594 



abundant hii^h up in the crown and in shady phiccs. i. c. those less exposed 

 to spring frosts. 



The especial susceptibility to frost of the base of the branch is ex- 

 plained by the fact that, on account of the greater number of buds originally 

 set there, more parenchymatous medullary bridges are present, which 

 traverse the wood ring. Tlie itarcnch}mat()us wood is more tender and 

 contains more starch. 'J"o this shcjuld also be ascribed the fact that bark 

 beetles like to settle in the crotches and that wood mice, as Xordlinger 

 states, fre(|uently eat only the base of the lateral branches in poplar suckers 

 [ropulns monilifera). Therefore the frost, i. e., the s])ring frost kills the 

 base of the branch most easily. 



In old. weakly growing trunks, the luxuriance of the o\ergrowth 

 edges decreases considerably and can become so slight that only narrow, 

 circular overgrowth edges are present, which push out slowly from under 

 the dead bark. This blight corresponds to that of the crotch injurv, since 

 in open canker, the first stage is not a cleft but a c()llai)sing, drying dead 

 bark surface. Hence, the expression "crotch hlujht" fre(|uently used by 

 many practical workers. 



c. Canki:r on Cherry Treks. 



In sweet cherries are usually found semi-cylindrical protuberances on 

 the twigs, or older branches. The outside of these swellings, often thicker 

 than one's fist, not infrequently seem depressed, as in blight ; the dead 

 bark is split and partially stripped from the blackened wood body, still 

 remaining attached as larger scales with up-rolled edges (cf. Fig. 139). 



The barrel-shaped sw'elling on the branch represents an abnormal de- 

 velopment of the overgrowth edges {n and n' ) of the wound (sp) which 

 does not close entirely, as is also found in the "closed canker of the ai^ple." 

 In the latter, however, the overgrowth tissue is a sudden, unusually lux- 

 uriant widening of the annual ring, while, in the cherry, the swelling of the 

 normal side of the twig shows a gradual transition to the excrescent oxer- 

 growth edge. On this account, the closed canker of the a[)])lc has the form 

 of knots but the completely developed canker of the cherry a gradually 

 increasing barrel-shaped thickening. Besides this typical form, various 

 transitions are found from the closed canker knots, on the one hand, to 

 the flat wound, on the other, which is termed blight. 



Conical swellings are found at the base of older branches of trees, 

 suffering from canker, which can offer all the transitional forms up to the 

 typical canker swelling. The initial stages are found on one side of the 

 branch in the form of a small frost wound alongside the first annual ring. 

 An especial emphasis should be laid here on the fact that the enormous 

 overgrowth tissue seems often to be developed from a medullary bridge. 

 This, therefore, points to some direct injury to the bud. The development 

 of the overgrowth edges is continued in subsequent years, when only paren- 



