230 TIMBER AND SOME OF ITS DISEASES. [CHAP. 



the healing tissue. As soon as the injury extends all 

 round, the stem dies it is, in fact, ringed. It is also 

 interesting to note that the zones 4 and 5 (and the 

 same would be true of 6 when completed) are thicker 

 than they would have been normally : this is partly 

 due to release from pressure, and partly to a con- 

 centrated supply of nutritive materials, due to the 

 stimulating action of the fungus. 



Much confusion still exists between the various 

 kinds of " canker " : some of them undoubtedly are 

 due to frost or to the intense heat of direct insola- 

 tion ; these are, as a rule, capable of treatment 

 more or less simple, and can be healed up. Others, 

 again, can only be freed from the irritating agents 

 (which, by the by, may be insects as well as 

 fungi) by costly and troublesome methods. 



I shall only select one case for illustration, as it 

 is typical, and only too well known. As examples 

 of others belonging to the same broad category, 

 I may mention the "canker" of apple-trees, 

 beeches, oaks, hazels, maples, hornbeams, alders, 

 and limes, and many others ; and simply pass the 

 remark that whatever the differences in detail in the 

 special cases, the general phenomena and processes 

 of reasoning are the same in all. 



Perhaps no timber disease has caused so much 



