X.] "CANKER": THE LARCH DISEASE. 229 



even extended, and there is hardly any limit to the 

 possibilities of damage to the timber thus exposed to 

 a multitude of dangers. 



In Fig. 34 is represented a portion of a tree stem 

 affected with " canker " : the transverse section shows 

 the periods of growth numbered i to 6 from within 

 outwards. When the stem was younger, and the 

 cambium had already developed the zones marked I 

 and 2, the cortex suffered some injury near the base 

 of the dead twig, below the figure i. This injury was 

 aggravated by the ravages of fungus mycelium, which 

 penetrated to the cambium and destroyed it over a 

 small area : in consequence of this, the next periodic 

 zone of wood (marked 3) is of course incomplete over 

 the damaged area, and the cortex and cambium strive 

 to heal over the wound by lip-like callus at the 

 margins. The accomplishment of the healing is pre- 

 vented, however, by the mycelium, which is continually 

 destroying fresh cells and extending the area of in- 

 jury : consequently the next zone of wood (4 in the 

 figure) extends even a shorter distance round the stem 

 than this one, and so on with 5 and 6, the cambium 

 being now restricted to less than half the circumference 

 of the stem — Le. from D to D, and the same with the 

 living cortex. Of course the injured area extends 

 upwards and downwards also, as shown by the lips of 



