1 62 THE PHYSIOLOGY OF PLANTS 



ing of the large surface which is exposed by the wound, but 

 it is by no means impossible. 



The process of healing, however, is different from that of 

 all the other operations which we have so far considered, in all 

 of which the exposed tissues became covered in by an over- 

 growth from the margins of the wound. But in the case of a 

 wound the upper margin of which may sometimes be a yard 

 away from the lower one, it is evident that the margins will 

 never be able to coalesce. Yet this large area, if it does 

 heal, will heal more rapidly than that exposed by ringing, 

 in which operation the knife removes the youngest layers of 

 wood. 



In peeling the stem, however, it is essential to leave the 

 new wood intact, for it is from this layer that the healing 

 tissues are formed (see Fig. 23). How very important it is 

 not to damage these layers of splint-wood we can see from 

 the appearance of those regions where the knife has made 

 longitudinal incisions to enable the bark to be lifted. The 

 operation, which can therefore only be executed when the 

 cambial activity is very great, and when a very delicate layer 

 of cells exists between the bast and the wood, is commenced 

 by making one or (in the case of thick stems) several longi- 

 tudinal incisions reaching down to the wood. These incisions 

 reach from an upper circular cut down to the point where the 

 lower cut is made. Where the circular and the longitudinal 

 cuts meet, we begin to lift the bark and then peel it off from 

 the underlying wood. If it comes off easily, the exposed wood 

 is smooth and moist. 



If it is smooth and free from adhering portions of fibrous 

 bast, there is a prospect of the operation succeeding. If the 

 surface is not clean, the healing is incomplete, or may not take 

 place at all. 



A few days will be sufficient to ascertain whether the tree 

 will live or die. If the latter is going to happen, the wound 

 turns grey and shows black lines. These are due to the de- 

 velopment of young fungal colonies, which attack and destroy 

 the woody tissues. If, however, the healing process is success- 

 fully established, the wound assumes a yellowish-green colour, 

 and in a few weeks we can ascertain by pressing with the 



