ARTIFICIAL WOUNDS. 197 



ment of a continuous layer of xylem and phloem, 

 no further trace of the injury is observable, unless 

 a speck of dead cells remains buried beneath 

 the new wood, and indicates the line where the 

 knife point killed the former cambium and 

 scored the surface of the wood in making the 

 wound. 



Stripping. Now suppose that, instead of a 

 mere slit with the knife-point, a strip of bark is 

 removed down to the wood. Exactly the same 

 processes of corking and lip-like callus formation 

 at the edges of the wound occur, but of course the 

 occlusion of the bared wood-surface by the meet- 

 ing of the lips occupies a longer time. Moreover, 

 the living cells of the medullary rays exposed by 

 the wound on the wood-surface also grow out 

 under the released pressure, and form protruding 

 callus pads on their own account. In course of 

 time the wood is again completely covered by 

 the coming together over its face of these various 

 strips of callus, but two important points of differ- 

 ence are found, as contrasted with the simpler 

 healing of the slit-wound. In the first place the 

 exposed wood dries and turns brown, or it may 

 even begin to decay if moisture and putrefactive 

 organisms act on it while exposed to the air ; and. 

 in the second place, the normal annual layer of 

 wood or layers, as the case may be formed 

 by the cambium only extends over that part of 

 the stem where the cambium is still intact, and is 

 entirely wanting over the exposed area. Thus, if 

 it takes two years . for the cambium to extend 



