INJURIES INDUCED BY PLANTS 



173 



stones, &c. remains for the present undetermined. Parts of 

 the stem older than 2,0 25 years appear to be incapable of 

 receiving infection. The mycelium of the fungus spreads by 

 intercellular growth amongst the cells of the cortex and of the 

 bast, from which it proceeds, by way of the medullary rays, into 

 the wood to the depth of about four inches. 



Wherever the mycelium obtains access, the starch-grains and 

 other cell-contents disappear, their place being taken by drops 

 of oil of turpentine, which form on the inside of the walls, or 

 saturate the wall-substance itself. The cells are, of course, 

 killed, death however being unaccompanied by browning of the 

 tissues. The whole stem, to a depth of some three or four inches, 

 is completely saturated with resin, a section of wood, as much as 

 one to two inches in thickness, being more or less translucent. 

 As the mycelium penetrates the resin-ducts as well, killing the 

 surrounding tissues, there is no doubt that a portion of the 

 turpentine finds its way down from parts of the stem situated 

 at a higher elevation. The 

 assumption that direct con- 

 version to turpentine of the 

 cell-contents and of the wall- 

 substance of the parenchy- 

 matous cells also takes place 

 is, however, justified by the 

 complete resinous saturation, 

 and by a frequent volumin- 

 ous outpouring of turpentine 

 from the cortex, which de- 

 taches itself from the tree 

 after death. 



Each year the mycelium 

 spreads from the diseased 

 part into adjoining tissues, 

 the rate of progress being usually somewhat more rapid longi- 

 tudinally than horizontally. In proportion as the mycelium 

 spreads, so is the passage of the plastic materials confined to 

 the sound side of the tree, in consequence of which the cam- 

 bium in that region is stimulated to such a degree of activity as 

 to produce exceptionally broad annual rings. Fig. 103 exhibits 

 the cross section of a stem which, when fifteen years old, was 



FIG. 103. Transverse section from the 

 upper part of the stem of a pine which, 

 seventy years previously, had been in- 

 fected at a by P. Pini corticola. The 

 crown of the tree died in the year im- 

 mediately preceding that in which the 

 section was removed, and at that time 

 the only portion of alburnum that was 

 not saturated with resin or attacked by 

 the fungus was the portion marked b. 

 The portion of the wood saturated with 

 resin is shaded in the figure. One tenth 

 natural size. 



