THE LIFE PROCESSES OF TEEES 21 



This process is called transpiration. The action also helps 

 to cool the leaves as does perspiration in animals. Transpira- 

 tion takes place in winter through the lenticles of the bark. 

 The main part of the mineral substances taken up remains in 

 the leaves and young twigs when the water is transpired and 

 is returned to the soil when the leaves are shed or when the 

 trees are cut and the brush left to decay. When wood is 

 burned these mineral substances reappear as ash. 



Digestion. — Carbonic acid gas forms the bulk of the food 

 of trees. -By the action of sunlight and chlorophyll, the green 

 coloring material of leaves, carbonic acid gas is decomposed 

 into oxygen and carbon. This process is called photosyn- 

 thesis. The carbon is retained and the oxygen exhaled into the 

 air. The carbon is then combined with the hydrogen and 

 oxygen of the water and with certain of the mineral salts 

 to form food substances like starch and sugar, which can be 

 utilized by the tree. In other words, the materials absorbed 

 through the roots and leaves must be digested in the leaves 

 before they become available as food. 



Circulation. — ^After food is thus prepared in the leaves it 

 is carried down through the inner, soft bark of the branches, 

 trunk and roots and then through the medullary rays into the 

 wood. Some of the food material as it is carried down the 

 tree is used at once in forming the annual rings, flowers, 

 fruit and other parts of the tree. Other portions are stored 

 away for starting the next year's growth. The expression 

 "the rise and fall of sap" in trees is thus seen to have no 

 real meaning. Water and mineral salts are constantly being 

 taken into the tree through the root hairs and carried to the 

 leaves through the new wood or sap wood of the trunks and 

 roots. The process goes on in the winter time as well as 

 in the summer although very slowly. Because of the greater 

 transpiration from the tree in the summer due to the leaves, 

 there is actually more water in a tree in winter than in sum- 

 mer. After food material has been manufactured in the 



