86 Minnesota Plant Diseases. 



threads of these parasites exude certain chemicals which are 

 able to attack the lignin of woody tissues and to dissolve out 

 the substances which make up this lignin. These cell walls 

 now have the same chemical constitution which they had before 

 they became lignified. But the fungus is also able to attack the 

 cellulose walls and the final result is a more or less complete 

 breakdown of the walls. The wood crumbles easily and is con- 

 verted into punk, which is characteristic of rotten wood. The 

 threads make their way from cell to cell, usually by boring 

 through the walls, whether they are wood walls or cellulose 

 walls, and in tissues attacked by these fungi one sees large 

 numbers of holes through the cell walls, where fungus threads 



oooo 



Fig. 38. — Two ways in which wood is destroyed by wood-rot fungi. On the right the wood 

 cells are destroyed from within outwards. On the left they are destroyed from the 

 middle of the wall toward the center of the cell. Highly magnified. After Hartig. 



have passed. From these points the dissolving substance 

 spreads over the cell wall completing the rotting process in 

 that vicinity. Since woody tissues can be colored characteris- 

 tically by using certain chemicals, one can determine by the use 

 of these chemicals just how far the rotting has proceeded. 



It is worthy of notice that fungi have different methods of 

 attacking and rotting woods, using different substances and ap- 

 plying them in various ways. The study of the rotting of 

 woods is still in its infancy, but it is now known that certain 

 wood rotting fungi can be determined by the kind of rot which 

 they produce. The wood-rot, therefore, often gives very def- 

 inite symptoms of determinable diseases. The study of wood- 

 rots is receiving considerable attention at the present time on 



