WOUNDS 235 



between the sound and diseased wood. One is inclined 

 to assume that when the cell-walls arc destroyed by the 

 mycelium of the fungus the turpentine in the interstices of the 

 micellae is again liberated and becomes volatile, and so 

 penetrates such cell-walls as are either wholly or for the most 

 part free from decomposition. As a matter of fact, those parts 

 of the wood which are the last to be attacked by the parasite 

 become completely saturated with resin, whereas mere traces of 

 resin are to be found in the decomposed portions. Thus, when 

 the alburnum has been destroyed, the duramen of old pine-stools 

 is very resinous. So far there is no proof to support the view 

 that the cell-walls are converted into resin during the decom- 

 position of the wood. 



When wounds due to pruning, barking, &c., expose the wood 

 of a dicotyledonous tree, the tree protects itself against the 

 unfavourable influences of the environment in two ways. In 

 the first place, the vessels become completely plugged up 

 by tyloses,* which both prevents the entrance of rain-water 

 and the evaporation of any water that may be present in 

 these organs. In the second place, gums are formed in abun- 

 dance in the neighbourhood of the wounded surface, and these 

 fill up and close the lumina of the organs, especially the vessels, 

 thereby protecting them to a certain extent against the 

 prejudicial influences of the environment. It is probably to 

 the direct action of the oxygen of the air that the brownness 

 of the wood under the surface of the wound is due, tannin and 

 its allies especially assuming a brown colour in the higher 

 stages of oxidation. 



The foregoing protective agencies are, however, insufficient to 

 afford absolute security to the exposed wood against decompo- 

 sition and decay. On this account wound-diseases are much 

 more liable to occur in dicotyledonous trees than in the resinous 

 conifers. 



In the previous section attention has already been directed to 

 wound-diseases due to parasites, and I shall again refer to this 

 subject when dealing with the pruning of trees. But besides 



* [Xyloses are ingrowths of the cells surrounding a vessel, which push 

 their way through the bordered pits into the cavity, and may there divide and 

 grow further. ED.] 



