586 

 Canker (Carcinoma). 



As "canker," I consider those wounds which develop their overgrowth 

 edges into excessive wood swellings. The character of the excrescence lies 

 in the exclusive, or predominant fonnation of parenchyma wood instead 

 of the normal prosenchymatous wood elements. The canker excrescences 

 have a typical form for each tree variety. 







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Fig-. 132. Overgrowing frost split in apple lii-anch, produced by artificial cold. 



a. Canker of the Apple Tree. 



The canker of the apple tree occurs in two forms, of which the more 

 common one is distinguished by a broad, central exposed wood surface 

 formed from the open, protruding, blackened wood body and is surrounded 

 l)y roll-like, strong calluses, developing outwardly each year like terraces. 

 At the centre of the wound is found frequently the remainder of a small 

 stump of a branch. This is indicated in Fig.. 133 by :;, while the nearest 

 overgrowth edge is indicated by «'. ^\'e see how the wound surface gradu- 

 ally increases since the first formed, still rather flat edge dies and turns 



