206 



BULLETIN 193. 



the position, and solvent action, of the mycelium. This becomes 

 more pronounced if the wood dries, if the tree is cut or if blocks 



of the wood are cut from the trunk 

 and allowed to dry. In the early 

 infection of the wood, and the 

 early stages in the advance of the 

 mycelium, before the larger strands 

 have become established, the open- 

 ings made by the mycelium are 

 very minute. They take the same 

 directions as the larger openings. 

 In fact the mycelium, instead of 

 making a general attack upon the 

 tissues, begins its advance at cer- 

 tain regularly separated points, 

 and then extends along in parallel 

 lines. If the wood is cut out at 

 this early stage, one can see these 

 minute perforations thickly scat- 

 tered over the exposed surface. 

 Sometimes, even in this early 

 stage, shrinkage of the wood will 

 have taken place. If not very 

 marked when freshly cut from the 

 wood, the shrinkage of the wood 

 on drying, marks it off in a beauti- 

 ful manner, by fine lines and holes 

 into cuboidal areas. 



In many of these cases of the 

 heart rot of trees, after the heart 

 wood is well affected, the mycelium 

 being well established and vigor- 

 ous, gradually encroaches on the 

 cambium or living area beneath 

 the bark. In this way, many of 

 the branches in the top of the tree 



62. Effect on wood of red 

 spruce by the mycelium of 

 Polyporus borealis. 



die, and in some cases later the cambium of the trunk may be 

 so destroyed as to kill the tree outright. 



