126 THE FIRST BOOK OF FARMING 



is called the heart wood of the tree. The lighter 

 wood is called the sap wood. It is through the 

 outer or sap wood that the water taken in by the 

 root is passed up to the leaves where the food which 

 it carries is digested and then sent back to the plant. 

 The returning digested food is sent back largely 

 through the bark. Between the bark and the wood 

 is a very thin layer which is called cambium. This 

 is the active growing tissue of the stem. In the 

 spring it is very soft and slippery and causes the 

 bark to peel off easily. This cambium builds a new 

 ring of wood outside of the old wood and a new 

 ring of bark on the inside of the bark. In this way 

 the tree grows in diameter. 



Now if the bark is injured, or any part of the stem, 

 all parts below the wound are cut off from the re- 

 turn supply of digested food and their growth is 

 checked. When such a wound does occur, or if a 

 wound is made by cutting off a branch, the cam- 

 bium sets to work to repair the damage by pushing 

 out a new growth which tends to cover the wound. 

 We can help this by covering the wound and keep- 

 ing the air from it to prevent its drying and to keep 

 disease from attacking it before it is healed. 



HOW THE WORK OF THE STEM MAY BE INTERFERED 

 WITH 



If there are any peach trees near by, examine the 

 trunks close to the ground, even pulling away the 

 soil for a few inches. You will very likely find a 



