TREE PHYSIOLOGY 37 



sections of the vital cambium layer which cou- 

 rageously come out into the open to perform cer- 

 tain vital duties. 



On a clear bright day each leaf is a busy little 

 factory for the manufacture of starch. The sun 

 furnishes the motive power and the air and ascend- 

 ing sap the raw materials. Carbon dioxide from 

 the atmosphere is the main ingredient. It is clev- 

 erly combined with sap-water to form starch. The 

 combination C 6 HioO 5 leaves a surplus of free 

 oxygen which promptly passes back into the air 

 through the stomates. This withdrawal of CO 2 

 and the substitution of oxygen is an important puri- 

 fication of the atmosphere from man's point of view. 

 With the oxygen, vast quantities of water are also 

 liberated, except in very hot weather when the clos- 

 ing of the elastic curtains or doors of the stomates 

 makes for the conservation of this important sub- 

 stance. On an ordinary day, when plenty of mois- 

 ture is to be had, a medium-sized oak will evaporatej 

 150 to 180 gallons of water. 



The minerals in the sap aid but do not enter into 

 the starch-making. Carried to other parts of the 

 tree, they are vital factors in construction work. 

 With the setting sun, the green leaf -laboratories 

 shut down. 



On the approach of winter, the leaves undergo 



