DISEASES OF TREES FROM MECHANICAL INJURIES. 159 



be so great as to more than fill the gap, and cause a slight protuberance on 

 the wounded side. Furthermore, when the tension of the cambium is 

 relieved by the removal of the cortex, its function of producing new wood 

 cells on its inner side is altered, and microscopic examination of the new 

 wood formed in wounds shows that the wood cells are shorter, and the ves- 

 sels decidedly less numerous than in normal wood. 



The description which I have given of the way in which the callus 

 arises, although you may perhaps think it somewhat complicated for a 

 popular lecture like the present, is, in reality, a brief attempt to sketch the 

 process in its main points only, omitting many details which are of inter- 

 est to specialists. What I have described is the normal mode in which the 

 healing process begins, and is to be seen in those seasons of the year when 

 the cambium cells are active. During the colder months of the year, how- 

 ever, the cambium is in a dormant condition, and if wounds occur at such 

 seasons, the cambium is not able to form a callus at once, and the process 

 . just described does not begin until the season of plant growth returns. 

 Meanwhile the exposed parts will probably have been more or less affected 

 by weathering, and the closing of the wound by natural processes is made 

 more difficult. In the case of large wounds the callus continues to increase 

 and overlap more and more the old exposed wood, but its activity dimin- 

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

 outer cells undergo the cork-transformation, and thus the delicate cambium 

 cells are soon covered with a protective bark similar to the normal bark of 

 the stem, and, as this bark increases in thickness, it exerts an increasing 

 pressure on the cambium cells beneath, which sufficiently explains why 

 the, at first, luxuriant production of new cambium cells gradually dimin- 

 ishes. The function of the cambium in the normal trunk, you will recol- 

 lect, 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 essentially 

 true of other wounds, and we can not now 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 away 

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

 Consequently the healing process is very much hindered. Again, when 

 trees 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 very often die and remain projecting as dead 

 plugs or pegs. This is in part owing to the disturbed nutrition of the 

 stumps, a subject too complicated to be described here. The fact is evident, 

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

 sidered 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 say that the provisions of nature are 



