Strange fruit

I've probably used that title before but how else to refer to these weird and slightly disturbing blood-red bobbles attached to the underside of oak leaves? They're not fruit at all but galls produced by the tree in response to tiny wasps whose injection of eggs  results in the creation of these structures. The gall acts as a nursery in which the wasps' larvae  develop and feed. When that stage of their life cycle is complete and the insect emerges the gall is redundant and drops away. No harm is done to the tree and it may even be that some protection is conferred by the gall wasp's attack.

I think they are cherry galls but sometimes incorrectly called oak apples.  

Previously in the land of the Gauls galls: 1, 2, and 3.

(With thanks to my sharp-eyed son who drew my attention to these.)

Edit After writing this I found the final lines from Yeats' mystical poem had lodged in my mind:

And pluck till time and times are done
The silver apples of the moon
The golden apples of the sun

These then must be the ruddy apples of the oak.