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Anonymous
There are various insects that will inject their larva into a twig and stem that results in these Galls. The egg/larva excretes a hormone that causes the abnormal growth.
If you see a little hole in the side of the gall it's because the larva has matured and burrowed its way out already. That's also the source of those channels you see in the cross sectioned ones.
When I was in grade school we went out and found these in the autumn and cut them open to find the nasty little larva inside.
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