Lichens are unique organisms, as they actually consist of two different organisms: an alga and a fungus. 

Algae are microscopically small plants that are capable of photosynthesis. They provide the fungal part of the lichen with sugar, whereas the fungal part provides the algal part of the lichen with water and protection! In other words, both organisms benefit from their co-operation. 

The co-operation of algae and fungi has reached a point where it has become hard to distinguish between the two different organisms. This is why we consider lichens single organisms. 

Lichens thrive best in rocky areas, where plants cannot grow. They can also grow on tree trunks. Lichens grow very slowly and they are sensitive to air pollution.