Lichens are a remarkable partnership between algae and fungi.
The algae in lichen are tiny green balls which make the food from sunlight to feed the fungi.
The fungi make a protective layer around the algae and hold water.
There are 20,000 species of lichen. Some grow on soil, but most grow on rocks or tree bark.
Fruticose lichens are shrub-like, foliose lichens look like leaves, and crustose lichens look like crusts.
Lichens only grow when moistened by rain.
Lichens can survive in many places where other plants would die, such as the Arctic, in deserts and on mountain tops.
Some Arctic lichens are over 4000 years old.
Lichens are tiny and slow-growing – some growing only a fraction of a millimeter a year. But they are usually long-lived.
Lichens are very sensitive to air pollution, especially sulphur dioxide, and are used by scientists to indicate air pollution.
The oak moss lichen from Europe and North Africa is added to most perfumes and after-shaves to prevent flower scents from fading. Scandinavian moss is lichen that is eaten by reindeer. It is exported to Germany for use as decorations.
Crustose rock lichens form dense crusts which attach themselves to the rocks by their whole undersurface.