THE STRUCTURE OF STEMS. 



225 



tirely. The corky layer continues to grow for a few 

 years, but it differs in different species. Its cells 

 are powdery in the birch, and so cause the peeling 

 of the more compact layers. The liber continues to 

 grow throughout the life of the tree. 



Fig. 397 represents a vertical and horizontal sec- 

 tion of a woody stem three years old, in which you 

 can trace the parts we have described. 



FIG. 39T. 



What must be the effect upon the bark of this 

 yearly formation of two new layers within it ? 



Examine the bark of such trees as you can get at, 

 and point out the results which you think follow from 

 this internal deposit of matter. 



In oak and chestnut wood the ducts of the inner 

 portion of each annual layer are large and numerous, 

 while the outer portion of the layer is dense and solid. 



