158 STATE HORTICULTURAL SOCIETY. 



many wounds are so large or so severe that even in the course of several 

 years nature can not 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 protective processes when trunks and 

 branches are wounded are the formation of cork cells 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 ceils 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-size branch is cut 

 off or broken off, the healing process is quite different. You have fre- 

 quently 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 whieh 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 attach- 

 ment 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 course 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 imsignificant 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 illustration, 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. 



It is a well-known fact that where the cambium is exposed in wounds, it 

 produces new cells more vigorously than the cambium of uninjured stems. 

 The reason for this, at first sight, anomalous state of things will be easily 

 understood if we call to mind the tension of any normal trunk. A trunk 

 may be regarded as a cylinder composed of a solid axis of wood whose cir- 

 cumference is formed of the actively growing cambium encircled by the 

 inner and outer bark, which taken together we may now, for convenience 

 sake, call the cortex. The different parts of this compound cylinder grow 

 under different tensions. On the one hand, the inner parts, as they grow, 

 exert a strong outward pressure on the cortex, while, on the other hand, 

 the cortex acts as a sheath which exerts a strong pressure on the parts 

 within. That when the normal pressure is interfered with, the relative 

 growth of the different parts of the stem is changed, is well shown if a 

 slit is made through the cortex to the region of the cambium. The cells 

 of the cambium thus freed from pressure from without grow more rapidly 

 than before in the direction of the slit, so that the wound thus made is 

 rapidly filled by the new cells thus formed, and the new growth may even 



