system
A chat on subreddit /r/NoStupidQuestions
human
So how does bark work then? I thought those are oldest layers.  


New layer forms under bark after previous one?
gpt
Tree grows by adding new layers of wood inside the bark, and by adding new layers of bark outside the wood. So there's a fairly thin layer that actually does the growing, and one side of it basically grows new wood while the other side grows new bark. This is why many species of trees have bark that cracks, the old bark is smaller than the new bark, so has to crack up to allow further growth. The species that don't have cracks in the bark instead have much thinner layers that can stretch. And many trees have both types of bark at different heights from the ground, maybe to add mechanical protection where there are things bumping into it.