120 



DISEASES OF TREES 



cortical tissues which in summer causes a temporary inter- 

 ruption to the progress of the parasite. In autumn the 

 mycelium again succeeds in entering the living bast, either 

 through the cambium region or by way of the wood, so that, 



as a matter of fact, the layer of 

 cork is only of slight service. In 

 proportion as the passage of the 

 plastic substances is confined to 

 one side of the tree, growth of 

 the wood and bast is stimulated 

 at that part (Fig. 59). Thus the 

 conflict between parasite and 

 host-plant may remain long un- 

 decided, and in the Tyrol I found 

 larches still alive with blisters of 

 a hundred years' standing. 



Should the parasite advance re- 

 latively quickly, and, at the same 

 time, should the growth of the tree 

 at the affected part be slow, then 

 the canker-spot soon embraces 

 the whole stem or branch (Fig. 59), 

 and the tree dies above this spot. 

 By artificial mycelial infection 

 one may, almost without fail, 

 produce a blister on any part of 

 a sound larch. 



Soon after the death of the 

 cortical tissues, the cushion-like 

 stromata of the parasite originate 

 in the form of small yellowish 

 white pustules of the size of a 

 pin-head (Fig. 58 c, Fig. 60 a). In 

 the interior of these stromata, 

 and partly on their surface as 



well, vermiform passages or roundish cavities are formed, the walls 

 of which are covered with innumerable club-shaped sterigmata, at 

 whose apex extremely minute cells originate. Whether these 

 organs, which appear to be incapable of germination, are 



FIG. 60. A canker-spot of two 

 years' standing, close to the collar, 

 and hidden by the grass. On the 

 upper portion, which is exposed 

 to air-currents, the stromata are 

 abortive ; but in the lower por- 

 tion, which 

 they have 

 vigorous ascocarps. 



