A lichen is a composite organism that consists of a fungus and a photosynthetic partner growing together in a symbiotic relationship.
Lichens occur in some of the most extreme environments on the planet—arctic tundra, hot deserts, rocky coasts, and toxic slag heaps.
They are also abundant as epiphytes on leaves and branches in rain forests and temperate woodland, on bare rock, including walls and gravestones, and on exposed soil surfaces. The roofs of many buildings for instance have lichens growing on them.
Lichens have also been used in making dyes and perfumes, as well as in traditional medicines. It has been estimated that 6% of the planet’s land surface is covered by lichen.