THE CULTIVATION OF THE OAK. 159 



cases : some of the fungi destroy the vessels, fibers, etc., 

 by dissolving their walls from inside, while others de- 

 stroy the part common to contiguous cells, etc., and 

 thus first isolate the elements and then complete the 

 destruction. A series of very interesting researches by 



FIG. 43. Oak timber destroyed by the fungus Hydnum Jirersidens: 

 a shows the medullary rays on the tangential section ; b, a mass of 

 felted mycelium. (R. Hartig.) 



Hartig has demonstrated that the presence of these tim- 

 ber-destroying fungi can be detected from the markings 

 and discolorations they produce in the wood ; those due 

 to Hydnum diversidens, Thelepliora Perdix, Polyporus 

 sulplmreus, P. igniarius, P. dryadeus, and Stereum 



