82 THE HEALING OP WOUNDS 



the branch could not make healing tissue of its 

 own. The stub is now a monument to the man 

 who pruned the tree. Fig. 74 shows how an- 

 other limb was cut, and although the wound is 

 not nearly so old as the other, it is being rap- 

 idly closed in. There are most important prac- 

 tical lessons, then, to be learned from this study 

 of knot-holes, two of which are that nature 

 is a most heroic pruner, and that limbs must 

 be sawn off close to the parent branch if the 

 wounds are to heal well. 



THE NATURE OF THE WOUND 



The foregoing paragraphs give the reader a 

 general view of the practical problems involved 

 in the expansion of trunks and the healing of 

 wounds. It will be profitable, however, to give 

 some of the problems more specific attention. 



The increase in diameter of the stem or trunk 

 is made by the growth of cells from the cambium, 

 which is a tissue lying upon the outside of the 

 woody cylinder and beneath the bark. From its 

 inside, the cambium produces wood, and from its 

 outside, it produces the soft or inner bark. As 

 the outer bark is ruptured by the expansion of 

 the stem, portions of the inner bark give rise to 

 the corky external and protective layers. A mere 

 abrasion or surface wound, which does not expose 

 the wood, is healed by the formation of new cork 



