INJURIES INDUCED BY PLANTS 



73 



the young immature leaves at the base of the shoot being killed, 

 subsequently shrivel up. The leaves on the middle and apex 

 of the shoot, being 

 reached somewhat later 

 by the slowly advanc- 

 ing mycelium, retain 

 their shape. 



The cushions which 

 are formed by the my- 

 celium on the under side 

 of the leaves are at first 

 white, but afterwards turn 

 brownish (Fig. 19, b U). 

 They only partially con- 

 ceal the bluish lines 

 which are met with on 

 the under side of the 

 leaves of the silver fir. 

 In the course of time 

 very minute perithecia 

 are formed on these 

 cushions (Fig. 20). 



The cushion originates 

 in the following way. 

 From the hyphae that 

 cover the leaf (Fig. 21, a) 

 numerous branches, , are 

 sent out towards the 

 epidermis, and these 

 form a fleshy cushion, c, 

 consisting of parallel 

 hyphae closely united 

 to each other. At the 

 point, d, where it reaches 

 the epidermis of the leaf, 

 each hypha sends a fine 



rod-like haustorium into the outer wall, e, of the epidermal 

 cells, and owing to the secretion of a ferment these cells and 

 the stomata, f t are killed and become brown. The cells of 



FlG. 1 8. Branch of the silver fir attacked by 

 Trichosphceria parasitica, a, healthy leaves ; 

 b, dead and brown leaves whose bases are at- 

 tached to the branch by the fungus-filaments. 

 On account of their not being fully formed when 

 attacked by the fungus the dead leaves towards 

 the base of the shoot have shrivelled up. 



