6o2 



in the form of spherical ((roups witli a shcath-like arraiiijt'ment of their 

 elements. The groups not infrequently unite and in this way cause a 

 greater firmness but no complete wood ring has ever been observed. It is 

 these isolated parenchyma and duct groups which in ])runing so greatly 

 resist the knife, that they are torn loose from their connection with the 

 other tissue before lieing cut through. Hence the easy crumbling of the 

 canker knot. 



Fig. 142. Rose Canker. Concentric overgrowth edges may be recognized, ri 

 like terraces around a central, dead wood surface. 



f. Canker of the Rose. 



Tn the culture of the newer climbing roses, which (according to 

 Crepin-Briissel) have resulted from a crossing of Rosa Indica with R. 

 multiflora and are called Polyanthus varieties, we have become acquainted 

 with a phenomenon which comes under the head of canker excrescences. 

 The adjoining Fig. 142 A and B, represents such canker swellings as are 



