ARTIFICIAL WOUNDS. 



195 



duce a layer of cork, the impervious walls of which 

 again protect the living cells beneath. This is 

 found to occur in all cell-tissues provided the cells 

 are still living, and it matters not whether the wound 

 occurs in the mesophyll of a leaf, the storage paren- 

 chyma of a Potato-tuber, the cortex of a root or 

 stem, or in the fleshy parts of a young fruit, the 

 normal effect of the wound is in all cases to call forth 

 an elongation of the uninjured cells beneath, in a 

 direction at right angles to the plane of the injured 

 surface, which cells then divide by successive walls 

 across their axis of growth : the layers of cells thus 

 cut off are then converted into cork, by the suberisa- 

 tion of their walls. Further changes may then 

 go on beneath the protective layer of wound-cork 

 thus produced, and these changes vary according 

 to the nature of the cells beneath : the cambium 

 forms new w^ood, the medullary rays similar rays, 

 cortex new cortex, and so on. 



Knife-wounds. Artificial cuts in stems are 

 easily recognised and soon heal up unless dis- 

 turbed. Several cases, differing in complexity, 

 are to be distinguished. The simplest is that of 

 a longitudinal, oblique, or horizontal short cut in 

 which the point of the knife severs all the tissues 

 of the stem down to the wood. The first effect 

 usually observed is that the wound gapes, especially 

 if longitudinal, because the cortex, tightly stretched 

 on the wood cylinder, contracts elastically. This 

 exposes the living cortex, phloem and cambium 

 to the air, and such tissues at once behave as 

 already described above : the cells actually cut die. 



