214 TIMBER AND SOME OF ITS DISEASES. [CHAP. 



the. cut surface is relieved from the compressing 

 influence of the cortex and bark, and owing to this 

 release of pressure it begins to grow out at the edges 

 into a cushion or "callus," as shown in Figs. 29 and 30. 

 A very similar " callus" is formed in the operation of 



1867-78 \ 



FIG. 28. The same in longitudinal section. P, the pith of stem and branch ; on 

 either side of this are the twelve annual z^nes of wood produced during the years 

 1867-78, as marked. The cambium, C, separates these from the cortex, B. 



multiplying plants by " cuttings," so well known to all : 

 the cambium at the cut surface of the " slip" or 

 "cutting" is relieved from the pressure of the cortex, 

 and begins to grow out more rapidly in the directions 

 of less pressure, and forms the callus. 



