74 THE LARCH CANKER 



thus producing an amphitheatre-like appearance in the 

 wood of a transverse section. The mycelium flourishes 

 chiefly in the cortex and the phloem, but can also attack the 

 wood to a limited extent. 



It has been shown that this fungus is not a parasite in 

 the sense in which the term is generally used. The mycelium 

 does not directly attack the living cells of the host, but first 

 kills them by secretions. The mycelium grows both in the 

 cells of the host and in the intercellular spaces. Fructifica- 

 tions are borne on the dead bark in the canker region. These 

 are larger than those on the dead branches in all their parts, 

 but it has been shown that this difference is due to growth 

 conditions, and the dimensional differences cannot be 

 made the basis of specific or varietal separation. 



Large quantities of resin are made by all attacked parts 

 of the tree. This oozes out on to the surface of the canker, 

 and often forms long streams down the trunk. 



Pure cultures of the fungus have been grown on various 

 media, including sterilized twigs of different conifers and 

 dead needles of larch, spruce, and Scots pine. In a damp 

 chamber larch needles may become matted together by the 

 mycelium which ramifies the mesophyll. It is suggested 

 that the matting of needles on the floor of larch plantations 

 may be largely due to the mycelium of this fungus, a process 

 which may help in the decomposition of the foliar debris. 

 Pure cultures on sterilized larch stems were grown up to 

 the stage of bearing apothecia, thus completing the cycle 

 from spore to spore. 



The fungus may gain admission to the living parts of the 

 tree in three ways : 



1. Through wounds, which may be caused by a variety 



of agents. 



2. By growing down from the dead branches into the 



living tissues of the trunk. 



3. By means of buds which have been killed by the 



shading of the upper branches. 



It has been shown that the second of these methods gives 



