THE PARTS OF TREES AND THEIR FUNCTIONS. 11 



cambium, the most of which, when hardened, forms a 

 layer of new wood, a small portion being converted into 

 bark. This process is kept up during the season, and in 

 many instances during the successive years of a long pe- 

 riod, depositing more or less wood each year, according to 

 the species and condition of the tree. 



It is an easy matter to determine the age of a tree by 

 counting the layers or rings in a crossjsection of the stem, 

 each of which represents a year's growth. (See fig. 2.) 



Fig. 2. 



It will be seen from the process of growth that the roots 

 and stem are mutually dependent upon each other, and 

 that the success of the tree depends upon their being vig- 

 orous and properly proportioned. The outer bark when 

 young performs an office similar to that of the leaves, but 

 in some trees it gradually becomes a hard substance, 

 which only aflfords protection to the more delicate, under- 

 lying parts. The heart or perfect wood adds stability to 

 the tree, and the pith while young promotes growth by 

 retaining moisture. In old trees the pith becomes dry, 

 often disappearing entirely, without any apparent dam- 



