Lichens are mutualistic associations between fungi and algae.
The fungus is an ascomycote or a basidiomycote while the alga is a green alga. It is believed that the alga contributes organic food from photosynthesis, while the fungus is water and mineral salts. The fungus can also conserve water enabling some lichens to grow in dry conditions where no other plants exist. Lichens are found on seashores, mountainpeaks, but not common in cities too much pollution. Lichens are a group of organisms which are among the most highly sensitive to air pollution.