BODY AND BRANCH DISEASES AND INJURIES 67 



trunk. It then extends its growth upward, downward and 

 radially, until it may spread out through the sapwood and bark. 



Nature of the process of wood-decay. 



The heartwood of trees consists of a complex arrangement 

 of empty and lifeless cell-walls. Previously, while this tissue 

 was still sapwood and when a portion of the cells were alive, 

 it carried on the function of transporting water and food 

 materials between the roots and leaves. During the process 

 of the development of the sapwood, the cell-walls were ligni- 

 fied. To this modification of the cell-walls is due all the 

 properties of strength, color and durability which make woody 

 tissue different from the tissues of the non-woody or herbaceous 

 plants. In the lignified condition, wood is immune to the 

 ordinary agencies which so easily destroy non-lignified tissue. 

 The wood-rotting fungi^ Jioweverj_„excrete_,£ertain .eazym^ 

 whkh abstract the ligninJrgmceU-wa^^^^ also in most cases 

 they are able to dissolve completely the basic structure of the 

 cell-wall by other enzymes. These two types of enzymes 

 effect the partial or complete solution of the cell- walls. In 

 either case, the wood is no longer of value as wood, since the 

 abstraction of the lignin destroys the properties of strength 

 and leaves a soft and spongy substance. In the entrance of 

 the mycelium into sound wood, preliminary changes take 

 place which liberate resins and gums and these may pass out 

 into the sapwood and bark. Sometimes the accumulation of 

 these substances retards or arrests the further spread of the 

 mycelium. Color changes usually accompany the decay, 

 whereby the rotted wood is left almost white or is colored 

 brown or yellow. These colors are due to decomposition 

 products which stain the wood. When the action is general, 

 a uniformly rotted area results, but, in many wood-rots, localized 

 nests of the mycelium cause the complete solution of small 

 areas of the wood. This leaves pockets or holes separated by 



