WOUNDS 



229 



cambium of the medullary rays, so that the clothing of the surface 

 of the wound is almost exclusively undertaken by the latter, 

 giving the impression that the medullary rays have grown out of 

 the wood. The healing tissue, which is originally homogeneous, 

 soon shows a certain amount of differentiation. The elements 

 which abut upon the old wood change into wood-cells, while 

 towards the outside a new bast region forms amongst the layers 

 of cells that are assuming the form of parenchymatous cortical 



11 78 75 



FIG. 134. Cross-section of the stem of an oak which, two years before being felled, 

 had ruptured at several places in the cortex in consequence of much-augmented 

 growth, x and y, two places where the cortex had ruptured ; a to f>, new in- 

 vesting layers formed by occlusion with their cortex, d ; c, callus ; e to e, 

 lower surface of the loosened cortex, the cambium of which has also produced 

 new growth. 



tissue. A portion of tissue between the wood and bast preserves 

 the character of meristematic cambium, while a new epidermis 

 forms on the surface of the cortex. 



In the accompanying woodcut (Fig. 134), which represents the 

 cross-section of an oak whose bark became separated from 

 the stem two years before felling, the portion of the surface 

 of the wound situated between b and b has dried up. Beneath 

 the shelter of the loosened cortex, e e, on both sides of the wound 

 new healing tissues (a b) have been formed on the wood, and 

 these have already attained an age of two years (1876-77). 



