Not many plants can survive on the limited nutrients and moisture afforded by cracks in rock faces or stone walls but many ferns can, usually the sub-family of spleenworts. The black spleenwort (Asplenium adiantum-nigrum) is certainly one of them. In fact it only grows on rocks and walls, mainly in the west of England.
You might ask "Why 'black' spleenwort, it looks green to me?". The base of the stipe (that is the stem to you and me) is black; or dark brown and so appears black. It may seem difficult telling all these ferns apart but of the ones occurring on rocks and walls this is the only one with a true 'fern' appearance forming a triangular shape with bigger and wider fronds (leaves!) at the base getting smaller as they go up the stipe (stem!). So although it might look like many other ferns by being this shape it is the only one of this shape that grows on rocks and walls. Does that make sense?
Given we do not have a lot of rocks and stone walls in Dorset black spleenwort is not that common here.