IX] HEALING OF WOUNDS BY OCCLUSION. 225 



of decomposed organic matter, it becomes a suitable 

 soil for the development of fungi which are not 

 parasitic — i.e. which cannot live on and in the normal 

 and living parts of the tree — but which can and do 

 thrive on partially decomposed wood. The spores of 

 such fungi are particularly abundant, and many of the 

 holes found in trees are due to their action. The 

 hyphae follow up the poisonous action of the juices 

 referred to above, living on the dead tissues ; and it 

 will be intelligible that the drainage from the pro- 

 ducts aids the poisonous action as it soaks into the 

 trunk. It is quite a common event to see a short 

 stump, projecting from the trunk of a beech, for 

 instance, the edges of the stump neatly rounded over 

 by the action of a callus which was unable to close up 

 in the middle, and to find that the hollow extends 

 from the stump into the heart of the trunk for several 

 feet or even yards. The hollow is lined by the 

 decayed humus-like remains of the timber, caused 

 by the action of such saprophytes as I have referred 

 to. Similar phenomena occur in wounded or broken 

 roots, and need not be described at length after 

 what has been stated. 



But, in addition to such decay as this, it is found 

 that if the spores of true wound-parasites alight on 

 the damp surface of the cut or broken branch, their 



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