xii.] PINE-BLISTER. 263 



When fairly thick stems or branches have the 

 mycelium on one side only, the cambium is injured 

 locally, and the thickening is of course partial. The 

 annual rings are formed as usual on the opposite side 

 of the stem, where the cambium is still intact, or they 

 are even thicker than usual, because the cambium 

 there diverts to itself more than the normal share of 

 food-substances: where the mycelium exists, however, 

 the cambium is destroyed, and no thickening layer is 

 formed. From this cause arise cancerous mal- 

 formations which are very common in pine-woods 

 (Fig. 40). 



Putting everything together, it is not difficult to 

 explain the symptoms of the disease. The struggle 

 between the mycelium on the one hand, which tries to 

 extend all round in the cortex, and the tree itself, on 

 the other, as it tries to repair the mischief, will end 

 in the triumph of the fungus as soon as its ravages 

 extend so far as to cut off the water-supply to the 

 parts above : this will occur as soon as the mycelium 

 extends all round the cortex, or even sooner if the 

 effusion of turpentine hastens the blocking up of 

 the channels. This may take many years to accom- 

 plish. 



So far, and taking into account the enormous 

 spread of this disastrous disease, the most obvious 



