Concept: The term ''Lichen'' was given by Theophrastus. Study of Lichens - Lichenology. Father of Lichenology - Erik acharius Explanation: Lichen: Lichen is a composite thalloid structure of algae & fungi. The algal part of Lichen is called phycobiont and the fungal part is called mycobiont. Both algae & fungi remain in symbiotic association i.e. they are beneficial for each other. The algal component prepares the food by photosynthesis while the fungal component helps in the absorption of minerals and nutrition from the soil Lichens are good indicators of air pollution. Lichens are sensitive towards SO2 (sulphur dioxide). They die in more concentrations of SO2. Therefore they are not found in those areas (polluted areas) where the concentration of SO2 is high. Important Points Lichens do not help in nitrogen fixation. Legumes and certain bacteria like Rhizobium help in nitrogen fixation.​ Thus, A. They show symbiotic relationships: Correct B. They are made up of algae and bacteria: Incorrect C. They are natural indicators of SO2: Correct D. They also help in nitrogen fixation: Incorrect Additional Information Other ecological importance of Lichen: Many Lichens are used as food and fodder. Eg –Parmelia, used as food in South India. It is known as Rock Flower. It is also used as dyes. Eg – Litmus(Acid-base indicator) is obtained from many members of lichens like Roccella tinctoria. The crustose and foliose lichens grow on rocks and secrete different types of acids. These acids break the rocks. In this way, these lichens help in soil formation.