Multiple lichen species growing on a rock in unpolluted air. 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. Lichens are slow-growing, and are usually found on stones and the bark of trees. They are sensitive to pollution, and are a good indicator of air quality. Photographed in Snowdonia National Park, North Wales, UK.