THE STRUCTURE AND GROWTH OF STEMS 167 



greatly enlarged; it now forms the principal part of the 

 stem ; the new wood has been added in the form of rings. 

 The pith is much reduced in area; it seems to be dis- 

 appearing. 



Now if we turn to the trunk itself, we find that the same 

 kind of change has continued. (See Figure 58.) The 

 pith is gone entirely. Where once there was but a cylinder 

 of wood, there is now a practically solid column which 

 makes up nearly all of 

 the stem. From the 

 center clear out to the 

 bark, ring after ring of 

 wood shows the growth 

 of many seasons. The 

 cortex is now nothing 

 more than a sap-filled 

 inner part of the bark. 

 The bark itself is thick, 

 and furrowed, and 

 corky. 



How have all these 

 changes come to pass ? 

 By what method has all this secondary growth been ac- 

 complished, all this wood been formed? What is the 

 meaning of the rings in the wood? What can we learn 

 of the process which thus produces timber for the world ? 



By the secondary growth of perennial stems the timber 

 supply of the world is maintained. There is no other 

 way. If the timber is cut more rapidly than this growth 

 can renew it, the supply is sure to become exhausted. 



In order to understand this formation of wood, it is 

 necessary to understand the structure of an individual 



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Fig. 58. — Section of the. trunk of a young oak. 



