x.] "CANKER": THE LARCH DISEASE. 235 



has long been known that the patches dry up and 

 cease to spread in the dry season. It should be 

 pointed out that it is one of the most general 

 properties of living parenchymatous tissue to form 

 cork-cells at the boundaries of an injury : if a slice 

 is removed from a potato, for instance, the cut 

 surface will be found in a few days with several 

 layers of cork-cells beneath it, and the same 

 occurs at the cut surface of a slip, or a pruned 

 branch, the " callus " of tissue formed is covered 

 with a layer of cork. 



If it is remembered that the cambium and 

 young wood are destroyed beneath the patch, it 

 will be at once clear that in succeeding periods of 

 growth the annual rings of wood will be deficient 

 beneath the patch. 



Next year, the cambium in the healthy parts of 

 the stem begins to form another ring ; but the 

 fungus mycelium awakens to renewed activity at the 

 same time, and spreads a little further upwards, down- 

 wards, and sideways, its hyphae avoiding the cork- 

 layer and traversing the young wood and cambium 

 below. During this second spring, therefore, a 

 still larger patch of dead tissue cortex, cambium, 

 and young wood is formed, and the cork-layer, 

 developed as usual at the edges of the wound, 



