i88 DISEASES OF TREES 



attains to the thickness of the finest tissue paper, and only where 

 it penetrates between the bark scales does it swell up to form 

 yellowish-white bodies varying in size from a pin-head to a pea. 

 Decomposition (red-rot) spreads from the roots up into the 

 interior of the stem to a considerable distance. It is only in 

 the case of the Scotch pine that the rot does not ascend into 

 the stem from the stool. 



Shortly described, the biology of the parasite is as follows. 

 The spores, which are formed in the hymenial layer of the 

 subterranean sporophore, do not as a rule spread from the place 

 where they originate, unless they are brushed off by a passing 

 object. As sporophores are especially liable to be formed on 

 diseased roots at the point where they abut on mouse-holes, 

 it appears to be a likely supposition that the mice, or other 

 burrowing animals, carry away the spores on their fur, possibly 

 for long distances, and afterwards rub them off on healthy roots. 

 The spores soon germinate in warm humid air, and the my- 

 celium, entering between the bark-scales, will probably reach 

 the living cortex at some point or other. From this time its 

 course of development is in two directions. It forces its way 

 into the wood, in which it very rapidly travels up the stem. The 

 contents of the parenchymatous cells are killed and turned 

 brown by the action of the ferment that is exuded by the 

 mycelium, while a violet colour in the wood is the visible 

 symptom of this stage of the decomposition. With the 

 disappearance of the protoplasmic contents of the cells the violet 

 colour is replaced by pale brownish yellow, except for a few 

 dark blotches which persist. The latter afterwards become sur- 

 rounded by a white zone, and simultaneously the wood gradually 

 becomes lighter and more spongy. Ultimately numerous holes 

 are formed, the tissues become dismembered, sodden, and pale 

 brownish yellow, but never dark brown. 



The hyphae of the fungus travel upwards in the lumina of 

 the elements of the wood, and easily pierce the walls of the cells. 

 As they send off lateral branches they also invade the cells of 

 the medullary rays, as well as adjoining tracheides. As already 

 mentioned, the first perceptible change in the wood occurs in the 

 contents of the living cells, which become brown and partly 

 disappear. This is succeeded by the conversion of the lignified 



