160 OUR NATIONAL FORESTS 



Diseases of the bark intercept the flow of food com- 

 ing down in the bark from the leaves. Diseases of 

 the sapwood cut off the water supply, which is 

 pumped upward from the roots. Those that attack 

 the roots also affect the water supply of the tree. 

 Diseases of flowers and seeds destroy the faculty 

 of reproduction. 



Certain parasites are able to enter the youngest 

 parts of trees, twigs and leaves directly, but the 

 majority of the fungi causing decay of the wood 

 can get into the interior of the living tree only by 

 way of a pin knot or wound. For this reason, 

 every wound caused by lightning, by fire, by man, 

 or by animals, constitutes a menace to infection. 

 Many coniferous trees cover their wounds by an 

 aseptic coat of pitch, which is very effective in 

 preventing the germination and growth of fungus 

 spores. But the less resinous conifers and the 

 hardwood trees do not cover their wounds very ef- 

 fectively; large wounds are not covered at all. 

 Upon exposure by a wound, the sapwood just un- 

 derneath the bark dies, dries out, and checks. 

 Spores of parasitic fungi enter the cracks, ger- 

 minate and infect the heartwood. The spores of a 

 heartwood-inhabiting fungus cannot germinate and 



