These are nail galls on the leaf of a Lime tree.
They are caused by a mites (Eriophyes lateannulatus) that is only about a tenth of a millimetre in size, which browse on the lower surface of the leaf.
Chemicals in their saliva cause the leaf cells to proliferate, creating these hollow galls.
The mites then move inside them to feed and breed in safety. They have no adverse effect on the tree.
It seems the mite Eriophyes tilliae prefers the Large-Leaved Lime and the galls are longer and pointed, whereas the mite Eriophyes lateannulatus prefers the Small-Leaved Lime and galls are shorter with rounded tips.
Common Lime can have either species of mite.