ORCHARDS 



231 



.FIG. 117. 



result of 



and beneath the bark. From its outside it produces bark 

 and from its inside it produces wood. It is the layer of 

 young, tender cells that makes the bark "slip" so readily 

 in early spring. A layer of new cells grows on the outside 

 of a tree every year. 

 The cells that grow in 

 the fall are thicker- 

 walled than those that 

 grow in the spring. This 

 makes the wood darker 

 in color, so that a ring 

 is formed at the end of 

 each season's growth. 

 It is these annual rings crotches 



that enable us to tell the age of a tree. The sap of a 

 tree passes up through the outer layers of wood, the 

 sap wood, while the elaborated food is distributed 

 through the cambium layers. 



The outer bark and the inner wood of a tree are dead. 

 This dead inner wood is protected by the cambium layer, 

 so that fungi and bacteria cannot reach it. When a limb 

 is cut off, or if the bark is removed, the dead cells are 

 exposed. These cannot heal the wound. The cambium 

 layer around the edges must grow over it. The safety of 

 the tree depends on having it heal over before it becomes 

 infected with molds. If the wound is large and is not 

 treated, some decay fungus is almost certain to become 

 established before it heals over. The tree may then heal 

 over and look all right, but the fungi will continue to grow 

 and will result in a decayed or hollow trunk. A hollow 

 tree usually continues to grow all right, as the inner 



