There are Cherry Galls, Cynips quercusfolii, on the fallen Oak leaves here in Bridford Wood.
They are the tree’s defensive reaction to having an egg laid inside them by a gall wasp. Gall wasps have a complicated life, with alternating generations with just females and then males and females. Sometimes the generations live on different species of tree and sometimes produce different looking galls. There are even wasps which lay their eggs inside the galls produced by other gall wasps and then the grubs eat the grubs. Sometimes I’m glad I’m human.
Here’s an Oak Medusa Gall I saw in Italy:
Spangle Galls on Oak from my childhood nature notebooks:
And Oak Apple Galls:
There are 380 different species of gall wasp in Europe alone. That’s a lifetime’s worth of study for someone, of something most people don’t even know exists.