INJURIES INDUCED BY PLANTS 



119 



the sieve-tubes, and killing and browning the tissues. The 

 mycelium also grows into the wood, and even penetrates as far 

 as the medulla. 



That portion of the cortical tissues which has been killed 

 during the first year dries up and appears as a depression, 

 especially after growth in thickness has been resumed by the 

 healthy part of the tree (Fig. 58). 



In summer the growth of the mycelium ceases, and an unusually 



FIG. 58. A canker-spot that has been re- 

 cently formed in the upper portion of 

 the stem of an eight-year-old larch from 

 the Tyrol. Infection has occurred above 

 the branch, b, where a crack has been 

 formed in the tissues, owing to the branch 

 having been depressed under a load of 

 snow. Numerous immature ascophores, c, 

 have already formed on the dead cortex. 



FIG. 59. Cross section of a well- 

 grown larch which has been at- 

 tacked by P. Willkommii. Infection 

 had occurred ten years previously 

 at the dwarf shoot, a. Each year 

 the mycelium advances in opposite 

 directions, in spite of the fact that 

 a layer of cork, b l>, is formed at 

 the beginning of each growing 

 season along the boundary of the 

 living tissue. In the immediately 

 preceding year a very small quan- 

 tity of wood had been formed. 



broad layer of cork is formed for the protection of the tree 

 along the boundary between the sound and diseased tissues. 

 These layers of cork (Fig. 59, b b) which form between the dead 

 and living tissues induce external rupturing of the cortex 

 at points along the boundary of the canker-spot (Fig. 60), 

 the result being that turpentine flows from the interior of 

 the tree. Year by year the canker-spot enlarges along its 

 whole periphery, rather more rapidly, however, longitudinally 

 than horizontally, and it is probably the vital activity of the 



