WHY SHOULD WOUNDS HEAL ? 85 



below the sapwood, or if the wood has not dried 

 out and died, the callus may make a vital connec- 

 tion with the exposed surface of the wound. 



The reader now wants to ask what purpose 

 the healing of the wound may serve the plant. 

 The healing serves as a protection. It prevents 

 or checks evaporation from the exposed parts, and 

 prevents decay by protecting the wood from the 

 weather and by excluding bacteria and fungi. A 

 rotten heart, or rotten wood of any kind, is a 

 diseased condition; and this disease is the work 

 of living organisms. 



The exposed wood dies. It cracks and checks. 

 The surface collects dust, which, with the dead 

 cells, makes a thin soil in which germs find con- 

 genial conditions for growth. Even after the 

 wound is covered by the callus, the mycelium of 

 the fungi may continue to extend itself in the 

 wood, often reaching the heart and causing the 

 trunk to become hollow. Normally, the heart of 

 a tree should never decay; but sooner or later, 

 most trees are exposed to injuries, either in top 

 or root, through which the organisms of destruc- 

 tion may enter. 



The pictures will help us to understand. Fig. 

 7i i -Imws a cross -section of a maple trunk in 

 which a nail is imbedded. The wood closed tight 

 about it and no harm resulted. Now, this was 

 the nail upon which a sugar-maker hung his 

 bucket. Just below it was the tap -hole; and this 



