Scanning electron micrograph of the foliose lichen, Xanthoria parietina, on a hawthorn twig, Crataegus monogyna. The picture shows cup-shaped apothecia, up to 2mm in size, arising from the thallus.The apothecia are the site of production of sexual spores called asci. Lichens are symbiotic organisms comprised of an alga and a fungus. X. parietina is a common and widespread species. resistant to air pollution, whether from Sulphur dioxide, Nitrogen oxides or ammonia. Lichens were used to assess sulphur dioxide levels in the context of acid rain resulting from industrial pollution. The current major air pollutant in rural areas is ammonia from agriculture, including intensive pig and poultry units, and slurry-spreading. Ammonia may cause biodiversity loss due to habitat enrichment by nitrogen. X. parietina can resist such pollution; this specimen was growing within 200m of a poultry farm.