Dorset's rivers have lots of bridges crossing them and many are built of limestone. It is always worth taking a look at these bridges to see how, despite being brick or stone, nature colonises them. Crustose lichens are usually well established on the upper stone surfaces but look over the sides and you will find plants, especially spleenworts, and frequently this one, the wall-rue (Asplenium ruta-muraria).
Wall-rue is common across Britain and is certainly well established on rocks, walls and bridges across Dorset. Some people think that these tiny plants harm the wall it is growing on by weakening the mortar but, in reality, the plant roots are so fine that they tend to cling on to the surface rather than eat into the substrate. The wall has no reason to rue the wall-rue!
As an aside, if you ever develop a dose of rickets then this is supposed to be a cure - how about that as a bit of chat to slip into the conversation at the next works party?