TREES AND TIMBER ^^s- 379 

 General Diseases 

 DECAY IN LIVE TREES ^^^ ^^'^ 



Wood decay is caused by fungi. The mycelium 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 

 crumbling 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 life 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; cutinized or supplied with resin, gum, or other 

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

 tious and does not offer attractive invitation to the fungus. 

 The protection thus constituted is normally ample; but in 

 case of removal of this natural protection and exposure of' 

 either sapwood or heartwood, especially the latter, the path 

 is open, and it is through wounds offering such exposure 

 that rot in standing timber usuall.y begins. Such rots might 

 therefore be appropriately termed "wound decays." 



The fungi which cause these decays are, in the main, 

 the larger fungi whose fructification is of the toadstool type 

 (Figs. 191, 194). After the decay within is well under way 

 the spore-bearing toadstools appear upon the surface of the 

 diseased parts, and are quite generally recognized by lumber- 

 men as a sign of rottenness within. 

 354 



