6oy 



preferred place of production. The bark is split and partially thrown back 

 like wings. 



With an abundant appearance of the canker swellings, first of all, the 

 foliage turns yellow, then the stem begins to die back slowly from the 

 browned eyes. By July, as a rule, the diseased branches on the same 

 shoot, side by side with bright, perfectly green ones, have died back entirely. 



If healthy plants are examined for such cankered stems, either small 

 reddish, or brown, long ridges are found, or gaping tears often one centi- 

 metre long. I observed the same phenomenon also on many petioles. The 

 sloping edges of such tears are covered also with cork. On these edges, 

 small beady excrescences appear in places which consist of parenchyma and 

 are formed from the primary bark close to the outside of the hard bast 

 cords. 



In the Rosaceae this tissue region proved to be extremely easily stimu- 

 lated. I found that, after very different injuries to the bark, which gener- 

 ally did not extend to the hard bast, strong branches responded to the 

 wound stimulus by a parenchymatous increase close outside the hard bast 

 cords. Often, in the canker of the blackberry, a place of predisposition for 

 the formation of canker may he noticed, for. in the spots where a wart-like 

 excrescence had appeared, even in young branch shoots, the mechanical 

 rings formed from the hard bast cords and other thick-w^alled connective 

 elements are proved to be unthickened. A thin-walled parenchyma had 

 appeared instead of the prosenchymatous and sclerenchymatous tissues. 



The parenchymatous, excrescent tissue in the primary bark increases 

 very rapidly and ruptures the overlying normal bark layers. In the interior 

 of the canker wart, a porous wood body is formed which is rich in ducts. 

 The formation of wood elements is repeated in the peripheral parenchyma 

 layers of the excrescence zone first produced since meristematic aggrega- 

 tions arise from which develop tracheal wood elements, arranged like bowls 

 or shells. 



The beginning of canker in the blackberry therefore is a parenchy- 

 matous excrescence in the primary bark body which grows outward, with 

 a cauliflower-like ramification. Only later does the tendency to hyper- 

 trophy extend backward into the inner bark, finally attacking also the wood 

 ring which, at first, seems to have a normal structure. As soon as the 

 swellings become older and the wood body participates in their formation, 

 it increases to 3 or 4 times its normal size. \Ve find similar processes in 

 dropsy, in the formation of tuber-gnarl. etc. The canker is more rare in 

 Rubus ; as yet I have found it only in four cases and always in narrowly 

 restricted places. 



C0RRE.SP0X])ING FEATURES IN CaNKER SweLLINGS. 



In a survey of all the known material relating to closed canker corre- 

 sponding features are found. ("Open canker" forms a transition to blight 

 and is included here). The production of a small tear forms universally 

 the beginning of the disease. It may be seen in all cases that the injury must 



