other 

 new parts of the tree. Other portions are changed from sugar to 

 starch and stored away as a reserve supply with which to begin the 

 next year's growth. Before being available for use it is again trans- 

 formed into sugar. 



