TREE PHYSIOLOGY 35 



minerals up from the roots to the leaves, where by 

 a digestive process they are chemically transformed 

 to meet the tree's needs. This material is then dis- 

 tributed as required or stored up by certain cells 

 for future use. Tree blood often contains sub- 

 stances valuable for the use of man. He does not 

 hesitate to appropriate large quantities, but, luckiry, 

 a tree does not bleed to death as easily as a human 

 being. 



The bark we have already alluded to as tree- 

 skin. It covers and protects every square inch of 

 the tree's permanent body. Without it the cam- 

 bium layer would perish. It is impervious to the 

 passage of water and gases except through pores 

 called lenticels. The mouths of these are fitted 

 with cork filters which makes them hard to see ex- 

 cept on very young branches. Through the lenti- 

 cels air is admitted and withdrawn from the cam- 

 bium layer in a sort of respiratory process. In 

 the absence of the leaves the bark also carries on 

 the work of transpiration, which is similar to per- 

 spiration in man. Bark is composed of dead wood 

 and cork cells which under expanding pressure 

 crack into the characteristic plates which are such 

 a great help in identifying trees. In the cork oak 

 from which we get our commercial supply, the cork 

 grows to a remarkable thickness and can be taken 



