WOUNDS are of two major types:
When microorganisms infect, they grow from compartment to compartment. The column of discolored or decayed wood that is seen macroscopically will be a composite of many columns as seen in these radial sections through drill wounds. (fig. 48)
In a vertical plane, each growth ring ("tree") will have its own column of discolored and decayed wood. The response of the "older trees" or innermost rings is slower and weaker than that of the "younger trees." (fig. 49)
The "holding strength" of Wall 1 decreases as the injury goes inward. The older "trees" have a weaker response. This action determines the shape of the column, shown here from a radial view.(fig. 50) In a sense, the wounds set the staqe. The DISCOLORATION PROCESSES begin immediately after wounding when the tree reacts-both by chemical reaction and by plugging. When the pioneer microorganisms invade, the discoloration processes may intensify. The DECAY PROCESSES begin when microorganisms begin to digest cell walls. Many factors affect the rate of the discoloration and decay processes-severity of the wound, position, size, time of year of wounding, wounding agents that import materials into the wound, and the types of microorganisms that infect.
Branches are often decayed by a wide variety of microorganisms that do not enter the trunk. Branch decay of this type is very beneficial as the tree is pruned when the branches fall. (fig. 54)
Also, concentrations of pigments and oxidized protective materials will usually be greater along the side of the column closest to the cambium. The column stays to the pith side of Wall 4. (fig. 55)
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