Lichens are symbiotic associations between algae and fungi. The autotrophic algal component is called phycobiont and the heterotrophic fungal component is called as mycobiont. 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 and in cities. Lichens are pioneers in the xerosere type of ecological succession on barren rocks.