DISEASES OF TREES LIKELY TO FOLLOW IX.IlIMEs. 147 



case of largo woiiuds the callus continues to increase and overlap 

 more and more the old exposed wood, but its activitj^ diminishes 

 from year to year. As soon as the callus ring has begun to form, 

 its outer cells undergo the cork-trausformation, and thus the delicate 

 cambium cells are soon covered with a protective bark similar ta 

 the normal bark of the stem, and, as this bark increases in thick- 

 ness, it exerts an increasing pressure on the cambium cells 

 beneath, which sufficiently explains why the, at first, luxuriant 

 production of new camlnum cells gradually diminishes. The 

 function of the cambium in the normal trunk, you will recollect, is 

 to produce new wood cells on its inner surface and new bast cells 

 on its outer surface, and the same function is retained when it 

 grows into a callus. "We find, therefore, that in the callus itself 

 new layers of wood are formed and overlap the old wood, and, if 

 the process goes on long enough, it happens that the old wood is 

 entirely covered by new layers of wood and a cortex somewhat 

 similar to that of the uninjured trunk. 



So far, we have supposed that we were dealing with a wound 

 made by cutting directly across a branch. What is true in this 

 case is essentiallj' true of other wounds, and we cannot noAv stop 

 to consider in detail the innumerable modifications depending on 

 the form of the wound. As a matter of fact it more frequently 

 happens, as when branches are broken by the wind or snow or by 

 external violence of any kind, that the wound is irregular or 

 splintered, and in such cases the cortex is often torn awaj' from 

 the branch below the wound and the cambium is crushed or injured. 

 Consequently the healing process is very much hindered. Again, 

 when ti'ees grow thickly together, or for other reasons, the lower 

 branches often die and break off at a certain distance from the 

 main trunk. In such cases the stumps of the branches ver}- often 

 die and remain projecting as dead plugs or pegs. This is in part 

 owing to the disturbed nutrition of the stumps, a subject toa 

 complicated to be described here. The fact is evident, however, 

 that such pegs do not heal over but rot away, and must be consid- 

 ered open wounds. 



Up to this point I have dwelt upon the nature of wounds and 

 the healing process adopted by nature, and you will now ask. Why 

 are all these elaborate changes necessary? What is the harm if a 

 wound does not heal over? As a rule it is safe to sa}^ that the 

 provisions of nature are alwaj^s adapted to some special end. and 



