146 TIMBEES AND THEIR USES 



cellulose. This evidently occurs by some solvent 

 action, and is due to a ferment excreted from 

 the fungus filaments, and the destroyed timber 

 becomes reduced to a brown mass of powder." 



Polyporus ignarius, in fact all the species of 

 Polyporus have very similar life histories. The 

 mycelium of P. ignarius penetrates the medullary 

 rays of its host and destroys the starch which 

 is always stored there. P. dryadens attacks its 

 host in a similar manner. " It occasionally 

 happens that an Oak is attacked by both these 

 fungi, and their myceUa become intermingled 

 in the timber ; when this is the case the starch 

 grains remain intact in those cells which are 

 invaded simultaneously hy the hyfhcB of both 

 fungi." 



P. dryadens and P. fulvus both destroy the 

 middle lamellae of their hosts, with the result 

 that the tracheids are isolated. 



The Yellow Toadstool, Agaricus melleus, is 

 another common external parasite of our forest 

 trees. It is found most commonly on Conifers, 

 Beeches and Oaks ; growing invariably at the 

 base of its host it has the appearance of a brownish 

 yellow mushroom ; its mycelium is shiny black 

 and grows in dense masses around and within 

 the tree roots. The black hyphse pass from the 

 roots up into the stem and rot sets in. 



A diseased tree may easily be recognized by 



