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.Lichens are good indicators of air pollution. Lichens are sensitive towards SO2(SULPHUR dioxide). They die in more concentration of SO2. Therefore they are not found in those areas (polluted areas) where the concentration of SO2 is high.