Growth 



329 



response or adjustment bringing with it most important 

 sanitary advantages. A wound long exposed is an almost 

 certain beginning of a heart-rot of some type. The 

 development of tissue covering the wound proceeds from 

 the cambium. A callus or cushion of vigorous meriste- 

 matic tissue is produced, and this is extended from all 

 sides, until the wound is completely closed, when new 

 woo^l-rings are laid down over it (Fig. 84). 



193. Budding and grafting. - The growth processes 

 immediately involved in budding and grafting are well 

 understood, but all of the relations of stock and scion are 

 not so clearly denned. In both budding and grafting, 

 the important principle is to unite the cambium of stock 

 and scion. When held firmly in contact by grafting-wax 

 or raffia, the meristems of the two individuals thus united 

 develop a callus, effecting a 

 close union ; and wood is 

 subsequently laid down 

 from each contributingpart, 

 cementing this union com- 

 pletely. In general, a 

 union so close as to insure 

 the life of the scion is only 

 possible when the plants 

 are closely related. If 

 stock and scion grow r at a 

 different rate, that is, if the 

 seasonal rings of one are 

 thicker than those of the 

 other, there will be a con- 



r IG. 85. Grafting : cambium of stock 

 Slderable difference in Size and of scion (on one side) in contact. 



