Fuzzy growth on underside of oak leaves

Every year about this time in autumn, I find a lot of mysterious little fuzzy peach colored balls on the ground under our oak tree. A few days ago I pruned an oak branch which was shading too many understory plants and I finally found out where those balls come from. There were a number of those fuzzy things attached to the undersides of the leaves.

I detached a fuzz ball which resembles the fuzz on tennis balls and could feel a hard kernel inside so I pulled off the fuzzy material and saw a green seed like object. Initially I thought it was a seed of some kind of parasitic plant but how a plant could attach itself to the host plant leaf was a mystery especially when the branches were several feet off the ground and I didn't see any vine like plants climbing the tree trunk. I thought it could be an airborne seed but how it would find a specific host was problematic.

I did a quick online search for oak galls and came upon some images that resemble the ones I took. Apparently it is the pupa of a wasp that lives in the gall! And the "seed" is not a seed at all. The gall is called a woolly oak leaf gall and the inhabitant is a parasitic gall wasp belonging to the genus Callirhytis. And here I was all set to plant the 'seed' to see what came up. ;)

I'm glad I solved the mystery of the fuzz balls which has been nagging me for about 3 years. I guess I should cut open a gall to see if I can find the pupa inside.

Here's an image of the Callirhytis wasp though this may not be the specific one that creates the woolly gall. And here are more photos of oak galls including the woolly oak gall.