TREES ' 359 



the life of the tree. The heart wood may decay out and the 

 tree will still flourish, though, of course, weakened mechan- 

 ically. 



The bark is separated from the wood by a thin zone called 

 the cambium or growing layer. This makes new wood on the 

 inside, and new bark on the outside. In spring it is especially 



FIG. 153. The Forest. 



active and large and can be easily broken, as is seen when 

 the bark is slipped from a willow twig in making a whistle. 

 The bark also forms annual rings, but because of the 

 wearing off on the outside, it is usually not very thick. In 

 some trees it wears away fast and the bark remains thin. 

 In others, notably the cork oak, it becomes thick. On 

 account of the growth of the woody part of the stem, the 

 old bark becomes too small and is fissured in a manner 

 characteristic of each tree. The bark on the young trees, 

 and on the branches and twigs of old trees, generally has a 

 different color from that of the old, and is of course not so 



