Lichens are made up of algal and fungal components, living together in an intimate symbiotic relationship. The plant body of lichens neither resembles algae nor fungi. Hence, lichens possess a stable thallus of specific structure. Lichens often grow in most inhospitable and uninhabited places. This is because they can survive high temperature and dry conditions for a long time. Lichens are also very good indicators of air pollution. Decrease in lichen population of an area is indicative of air pollution particularly $SO _{2}$ pollution.