Wildlife has a wonderful ability to colonise man-made structures that mimic natural habitats.To many organisms walls are nothing more than exposed rock faces or low, inland cliffs. These are relatively rare natural habitats in much of the British Isles but their artificial equivalents - stones walls - are everywhere, in towns and the countryside sometimes stretching for miles across the landscape and built from a wide variety of rock types, sometimes held together with mortar, sometimes merely piled one on top of another.
Natural cliffs are a habitat colonised by specialists - plants and animal that can thrive where there's little soil, often extreme changes in temperature, shortage of water and extremes of sunlight and shade. So cliffs' artificial equivalents are similarly colonised by some interesting, highly adapted organisms. This wall, for example, around Romaldkirk church in Teesdale, has been colonised by a wall specialist - yellow corydalis - which has an interesting method of sowing its seeds.
Yellow corydalis Pseudofumaria lutea comes from the lower slopes of the southern Alps in Italy and Switzerland and has grown in Britain since at least 1596. Two hundred years later it was first recorded in the wild and since then it has spread throughout almost all of southern and central England, most of Wales and parts of Eastern Scotland, Northern Ireland and to a few locations in Ireland. It's still extending its range. You can view a distribution map here.
It's uncommon to find it growing anywhere other than on walls, where it thrives - and that's probably because its seeds are equipped with a structure called an elaiosome - a protein and lipid-filled extension which is irresistible to ants - so they take the seeds with their food source back to their nests, and in so doing lodge the seeds (which they don't eat) in cool, damp crevices deep within walls. For those who like botanical jargon, there's a name for this ant-aided seed sowing - myrmechocory.
For more on dry stone walls, click here.