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MISSOURI BOTANICAL GARDEN. 



A careful' examination showed that cankers always origi- 

 nated aroimd a small, dead branch. On plate G, five stages 

 of the canker development are shown. Figure 1 represents 

 a living branch bearing a small, lateral branch which had 

 recently died. Figure 2 shows a similar branch on which the 

 lateral branch has been dead for a longer period, and figures 

 3, 4 and 5 show successive stages in the canker development. 

 From an examination of an extended series of cankers in 

 various stages, the conclusion is drawn that the development 

 of these cankers was primarily due to an attempt on the part 

 of the branch to heal over a dead branch stub. Woody 

 plants, both trees and shrubs, differ very materially in the 

 manner in which they heal over branch stubs. In the ma- 

 jority of forest trees, when a branch tlies, the base of such a 

 branch usually remains in a sufficiently intimate relationship 

 to the parent branch so that the dead tissue is confined strictly 

 to the branch up to the point where the living branch tissue 

 comes into contact with the living tissue of the trunk or 

 parent branch. In other words, that part of the branch situ- 

 ated within the trunk, to use a common expression, is sup- 

 plied with water and food and remains alive. When the 

 healing process begins, the tissue immediately surrounding 

 the inserted branch starts to develop with greater rapidity, 

 resulting in increased wood formation, and where the branch 

 has broken off short, a marked callus soon arises which in 

 time will cover the dead branch stub. The extent to which 

 the tissue at the base of the branch stub assists in this healing 

 process will vary considerably. In the genus Picea, for in- 

 stance, the base of the branch stub remains alive for a very 

 considerable period of time,giving rise to the well-known swell- 

 ings commonly found on spruce trees at the base of dead 

 branch stubs. Pines and many of the hardwood trees, on 

 the other hand, form no such swellings at the base of the 

 branch stub, because the branch in this case dies down close 

 to the bark of the trunk. Owing to this difference in the 

 behavior of the base of tiie dead branch, various species of 

 forest trees heal over branch stubs with different rapidity 

 and with varying degrees of success. In the white pine, for 

 instance, a branch dies and in the course of time usually 



