WOUNDS 



255 



along the upper and lower edges. The upper edge, however, 

 is greatly favoured as compared with the lower edge, because 

 the plastic substances during their passage down the stem are 

 conveyed directly to the former, whereas the latter lies out of the 

 stream as it were, and is but sparingly supplied with nutriment 

 (Fig. 147). 



There is, however, a much more important reason for the 



FIG. 146. A fractured oak-branch. The wound is being gradually occluded by a 

 callus which is slowly advancing beneath and pushing off the thick cortex. At 

 a the new growth shows curls, while at b it pushes its thin non-vascular edge 

 forward regularly over the dead wood. The dead wood is represented at c, 

 One fourth natural size. 



slow formation of callus on the lower part of a wound. In 

 that region the cortex is, as a rule, loosened from the wood 

 during the operation of pruning. At the time when the 

 cambium is active, it is quite impossible to prevent the cortex 

 being loosened, the friction of the saw being sufficient to 

 account for it. But the main cause is to be traced tp the fact 

 that, in order to prevent the cortex being torn off, a cut is first 

 of all made underneath, and during the sinking of the branch 

 the lower edge of the wound is subjected to severe pressure. 



