Lichens have been used by humans as food and as sources of medicine and dye.They also provide two-thirds of the food supply for the caribou and reindeer that roam the far northern ranges.Scientific knowledge about lichens has expanded significantly during the past few decades, and new discoveries continue.Most lichen species grow best where there is sufficient light and moisture within a moderate temperature zone.Lichens are long-lived and grow relatively slowly, and there is still some question as to how they propagate.Most botanists agree that the most common means of reproduction is vegetative; that is, portions of an existing lichen break off and fall away to begin new growth nearby.Join Britannica's Publishing Partner Program and our community of experts to gain a global audience for your work! Lichens are found worldwide and occur in a variety of environmental conditions.A diverse group of organisms, they can colonize a wide range of surfaces and are frequently found on tree bark, exposed rock, and as a part of biological soil crust.
As symbionts, the basis of their relationship is the mutual benefit that they provide each other.
The phycobionts also produce vitamins that the fungi need.
Fungi contribute to the symbiosis by absorbing water vapour from the air and by providing much-needed shade for the light-sensitive algae beneath.