The ruler markings in the following picture are millimeters. These are small galls.
Valley Oak leaf galls. Columbia CA August 2014
This Valley Oak leaf has two species of leaf galls: The two red, dunce-cap galls, on the right, and the small round gal on the left. Duncecap galls, Valley Oak, Columbia California, 21 August 2014 DunceCap leaf galls from Valley Oak, Columbia CA. August 21, 2014.
This is a closeup of the small round gall on the valley oak leaf
