Shade Tkee and Timber Destkoying Pungi. 



24:1 



y 



aged maples there are comparatively few hraiiclies and these near 

 the top of the tree, tlie lower branches having died and dropped 

 off. The same can he said of the beeches, l)irches and otlier l)road 

 leaved trees. Even on tliese topmost branches there are compara- 

 tively few leaves, because of the low liglit reaction. This means 

 then that a comparatively small amount of the carbohydrates neces- 

 sary in the formation of cell walls and woody tissue is manufac- 

 tured. Consequently growth and the formation ol" wood goes on 

 slowlv. This interferes in a striking; way with the healini'' nroc- 

 esses needed to cover up the wounds caused by the falling limbs. 



AVhen there is an abundance of foliage and light carbodydrates 

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

 liealing tissue is iirmer than the 

 normal wood, and working from 

 within and close to the branch, 

 sOon heals over and excludes the 

 mvcelium of the timber destrov- 

 ino- funo-i. On the otlier hand, 

 wlien there are few leaves and a 

 small amount of starch is formed 

 the healing process goes on 

 slowly, and before the opening 

 caused by the falling lind) can be 

 closed, the portion of the branch 

 exposed undergoes first weather- 

 ing, and later the myceliam lays 

 hold and enters, reaching' the 

 heart of the tree before the bar- 

 rier of healing tissue is formed. 

 From observations durinof several 



79. — Ve?-i/ young fruit body of Poly- 

 •porus icjmavius forming at a 

 branch icound. 



seasons it seems that this is a very common mode of entrance for 

 the timber destroying fungi in the Inroad leaved trees. 



A number of cases observed at Clearwater offer strikino: exam- 

 pies 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 percentage of 

 newly formed plant substance. Then for a series of years the heal- 



