What is Lichen? CBSE class 9

Some fungal species live in close symbiotic association with blue green algae (or cyanobacteria). They are so much associated with each other that they form an independent group called lichens. Thus, lichens aremade up of fungi and algae which live in mutually dependent association. 

The algal partner synthesises food material and the fungal partner provides protection to alga.Such an association when two organisms live together and benefit each other is called symbiotic association. The growth of lichens is very slow. They form coloured patches on the bark of trees, barren rocks and wet walls. they usually grow in pollution free areas, so they are a kind of pollution indicators.