46 The Tree and How it Lives 



which is slipped over the body of the tree each 

 year. It covers the tree between the old wood 

 of last year and the bark, which is pushed out 

 farther and farther as the tree grows in thickness. 



You can tell the age of almost any tree if you 

 know how to ask. Look at the broad, flat stump 

 of an old oak just cut down. On the trunk of the 

 oak its growth for many years is plainly to be 

 seen. You will see some little rings on the stump. 

 These rings are narrow and very close to one 

 another, and they show you where the coat of 

 new wood was added to the tree each year. The 

 inside of the ring, since it is grown earlier in the 

 season, is called the "spring wood," the other 

 grows over it later on, and is known as the "sum- 

 mer wood." Both are formed by the cambium, 

 the living wood. It is only a narrow layer of a 

 few cells; but this layer grows and, as it grows, 

 divides. It makes the new wood each year the 

 annual ring. Inside, next to the cambium, is a 

 layer of sapwood several years old. A little 

 farther in is the heart wood, which is no longer 

 alive. But the younger wood helps the cambium 

 to carry the tree's food. 



In the rings covering the stump the life his- 



