THE FUNGI : ASCOMYCETES AND BASIDIOMYCETES 313 



trees. In badly infected woods it is not uncommon to find many of the 

 trees in various stages of the disease. Firstly, there are trees in which the 

 number of leaves and branches appears to be below the normal and in which 

 many of the lower branches are dead. Secondly, there are those in which 

 early stages of the fruiting bodies can be found and whose leaves have fallen 

 by August. Thirdly, there are trees entirely defoliated and consisting of 



Fig. 305. — Polyporns betulimis. Birch tree which has been killed 

 by the Fungus, lying on the ground and bearing numerous 

 fruiting bodies. 



little more than gaunt, branchless trunks bearing fruiting bodies, and, 

 finally, there are fallen trees on which the fruiting bodies may still be 

 developing (Fig. 305). 



The mycelium develops just below the bark and forms a complete layer 

 of tissue about the thickness of a piece of paper. It is white in colour and 

 becomes more solid as the parasitism becomes more severe, until it forms 

 a more or less complete investment of the central woody cylinder. It is 

 made up of countless very fine, septate hyphae (Fig. 306). 



We know little about the way in which the fruiting body is initiated, 

 but it is apparentlv derived from this mycelium. It first appears as a more 



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