Various 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 (fruticose) and leafy (foliose) lichens tend to be more sensitive to atmospheric pollution than species which grow on surfaces as a crust (crustose). Photographed at Llanfaglan, North Wales, UK.