196 DISEASES OF TREES 



POLYPORUS BOREALIS l * 



This fungus produces an exceedingly characteristic form of 

 white-rot in the spruce. In the Salzburg and Bavarian Alps, 

 and in the spruce woods near Munich, it is the commonest form 

 of decomposition in the spruce, and I have also noticed it in the 

 Harz. Infection takes place, and the sporophores are produced, 

 above ground. On account of their white colour the sporo- 

 phores are conspicuous even at a distance. They are annual, 

 more or less bracket-shaped, and frequently superimposed the 

 one above the other and grown together. They are very 

 watery, somewhat sodden on the upper surface, and destitute 

 of zones. 



The colour of the wood is but little altered by the decompo- 

 sition. It becomes brownish yellow, and horizontally disposed 

 holes filled with mycelium appear in vertical rows in the spring 

 wood. These holes, which stand I to ij mm. apart, impart to 

 the wood an appearance which reminds one of the finest graphic 

 granite. The wood constantly becomes lighter and more friable, 

 but the peculiar appearance is retained till the end of the final 

 stage of decomposition. 



Should the wood be exposed, without drying, when decom- 

 position is beginning, the mycelium will grow outwards to form 

 a white skin, the mycelial strands of which are chiefly disposed 

 in a horizontal direction. 



Growth and decomposition are in several ways characteristic. 

 The hyphae, which in the first stage of decomposition are yellow 

 and stout (Fig. 124, a, #), are replaced by more delicate fila- 

 mentous mycelia as decomposition proceeds, until at last the 

 hyphse which are formed can only be seen by the. aid of a very 

 powerful microscope. The mycelium has a marked tendency to 

 grow to some extent in a horizontal direction, at right angles to 

 the long axis of the elements (Fig. 124, t\ the chief result being 

 the formation of the above-mentioned horizontal holes in the 

 wood. Why these are formed only at definite distances from 

 each other I have not been able to determine. Dissolution of the 



1 R. Hartig, Zersetzungsersckeinungen, pp. 54 et seq. 

 * [This is quoted as a British species. ED.] 



