INJURIES INDUCED BY PLANTS 



193 



disease not usually appearing upon trees younger than fifty 

 years. 



The mycelium shows a preference for growing longitudinally in 

 the stem, while its distribution horizontally is most pronounced 

 in some particular annual ring. On this 

 account decay often takes the form of ring- 

 shake that is to say, it is most pronounced 

 in peripheral zones which encircle a part or 

 the whole of the stem. At first the wood 

 becomes somewhat deeper red brown in 

 colour, and then white blotches or holes 

 appear here and there. In the case of 

 the pine especially, these are largely con- 

 fined to the spring wood of some par- 

 ticular annual ring, in which they enlarge 

 parallel to the longitudinal axis of the 

 stem, the result being that the resinous 

 zone of autumn wood may persist alone 

 for a long time, until it also is destroyed 

 by decomposition. 



A resinous zone is formed along the 

 boundary between the alburnum and the 

 decomposed wood, and this interferes with 

 the outward progress of the mycelium of 

 the fungus. In those specimens which I 

 have examined, this zone is absent only in 

 the comparatively non-resinous silver fir 

 and in spruce-branches, and thus, in their 

 case, the fungus is able to reach and pene- 

 trate the cortex with ease. The action of 

 the ferment of the parasite produces white 

 spots, similar to those that have been de- 

 scribed in the case of T. radiciperda. The 

 lignin is extracted from the cell-walls, and 

 pure cellulose is left. The middle lamella 



is completely dissolved as soon as the lignin disappears, and 

 thus the tracheids become isolated before being finally dissolved 

 (Fig. 123, a a to b\ The lamella which is nearest to and bounds, 

 the lumen persists the longest, and before it is dissolved the 



O 



FIG. 123. A tracheicl 

 of P. sylvestris which 

 has been decomposed 

 by T. Pint. The prim- 

 ary cell-wall has been 

 completely dissolved as 

 far as a a. In the lower 

 portion of the figure the 

 secondary and tertiary 

 walls consist of cellu- 

 lose alone, in which 

 granules of lime are 

 distinctly recognizable, 

 b ; filaments penetrate 

 the walls and leave 

 holes behind, d, e. 



