Why do the sides of old branch scars bulge out?
Answer:
The outer edges are still living tissue, and so they continue to grow. The inner part is dead wood. The wood did not die because the branch was cut. Because we usually only see the living, outer layer, it is easy to forget that the insides of trees are made up of dead wood. Although it is dead, it provides strength and support, but if it decays away, the resulting hollow tree can still thrive.
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