NATURE OF PLANTS 



101 



is made that extends across the cambium as in the cutting off of a 

 limb or by a cut into the trunk that removes the tissues as far as 

 the xylem it will be noticed that the exposed cambium is stimula- 

 ted to form a mass of delicate cells, the callus, that gradually ex- 

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

 forming its outer cells become changed to cork cells which join 

 onto the old cork while a new cambium layer is also developed on 

 the inner side of the callus, which unites with the old cambium, 

 thus continuing the cambium of the stem over the surface of the 

 wound (Fig. 65, .4). In this way wounds are covered or healed 



x cm 



X 



c c 



A. B. c 



FIG. 65. Section of a portion of a stem showing the healing of a wound 

 caused by the cutting off of a lateral branch: A, formation of the callus, cl, 

 owing to the renewed activity of the living cells exposed by the wound; c, 

 cortex; cm, cambium; x, region of the xylem. B, a similar stem three years 

 later c, cortex; cm, cambium; x', xylem added since the wound was made; 

 o, position of the cambium at the time the branch was cut off; x, original 

 xylem of the stem. C, trunk from which three branches have been removed, 

 showing the gradual covering of the wounds by new tissue. 



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

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

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

 callus that unites the tissues of the scion and stock (Fig. 66). 

 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 



