Lichen growing on a gravestone carving. Lichens are formed from the a symbiotic association between a fungus and an alga. The alga is contained within the hyphae (filaments) of the fungus. They are therefore protected from harsh conditions, and in particular from dessication. The alga supplies the fungus with nutrients that it makes by photosynthesis. Lichens are usually found growing on stones and the bark of trees. Lichen are sensitive to pollution, and are a good indicator of air quality. Photographed on Anglesey, North Wales, UK.