228 TIMBER AND SOME OF ITS DISEASES. [CHAP. 



semi-parasitic fungus, as it tries to heal over the wound. 

 The local disturbances in growth kept up by the 

 mycelium feeding on the contents of the cells of these 



Cfam. 



Cor. 





FIG. 34. Piece of tree stem affected with " canker." The injury commenced after 

 the two inner zones of wood (i and 2) had been developed : it extended further in 

 successive periods of growthj as shown by the receding zones 3, 4, 5, and 6, until 

 all the cambium and cortex was destroyed except the pieces D to D. Cam, cam- 

 bium ; Cor, living cortex ; D Z>, dead tissues. At each period of growth the 

 attempt has been made to heal over the wound, as shown by the successively 

 receding Hps. 



tissues lead to the irregular growths and hypertrophies 

 referred to ; the wounds are kept open and " sore," or 



