HOW WOUNDS HEAL 



127 



the mere surface of the stem is injured, new cork 

 cells will develop from the inner bark; when the cambium 

 is reached and when the wound is deeper than the cam- 

 bium, new tissue is produced by the cambium and the 

 attempt made thus to heal and bridge over the wound. 

 Outer bark cells and wood cells beneath the cambium arc 

 helpless to perform any healing function, because they 

 are lifeless or nearly so (Chapter II). In other words, 

 when a limb is broken or cut off, or when it is exposed 

 by peeling off the bark, the heart wood cannot heal itself 

 but must rely upon the cambium. 



117. Callus. — The tissue which is formed by the cam- 

 bium and the inner bark over a wound is called a callus. 

 Always such growths begin 

 around the edges of the wound 

 and gradually or rapidly grow 

 toward the center. In no case 

 does the callus have any physio- 

 logical connection with the 

 heart wood of the original 

 trunk or of the limb broken or 

 cut off (Fig. 83). The callus 

 cap has often been likened to 

 the metal or glass cover placed 

 over a fruit jar. 



118. The purpose of the 

 callus is to serve as a protection 

 to the tissues beneath. Part 

 of this protective function is to 

 check loss of moisture from the 

 surface of the wound, but its 

 more important role is to prevent decay by protecting 

 the wood from weathering and, most important of 

 all, the entrance of rot-producing fungi and bacteria. 

 Once such organisms gain an entrance into the heart 

 wood, the limb and the tree so affected are doomed 

 sooner or later to breakdown and death. When limbs 



FIG. .S6 

 CANNON BALL IN OAK TREE 

 Even the trees try to heal the 

 wounds inade by war. This speci- 

 men is in the Jenny Wade mu- 

 seum at Gettj'sbur;;, Md. Many 

 trees still standing on the battle- 

 field have completely buried the 

 shots that wounded them. 



