DISEASES OF SHADE AND FOREST TREES. 



113 



thin, tough, leathery, shelving sporophores. (Fig. 

 42. ) They are variable in size, frequently very numerous 

 and overlapping. The upper surface is marked with con- 



Fig. 42. Polystictus versicolor on dead bark. (From Report 

 of* Perm. Forestry, 1902, Plate XXII.) 



centric zones of various colors while the under surface is 

 usually white. The pores of the under surface, within which 

 the spores are borne, are very small and regular. 



Another sap rot is caused by Polystictus per game nus Fr. 

 It is usually found on dead trees and is quite common on 

 trc< s that have been injured by fire. It also occurs on living 



8 



