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tical jacket, which, before the wound was made, was tightly 

 stretched and exerted considerable pressure on the tissues 

 below it. The living cells of the cortex and the cambium at 

 the edges of the wound at once protect themselves with a 

 layer of cork and then rapidly develop a callus as above 

 described, which pushes out towards the surface of the 

 wound, i.e. in the direction where there is the least 

 resistance, and thus forms a thick pad-like rim around the 

 wound. A layer of cells in this callus, continuous with the 

 cambium of the stem, retains its embryonic character and acts 

 as a cambium, .the tissue formed on the inside of the layer, 

 abutting on the old wood, becoming differentiated into wood 

 and that outside the cambial layer developing into cortical 

 tissue. This wood formed in the callus is called wound-wood 

 and, owing to the diminished pressure and abnormal conditions 

 under which it is developed, it usually differs considerably 

 from normal wood in its structure. The cortex of the 

 callus usually remains thin and exerts far less pressure on 

 the tissues beneath it than old cortex would do. These 

 callus cushions may consequently continue to grow very 

 rapidly for several years, and the whole surface of the wound 

 may thus become quickly and completely covered over, 

 owing to the various cushions coming in contact with 

 one another and coalescing. In many cases no thick, outer 

 bark consisting of layers of dead elements has been 

 formed on the callus pads when thev thus come in contact, 

 and, as they continue to grow and press against one another, 

 the thin cork layers lying between them are ruptured and 

 squeezed out, while the cambium and living cortical tissues 

 of the adjacent lips come into direct contact and grow together. 

 The cambium of the various cushions thus uniting, a con- 

 tinuous cambial layer is formed over the whole surface of the 

 wound, and then further growth in thickness continues in the 

 normal way as if nothing had occurred, and very soon no 

 external sign of the injury can be seen. If thick layers of dead 

 bark have been formed on the callus when the various cushions 

 come into contact, the complete coalescence of the latter is 

 rendered much more difficult and may be much delayed. In 

 some species the living cells of the medullary rays on the sur- 

 face of the exposed wood also take part in this healing process 

 and form callus pads, in addition to those developed at the 

 margins* of the wound. A wound which is healed in the man- 

 ner described is said to have been occluded, and this healing 

 process is termed occlusion. 



