TREES AND TIMBER i 



General Diseases 

 DECAY IN LIVE TREES 



Wood decay is always caused by fungi (perhaps rarely 

 aided by bacteria) whose myceUum penetrates through 

 or between the wood cells, producing enzymes which 

 soften the cells or disintegrate the middle layer between 

 cells, thus destroying stability of the aggregate as by the 

 crumbHng of the plaster or the brick of a wall. Within 

 the tree this disintegration may occur either to the heart- 

 wood or to the sap wood, or to both. 



The Ufe of a tree may be much reduced by decay of the 

 heartwood, the main mechanical support. Decay of 

 the sapwood further hastens death by interfering with 

 the rise of the sap. 



Upon the living tree the natural protection against fun- 

 gous invasion is the bark, consisting of cells with specially re- 

 sistant walls ; cutinizedor supplied with resin, gum, or other 

 repellents. Moreover, the dead bark is not generally nutri- 

 tious and does not ofifer attractive invitation to the fungus. 

 The protection thus constituted is normally ample ; but in 

 case of removal of the natural protection and exposure of 

 either sapwood or heartwood, especially the latter, the path 

 is open, and it is through wounds offering such exposure 



' The facts concerning tree and timber rots are drawn largely from 

 the writings of von Hchrcnk and his associates. 

 409 



