WOUNDS 231 



The process by which callus is formed begins in the soft bast, 

 and in the embryonic tissue along the edge of the wound 

 namely, the cambium (Fig. 132, g). It is a purely mechanical 

 process, and results from the bark-pressure on these tissues 

 being reduced. The annual growth in thickness of the stem 

 produces distension of the cortex and bast, which, however, 

 is balanced for the most part by the living cells of these tissues 

 dividing and growing, and so keeping pace with the increase in 

 the periphery of the stem, while the dead external portions 

 become fissured longitudinally. Nevertheless there is always 

 a certain amount of tension in the cortical mantle, whereby a 

 considerable pressure is exerted on the cambium. Should this 

 pressure on the cambium be locally reduced by a wound 

 reaching to the wood, the processes of cell-division and 

 growth are accelerated not only along the edges of the wound 

 but also at greater distances. In Fig. 132 this is visible as far as 

 g. Wherever the pressure has been reduced (in Fig. 1 34 this 

 may be perceived at' a distance of some inches from the points 

 a a), the normal cambium changes into "wound-cambium" 

 with short cells, which produces a luxuriant growth of " wound- 

 wood," destitute of vessels and without distinct medullary rays. 

 The process of cell-division proceeds most energetically in the 

 direction of the surface of the wound, where of course there is 

 absolutely no counter-pressure, and one may perceive the 

 cushion-like callus appearing between the wood and the cortex 

 Either in the year in which it originated, or not till later, the 

 wound-wood assumes a normal character, whereas the cortex of 

 the callus remains thinner and more expansive for a series of 

 years, and exerts less pressure than old cortex or bark. The 

 increased rate of growth is consequently not confined to the 

 first year, but is often maintained till the various callus-cushions 

 which advance from the edges of the wound come into contact 

 and coalesce. 



This coalescence is retarded, if not rendered absolutely 

 impossible, in the case of trees which at an early stage clothe 

 the callus with dead bark. 



Should the cortex of callus-growths that have come 

 into contact be thin, living, and free from dead bark, it is 

 squeezed out during further growth, so that cambium abuts upon 



