Wart like looking bumps on green leaves. Not sure if this is part of the plant, or something an animal left behind. Was only on a few of the leaves. Galls or cecidia are outgrowths on the surface of lifeforms. Plant galls are abnormal outgrowths of plant tissues and can be caused by various parasites, from fungi and bacteria, to insects and mites. Plant galls are often highly organized structures and because of this the cause of the gall can often be determined without the actual agent being identified. This applies particularly to some insect and mite plant galls.
I am thinking it maybe a form of these: Knopper gall (Andricus quercuscalicis) But not positive. http://en.wikipedia.org/wiki/Gall