28 DISEASES OF CULTIVATED PLANTS 



drops off the wound is already healed and closed against 

 infection by fungus spores, etc. In some instances a deposit 

 of gum at first spreads over the wound, and the production 

 of periderm follows by degrees. Such wounds are natural, 

 and do not expose the tree to those dangers that follow 

 artificial wounds, over which the tree has no control, and for 

 which it was not prepared. 



Amongst trees that annually shed some of their twigs are, 

 oaks, poplars, willows, elms, horse-chestnuts (Catalpa, Ailan- 

 thus], etc. 



Gager, Journ. N. Y. Bot. Garden, 8, p. 252 (1907). 



Pruning. My only object in mentioning this subject is to 

 indicate the great risk of injury following an improper method 

 of pruning. If a branch is properly removed the wound is 

 in course of time protected by a callus, but before this protec- 

 tion is completed, even under the most favourable conditions, 

 the spores of fungi may alight and germinate on the wounded 

 surface, enter into the tissues, and start a disease which can 

 never be eradicated. On the other hand, if a branch is 

 improperly removed, a callus may form only in part or not at 

 all, and the danger indicated becomes almost a certainty. 



The advice of those best able to speak with authority is : 

 prune trees as little as possible. 



Hartig says : ' The rate at which a wound is occluded 

 [ = protected by a callus] 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 dicotyledonous trees, 

 especially the oak, branches of a greater diameter than four 

 inches should not be removed.' 



Schlich says : ' As a general rule plants should not be 

 pruned unless it is absolutely necessary. Every cut produces 

 a wound, exposing the plant to disease, which may ultimately 

 render it unfit for the purpose for which it has been grown. 

 Recent researches have shown that the unhealthy condition 

 of timber trees may be due to the spores of fungi entering the 

 tissues through wounds received at a very early age.' 



