22O 



BULLETIN 193. 



of cell walls and woody tissue is manufactured. Consequently 

 growth and the formation of wood goes on slowly. This inter- 

 feres in a striking way with the healing processes needed to cover 

 up the wounds caused by the falling limbs. 



When there is an abundance of foliage and light, carbohy- 

 drates are formed in sufficient abundance to heal at a rapid rate. 

 The healing tissue is firmer than the normal wood, and working 



from within and close to the 

 branch, soon heals over and 

 excludes the myceliutn of the 

 timber destroying fungi. On 

 the other hand, when there 

 are few leaves and a small 

 amount of starch is formed 

 the healing process goes on 

 slowly, and before the open- 

 ing caused by the falling limb 

 can be closed, the portion of 

 the branch exposed undergoes 

 first weathering, and later the 

 mycelium lays hold and enters, 

 reaching the heart of the tree 

 before the barrier of healing 

 tissue is formed. From obser- 

 vations during several seasons 

 it seems that this is a very 

 common mode of entrance for 

 the timber destroying fungi in the broad leaved trees. 



A number of cases observed at Clearwater offer striking exam- 

 ples of the slowness with which the healing process goes on at 

 branch wounds. The healing tissue formed slowly and did not 

 close up against the base of the branch because of the small per- 

 centage of newly formed plant substance. Then for a series of 

 years the healing would cease and an area around the knot 

 would die back. Then for another series of years the healing 

 would begin and advance over a portion of this dead area, when 

 another period would intervene during which a still greater 

 area would die back. In this way large and ugly, open ulcers 



79. Very young p-uit body of Poly - 

 porus igniartus forming at a 

 branch wound. 



