What is it?

[salden]

Interesting looking "stuff". It is on the leaves of my pin oak in the backyard. It is only on the leaves, but it is on plenty of them. Whatever it is, it is on both sides of the leaves like it has penetrated the leaves on both sides in the same place.

[rjlittlefield]

I'm not familiar with this exact kind, but in general this looks like one of the many oak leaf galls caused by insects -- wasps or flies, usually. The bugs produce chemicals that prompt the plant to produce swellings that provide food and shelter for the bugs.

The usual writeup says that they're unsightly but don't seem to hurt the tree.

If you took one apart, you just might be able to find the little critters living in it.

--Rik