Lichens: Lichens are symbiotic associations, i.e. mutually useful associations, between algae and fungi. The algal component is known as phycobiont and fungal component as mycobiont, which is autotrophic and heterotrophic, respectively. Algae prepare food for fungi and fungi provide shelter and absorb mineral nutrients and water for its partner. Lichens are very good pollution indicators – they do not grow in polluted areas. They cannot grow in the atmosphere having a high concentration of oxides of sulphur.