6o8 



lia\c taken place in tlic early sprinj^ and lliat the riclil\- collected material 

 enabled the parts surrounding- the wound to form enormous excrescences 

 most quickly. The parenchymatous character of the new structures causes 

 a great sensitiveness to injurious atmospheric influences and especially to 

 frost. Low temperatures, therefore, are able to injure the canker tissue in 



the next period of growth. The injured 

 tissue complex can respond repeatedly with 

 excre^cenl tissue, because, with its paren- 

 ch}'niatous nature in the pre\ ious period of 

 .s';rowth. it has stored up \ery abundant re- 

 ser\c substances in the form of starch. 



The canker forms in the indixidual 

 genera of the Rosaceae differ onl\- in the 

 manner of reaction to the wound stimulus 

 and agree in that they prefer the bud and 

 its immediate surroundings as the place of 

 production. The reason for this may be 

 sought in the division of the trunk at tlie 

 place of insertif)n of a bud. The wood 

 ring is always more slender here and hnall\ 

 tra\ersed by a parenchymatous pith bridge. 

 The initial stages of the canker knot, 

 so far as observed, i. e., the small tears 

 usually arising near the buds, have been 

 produced by artificial frost, but not the 

 luxuriant overgrowth structures. This cir- 

 cumstance may possibly be traced back to 

 the fact that a period in the spring had 

 been chosen which was too late for the 

 action of the artiiicial frosts. 



In the health}- branches of cankered 

 trees an alinormallv increased formation ot 

 the mcdullar\' rays has often been obserxed, 

 and this nia\ indicate the explanation of 

 the tendency to canker excrescences of 

 certain cultural varieties, or diff'ercnt indi- 

 viduals in certain habitats, since those ex- 

 amples will answer most easily to a wound 

 stimulus by hypertrophy, if their medu! 

 lary. or rather bark rays, grow luxuriantly 

 in a healthy condition. 



El.I<;ilT (."^I'liACELUS). 



In contrast to the term "canker" which in general [)ractice is used for 

 die heterogeneous phenomena of a gradually extending disease, one under- 

 stands pretty generally by the term "Blight" the occurrence of dead, black- 



J 



