FORMATION OF WOOD 267 



lignin, a substance, moreover, wHch like cellu- 

 lose contains carbon, liydrogen and oxygen 

 though not in the same proportions. 



A closer examination of our cross section will 

 show that there are two groups of wood tissue. 

 On the interior next to and often projecting 

 into the pith, there are a number of bundles of 

 wood tissue, differing in appearance from the 

 main wood tissue. These inner bundles of wood 

 are the first formed wood elements or pro- 

 toxylem, formed by the procambium. The 

 greater part of the wood, however, arises from 

 the cambium ring. The cells of this first formed 

 wood are always smaller in diameter than those 

 of the later formed wood. 



Before we consider in detail the various 

 elements composing the wood, let us examine 

 the annual rings (Fig. 72) for a moment. These 

 rings are conspicuous in the majority of de- 

 ciduous trees and inconspicuous, irregular or 

 absent in most evergreens, for a reason which 

 will be apparent in a moment. We have said 

 that one of the functions of wood is to serve 

 as a channel for water and dissolved substances 

 from root to leaf. That being so, it is clear 

 that where there are no leaves on the tree there 

 is no need for these watery solutions to pass up 

 the tree ; on the other hand, when the tree is 

 in full leaf the call on the conducting tissue is 



