86 



HEALING OF WOUNDS 



or by a cut into the truk that removes the tissues as far as the 

 xylem it will be noticed that the exposed cambium is stimulated 

 to form a mass of large cells, the callus, that gradually extend 

 over the wound if this is not too large. While the callus is still 

 in process of forming an outer protective layer of cork tissue is 

 developed and also a new cambium layer is formed on the inner 

 side of the callus which finally unites the old cambium, thus 

 continuing the cambium of the stem over the surface of the wound 

 (Fig. 64, A). In this way wounds are covered or healed and 

 the tissue exposed by the cut is gradually buried deeper by the 

 annual addition of new tissue derived from the cambium (Fig. 

 64, B, C). Grafting is made possible owing to the formation of 

 callus that unites the tissues of the scion and stock (Fig. 65). 

 While grafting is performed in a variety of ways the process 

 consists essentially in all cases of bringing the cambium and 

 cortex of the cutting or scion in contact with the corresponding 



FIG. 65. A common method of grafting: A, insertion of two scions into 

 cleft of stock at cambium region. B, wound protected with wax to prevent 

 drying out of tissues. After L. H. Bailey. 



region of the stock. The junction of the scion with the stock is 

 sealed with grafting wax to prevent decay and the drying out 

 of the exposed cells while the callus is joining together the corres- 

 ponding tissues in the two parts. When the wound is healed 

 only a slight ring or swelling indicates the point where the scion 

 was inserted so complete is the union of the vascular and cor- 

 tical regions. Strange to say, although the organic union is 

 complete, each part, with very rare exceptions, continues a sepa- 

 rate and distinct existence. For example, a scion from a plum 



