CHAPTER VI 



HOW WOUNDS HEAL 



113. Natural wounds in the bark, such as those caused 

 by growth in girth, usuall_v heal quickly and without 

 danger to the life of the tree. From what has been said 

 (Chapter II) it is apparent that the bark must necessarily 

 l)ecome tighter and tighter over the woody CN'linder as 

 the cambium (Ic\elo]is new cells. This internal pressure, 

 which often reaches •")() pounds to the square inch, causes 

 the rupture of the bark in countless exogenous trees and 



riG. 7S— HOW BARK BECOftlES ROUGH BY SPLITTING AND HEALING 



shrubs, and we thus see the wrinkled, contorted or scaly 

 stems characteristic of old trees, such as oak (Fig. 78), 

 hickory and sycamore. The splitting is due to the non- 

 expansion of the outmost or dead layers of bark ; the 

 wounds are healed by the development of new cells from 

 below. 



114. Bark-bound trees. — In some cases, for instance, 

 neglected orchards, the bark may become so tough, hard 



