6oi 



Similar formations are found in the canker of the (/rape. Only I have 

 found as yet that the disturbance, setting in at the beginning of the second 

 year, and corresponding to the holes (d), consists of a broader tangential 

 elevation, circular in form. It give the impression that, at the beginning 

 of the period of growth, the bark was raised from the wood body for a 

 considerable distance. My repeated experiments with artificial frost show 

 that this process can actually occur and, in fact, it is met with rather fre- 

 quently in various trees. As a result of this lifting of the bark, a tangential 

 hole is produced on the grapevine, usually at the place where, on Spirea, 

 the slender, radial cleft is found. The raised bark forms, first of all, wood 

 parenchyma and this soft wood l)ody passes over very gradually, in the 

 course of the following summer, into normal wood. Here, however, some 

 of the broad medullary rays are found aboxe the raised part which have 

 developed especially and at the end of the year project as delicate tissue 

 caps. 



In the grapevine, as in Spirea in canker formation, these are not neces- 

 sarily overgrowth edges, as is always the case in the canker of the apple; 

 in the former, tissue cushions of a w^ood body which has become parenchy- 

 matous develop to canker knots. These cushions at first appear uninjured 

 and are at any rate caused by some previous disturbance. This explains 

 the theory expressed by Blankenhorn, on the canker of the grapevine, viz.. 

 that the stoppage of plastic materials (for example, with too strong orun- 

 ing), can cause the canker excrescence. 



The formation of the canker excrescence often indicates some modi- 

 fication, inasmuch as the canker cushions, produced in the first year, are 

 partially killed by the frost. Then the central, most delicate part suffers and 

 represents a black, dried core. In the following spring only the edges 

 grow further, just as do overgrowth edges, and line the cleft, as is shown 

 in Fig. 141 B. It has been said that the parts of the edges of the growing 

 canker knot continue growing "after the manner" of overgrowth edges. 

 Actual overgrowth edges, spirally cur\ed, are found only rarely (as in the 

 canker of the grape). 



Fig. 141 B shows that the wood ring of the third year passes over 

 imperceptibly into the canker swellings. Therefore, the canker swelling 

 is actually a wood formation but this wood, because of the enormous rapid- 

 ity of the tissue formation, is a structure so soft and so similar to the 

 likewise excrescent bark tissue, which is dying back from the outer side, 

 that it is often difficult to determine the boundary between them. This 

 porous wood, which I have found so very soft only in the canker of the 

 rose, forms, on the dead swelling, the brown,, tinder-like ground mass, of 

 which we spoke at the beginning. The firmer, lighter colored parts are the 

 islands of thick-walled w^ood cells and ducts (Fig. 141 B, i) increasing in 

 breadth and thickness at the periphery. In canker knots of different sizes, 

 the groups of ducts (i) are sometimes found in the form of wedge-like 

 lamellae, becoming thicker toward the outside, sometimes (as in Fig. 14JB) 



