77^ 



more the characteristics of parenchyma wood, the less the pressure of the 

 bark girdle on the cambium at the time of ringing and the longer this loosen- 

 ing lasts. 



We have seen in canker wounds how this porous structure at the edge 

 of the wound causes more and more a new loosening of the bark, a new 

 excrescent production of porous tissue and the final exhaustion of the 

 branch, due to this production. 



Every overgrowth edge formed about open wounds on the trunk, there- 

 fore, begins with the formation of short-celled, wide-lumined wood elements 

 which, sharply bounded, lie against the normally exposed wood. The wood 

 elements also pass gradually over into a normal structure, according to the 

 increase of the overgrowth edges and, therefore, the stronger bark pressure. 

 Tf, finally, the overgrowth edges coalesce and the bark again becomes a 

 uniformly connected girdle around the trunk, or branch, the normal amount 

 of hark pressure again sets in and with it the normal direction of wood 

 cells and ducts. Every year normal wood is deposited above .the closed 

 wound. 



Scarification Wounds. 



The best example of the changes in tissues during the process of wound 

 healing is found in the cicatrization of scarification wounds. By the term 

 "scarification," as is well known, is understood the cutting through the bark, 

 lengthwise of the stem, down to the wood body, without the removal of any 

 substance. If the tree is slit in this way, the edges of the wounds pull apart 

 (Fig. 173). Naturally, the two edges of the wound are nearer at the end 

 of the incision (Fig. 173 a). The process of healing is completed most 

 rapidly there. Fig. 174 shows the cross section of a healed incision on a 

 sweet cherry tree, from the end of the wound, i. e. from the region marked 

 a. We see at h the old wood, which was cut at w, and shows that part of 

 its ducts and wood cells died because of the effect of the air. The cambial 

 zone (c) which at the time the incision was made lay above h has formed, 

 during the process of healing, new bark {nr) and new wood {nh). The 

 newly formed wood zone, however, does not resemble the normal wood 

 jjroduced beneath the uninjured bark either in position, or in structure. 



It forms one part which, projecting outwardly, is three-cornered in 

 shape, its highest point coming nearest the groove {s) formed by the previ- 

 ous incision. This three-cornered convexity is caused by the development 

 of parenchyma wood {hp) which exceeds that of the tissue lying farther at 

 the side. This production of wood was the first activity of the two edges of 

 the cambium which were separated by the incision {s). Here the bark 

 pressure was the weakest, the cell increase the greatest, but the elongation 

 the least. Only after the new bark, formed from the young, inner bark and 

 the cambial zone, has attained at j a greater power and greater resistance 

 because of the newly produced cork layer {k') does the bark pressure 

 gradually increase. Its influence on the cambial zone producing the wood is 

 stronger and the form of the wood elements gradually becomes more like the 



