INJURIES INDUCED BY PLANTS 195 



This fungus produces a kind of white-rot in silver firs and spruces, 

 and is very frequently encountered in association with jEcidiiim 

 elatinum. Apparently infection is most liable to occur naturally 

 on .those parts of the cancerous swellings where the cortex has 

 ruptured and exposed the wood. The mycelium, which is at 

 first very vigorous, is yellowish in colour, and produces numerous 

 short lateral branches, which are twisted in a worm-like manner, 

 and are apt to fill up the cavities of the bordered pits of the 

 tracheids. This vigorous mycelium gives off" a few exceedingly 

 delicate lateral hyphae, which bore through the walls, in which 

 they form very minute holes. Only in the later stages of de- 

 composition is the disappearance of the middle lamella effected, 

 after which the inner walls are also dissolved, having first been 

 greatly attenuated and then temporarily isolated. At this stage 

 the mycelium is of extraordinary fineness. The wood of the 

 silver fir appears yellowish, clear oblong patches being observable 

 if carefully looked for on a smooth surface. The vigorous 

 yellow hyphae induce the formation of narrow dark lines at the 

 boundary of the sound wood. 



As the silver fir cannot form a strongly resinous zone, it is 

 unable to prevent the progress of the mycelium into the 

 youngest layers of wood. The mycelium consequently grows 

 outwards with ease into the cortex, and, having advanced far 

 enough, it produces the sporophores on the surface. These are 

 at first hemispherical, but in the course of years they become 

 more and more bracket-like in shape. Externally they are 

 yellowish brown on the hymenial surface, but elsewhere they are 

 ashy grey, almost smooth, display no zones, and are beset with 

 exceedingly minute punctures or pits. The interior, which is 

 tawny and lustrous, shows distinct zones, except in the region of 

 the pore-canals, which increase in length each year at their 

 lower extremity. 



As it is found that silver firs with cancerous swellings sooner 

 or later break at the diseased spot owing to snow or storms, 

 it has become the custom in many districts for instance, 

 in the Black Forest in Wurtemberg to fell all cankered trees 

 during the thinnings, even when such trees belong to the larger 

 class. In this way the spread of Poly poms Hartigii can best be 

 prevented. 



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