595 



chyma wood is ff)rmed in which starch is rapidly and abundantly deposited. 

 If the canker swelling has become considerably extensive, the branch dies, 

 as a rule, above this swelling; in this, stroma- forming fungi (usually from 

 the family of the Valseae) greatly co-operate. They appear in the form 

 of small warts. 



If the young branches (i to 2 years old) of trees sufifering from 

 canker are examined, blight-like places, often se\eral centimeters long, are 

 found, with lip-like overgrowths instead of individual buds, while, on the 

 [)arts of the branch above and below 

 these places, the buds have developed 

 to short shoots. It is evident from 

 this that the injury to the branch 

 must take place before the breaking 

 of the bud. 



Since, however, no injury of any 

 kind can be ascertained in the year in 

 which the liranch is formed, but will 

 lie found only in the following spring, 

 it must have arisen in the winter or 

 at the beginning of spring; the as- 

 sumption is. therefore, pertinent that 

 the bud, as it unfolds in sprouting, is 

 killed by the frost and that the accu- 

 mulated plastic material is now used 

 in the formation of the excrescent 

 edges of the wound. Since the tissue of 

 these overgrowth edges remains as 

 soft as the parenchyma and is almost 

 always found filled with starch, it is 

 clear that, in the following winter, its 

 edges succumb very easily to injury 

 from frost and new excrescences are 

 produced from the deeper lying zones 

 which remain healthy. A consider- 

 ation of the cross-section in Fig. 139 

 makes clear the whole process. This 

 shows that the clefting of the axis 



has begun at a short distance from the pith body (/;;) and in the 

 second annual ring. The third annual ring has already furnished 

 luxuriant overgrowth edges (/) which, in turn, split the following year 

 (sp'). These secondary clefts cause secondary overgrowth (/'). The 

 barrel shaped canker swelling, however, is formed chiefly by the ex- 

 crescent wound edges of the main cleft, which are radiatingly arranged 

 {k). Thus an annual ring inside the canker swelling is divided into 

 several rings, as in the closed canker of the apple. The bark body 



I'^S. 139. Cherry canker frost cleft 



with overgrowth edges in longitudinal 



view and cross-section. 



