4$2 PLANTS AND THEIR RELATION TO MAN 



FIG. 319. The bark of this 

 tree has been injured by 

 wire. 



girdle may be loose at first, it becomes 

 tighter as the tree grows, and finally in- 

 terferes with the food supply of the 

 roots. The person who carves initials 

 on a tree should be careful not to cut 

 deep enough to reach the bast layer. 

 A few deep cuts may not injure a tree, 

 but many deep cuts often ruin fine 

 trees. The bark of the birch tree is 

 easily peeled off and is frequently used 

 in the making of fancy articles, such as 

 jewelry cases, sewing baskets, and post 



cards. Inhabitants and visitors in regions where birch trees 



grow should make it their business to protect these trees 



rather than to destroy them, and should refrain from ruthlessly 



stripping off the bark. 



How small stems grow into large stems. The growing part 



of the stem is the cambium layer. In the spring and the 



summer the cambium cells 



multiply and form a large 



layer of wood on their 



inner surface and a small 



layer of bark on their outer 



surface. New wood is 



formed very much faster 



than new bark, and old 



trees contain much more 



lumber than bark. Each 



year new wood adds to the 



thickness of the stem and 



enlarges it, and the slender 



trunk of the tiny sapling 



r to FIG. 320. A piece of wood in cross section, 

 becomes the large trunk of showing the annual rings. 



