WOUNDS 



227 



periderm possesses sufficient power of cell-division to enable it 

 to keep pace with the increasing thickness of the stem. In the 

 case of a wound, however, its reproductive capacity is confined 

 to the development of a periderm close beneath the surface 

 of the exposed tissues. This layer of cork, which is also formed 

 along the boundary between the sound and dead tissues when 

 plant-parasites induce diseases of the cortex, is called " Wound 



FIG. 132. The formation of callus on the edge of a wound on an oak-branch, a, 

 periderm ; b, collenchyma ; c, outer cortex ; d, primary bundles of hard bast ; 

 e, cortical parenchyma ; f, soft bast ; g, cambium ; /i, wood ; z, " wound-cork ; ' 

 formed by the outer cortex ; k, callus. 



Cork" (Fig. 132, i]. Its formation does not depend on the 

 season of the year, for even in winter, should the weather be 

 favourable, it is formed soon after the occurrence of a wound. 



Only that portion of the cortical parenchyma which is situated 

 nearest to the cambium, or the soft bast, or in other cases merely 

 the deepest-lying and youngest organs of the soft bast, take part 

 in the reproductive processes that are about to be discussed. 



As wood consists for the most part of empty elements viz. 

 fibres, tracheids, and vessels it possesses only a very limited re- 

 productive capacity. The cells of the wood that retain vitality 

 consist of the parenchyma of the medullary rays and the wood- 



Q 2 



