I find these very round lumps on the gum leaves fascinating. Apparently they are caused by a wasp which lays its eggs in the leaf. It is not clear how exactly the gall is formed, but it creates a safe place for the wasp to develop while it is vulnerable. I assume at some point the wasp emerges, as I have seen galls which appear to have burst open, and plainly there are several tunnels visible inside the exploded gall. I have a collection of these leaves on my sideboard...maybe i should put them outside!?!