74 



DISEASES OF TREES 



FIG. 19. Lower side of a 

 leaf of the silver fir at- 

 tacked by T. parasitica. 

 At a the colourless my- 

 celium spreads from the 

 branch on to the lower 

 side of the leaf, on which 

 it forms white cushions, 

 bb. 



FIG. 20. Part of the leaf 

 of a silver fir, on the left 

 side of which the cushion 

 bears a number of small 

 perithecia. 



FIG. 21. Mycelial cushion of T. parasitica on the under side of the leaf of a silver 

 fir. The filamentous mycelium a sends down numerous branches at b to form 

 a cushion c, consisting of parallel hyphse ; where the hyphre reach the surface of 

 the leaf each sends a rod-like haustorium, d, into the outer wall of the epidermal 

 cells, e e ; at ^the cushion has been somewhat raised from the leaf, so that several 

 of the haustoria have been pulled out of the epidermis; the epidermal cells//" 

 have become brown. Although the filamentous mycelium h has penetrated the 

 chlorophyllous cells of the leaf-parenchyma^^, these do not become brown till 

 somewhat later ; the mycelial cushion grows into the depressions at the entrance 

 to the stomata z, where however it is unable to form haustoria ; at these places 

 it becomes coated with the adhering waxy granules. 



