234 TIMBER AND SOME OF ITS DISEASES. [CHAP. 



of a healthy living larch, the mycelium to which 

 they give rise kills the cells of the cortex and 

 cambium, penetrates into the young wood, and 

 causes the development of a patch which every one 

 would recognize as that of the larch-disease. It 

 is thus shown that the fungus is the immediate 

 cause of the patch in which it is found. 



The next fact which has been established is that 

 the fungus can only infect the cortex through some 

 wound or injury such as a crack or puncture 

 and cannot penetrate the sound bark, &c. Once 

 inside, however, the mycelium extends upwards, 

 downwards, sideways, and inwards, killing and 

 destroying all the tissues, and so inducing the out- 

 flow of resin which is so characteristic of the 

 disease. The much-branched, septate, colourless 

 hyphae can penetrate even as far as the pith, and 

 the destroyed tissues turn brown and dry up. 



After destroying a piece of the tissues in the 

 spring, the growth of the mycelium stops in the 

 summer, the dead cortex dries up and sticks to the 

 wood, and the living cortex at the margins of 

 the patch commence to form a thick layer of cork 

 between its living cells and the diseased area. 



It is this cork-formation which gives the appear- 

 ance of a raised rim around the dead patch. It 



