WOUNDS 



257 



formed after the cortex has been separated from the wood by 

 the advancing callus. It is in fact the Achilles heel of the 

 branch-wound. In pruning, the main object must be to prevent 

 its formation, but this is possible only if pruning be confined 

 to autumn and winter, when growth is at a stand-still, and when 

 the cortex is least liable to be detached from the wood. If 

 one also takes the precaution to support the branch during 

 sawing, and at the moment of separation to push it clear of the 

 wound, danger is reduced to the minimum. 



The rate at which a wound is occluded depends entirely upon 

 the vigour of the tree and the size of the wound. A callus 

 forms on young trees, with their relatively broad annual rings, 

 faster than upon old trees, and the faster too the higher on 

 the stem the wound is situated, because with few exceptions 

 the breadth of the rings increases as we ascend. It is equally 

 apparent that occlusion will be accomplished sooner where the 

 situation is good than where bad. In the case of dicoty- 

 ledonous trees, especially the oak, to which my investigations 

 have hitherto been confined, branches of a greater diameter than 

 4 5 inches should not be removed. 



The effects of pruning as regards the health of the tree 

 depend chiefly upon the period of the year in which the opera- 

 tion is performed. So far as my observations go, it is always 

 highly dangerous to prune the spruce during summer, as 

 rapidly advancing wound-rot is an almost invariable con- 

 sequence. It may be mentioned, however, that in all the cases 

 which I examined, the cortex had been injured during the 

 process of pruning. This may be avoided by pruning during 

 autumn or winter, and as the cut surface becomes immedi- 

 ately covered with a resinous exudation the wound is almost 

 certainly safe from rot. It is only in the case of the older 

 branches, where the heart-wood emits no turpentine, that parasitic 

 infection is liable to occur. It thus appears to me that conifers 

 may be pruned in autumn and winter, if the wounds caused by 

 the removal of the larger branches be coated with tar, but 

 since in the case of these trees the branches are generally small 

 this will seldom be necessary. 



When the wounded surface of a dicotyledonous tree has not 

 been tarred, one first observes a brown colour penetrating to a 



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