268 



Mycologia 



stages in their dissolution are readily found in radial section. 

 They are represented in plate 13, figure 3. The disintegration 

 takes place in a more or less centrifugal direction but evidence of 

 dissolution in the outer ring of the border is evident before the 

 more central parts are entirely destroyed. 



After entering the tracheids, growth takes place in a vertical 

 direction and so the mycelium rapidly reaches hitherto unattacked 

 medullary rays. At such points one to several hyphal branches 

 are given off to the medullary rays or, as seen in plate 13, figure 

 I, the main hypha may enter the ray and pass through and out 

 on the opposite side, following the same vertical direction as before 

 but in another line of vertical tracheids. In this manner it is not- 

 impossible to trace the hyphae for rather long distances in a 

 vertical direction. 



In the meantime the rays first attacked have been practically 

 all destroyed. Soon after the ray parenchyma cells disappear 

 the ray tracheids show evidences of disintegration. Their com- 

 plete dissolution usually occurs about the time the bordered pits 

 in the vertical tracheids of the near vicinity are being disin- 

 tegrated. In many cases, however, their lateral walls persist 

 much longer and apparently result in binding together a small 

 bundle of tracheids. These bundles may persist even after the 

 spring wood of a given annual ring has been destroyed so that 

 they project into the resulting cavity and are quite conspicuous, 

 as mentioned under the description of the macroscopic appear- 

 ance of the decayed wood and as shown in plate 12, figure 2 (left). 



When a fairly advanced stage of the decay is studied it is seen 

 that the medullary rays have practically entirely disappeared 

 and the vertical tracheids are in a state of collapse. Prior to this 

 time no free cellulose can be detected in these tracheids. In 

 later stages of decay, however, the reaction to the chlor-iodid-zinc 

 solution is distinctly positive and the remains of the tracheid wall 

 as seen in surface view is colored a uniform lavender-violet. 

 This, of course, means that the lignin has been removed from 

 •these walls. It probably does not indicate that cellulose destroy- 

 ing enzymes are not present later, else the wood would not even- 

 tually be entirely destroyed. On the other hand, it seems justifi- 

 able to conclude that the mycelium at practically every stage of 



