Multiple lichen species growing on a gravestone 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. Shrubby and leafy lichens, seen here gowing on the top of the gravestone, tend to be more sensitive to atmospheric pollution than crustose species. Photographed at Rhoscolyn, Anglesey, North Wales, UK.