What's inside these tree balls?

Oak galls are found on oak trees, and they grow when a certain type of wasp injects the tree with chemicals. They're kind of like wood tumors, and if I crush one, it's kind of like smashing a ball of wood. What I think is really cool though, is what happens, when it's added to a solution of iron. he tannins in the oak gall react with iron to form a super dark complex. In Europe, this was one of the first, and most popular ways to make ink.

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