Explanation: A lichen is a peculiar combination of an alga and a fungus– the two live deriving mutual benefit.

• They are group of greyish green plants which grow on rocks, tree-trunks, dead wood, etc.
• The algae manufacture food which becomes available to the fungus, and the absorbs and retains water and thus keeps the algal cells moist. A example of symbiosis.
• They are most common in wetlands, rare in rivers and streams and are not found in ground water.

Source: Shankar IAS Environment book