DISEASES OF TREES LIKELY TO FOLLOW INJURIES. 145 



years nature cannot close them. In such wounds disease is likely 

 to arise, which will infect the whole tree, unless man comes to the 

 aid of nature. Let us then consider the question of the manner 

 in which nature acts and to what extent. The two natural protec- 

 tive processes when trunks and branches are wounded are the 

 formation of tork ,"£lls and the formation of a callus. The two 

 processes may go on -together. If the wound is slight, as when 

 the outer bark is scraped or gnawed off, so as to expose the more 

 delicate cells beneath, a new formation of cork may be sufficient 

 to close the wound. But when, as is very frequently the case, 

 both the outer and inner bark are torn away, exposing the wood, 

 or when a good sized branch is cut off or broken off, the healing 

 process is quite different. You have frequently seen the scars left 

 when branches have been cut away and know that the edges of the 

 wound swell and form a thick, rounded rim which in course of 

 time seems to contract around the wound, and, if the wound is 

 of moderate size, finally covers it. This thickened rim is what 

 is called the callus, and it originates mainly in the cambium which 

 was exposed when the wound was made, and to some extent in 

 the adjacent cells of the inner bark. 



To understand what takes place it will be best for us to suppose 

 a simple case of wounding, such as that of a branch six inches in 

 diameter, let us say, which has been carefully sawn across so as 

 not to loosen the attachment of the bark to the wood. The 

 greater part of the exposed surface here would consist of the wood 

 proper with a comparatively narrow circle of the coarse outer 

 bark and the more delicate inner bark. Between the wood and 

 the bark is, of course, the cambium, represented by the circum- 

 ference of a circle quite insignificant in thickness compared either 

 with the bark or the wood. 



Of the exposed parts the wood itself is practically unable to 

 take any active part in the process of healing. It presents a 

 series of open tubes, which are incapable of producing new cells. 

 The cells of the cambium and, to a less extent, those of the inner 

 bark and of the rays which lie near the cambium, are able to 

 produce new cells, and hence, in the case we have chosen as an 

 illusti'ation, there would arise a ring of new growth just around 

 the wood and beneath the bark. This raised ring of new growth 

 is the beginning of the callus. 

 10 



