190 



PLANT DISEASES 



curved; flesh very thick, soft, pure white; pores on the 

 under surface very minute, white. Varying in size from 

 three to eight inches across. 



When the fungus first attacks the living wood it pro- 



Fig. 43. Polyporus betulinus. Fungus about one-quarter 

 nat. size ; section of same. 



duces a brown discoloration, due to a ferment present in 

 the mycelium ; by means of this substance the thickening 

 layers of the cell-walls are dissolved and absorbed, the 

 middle lamellae of the cells alone remaining. Finally 

 cracks are formed in the soft, decayed wood, containing 



