Lichen
A lichen is a composite organism that arises from algae or cyanobacteria living among filaments of multiple fungi in a symbiotic relationship. The combined lichen has properties different from those of its component organisms. Lichens come in many colors, sizes, and forms. The properties are sometimes plant-like, but lichens are not plants.
Lichens may have tiny, leafless branches (fruticose), flat leaf-like structures (foliose), flakes that lie on the surface like peeling paint (crustose), or other growth forms. Lichens do not have roots that absorb water and nutrients as plants do, but like plants, they produce their own food by photosynthesis. When they grow on plants, they do not live as parasites, but instead use the plants as a substrate.
Pronunciation and etymology
In American English, "lichen" is pronounced the same as the verb "liken" (/ˈlaɪkən/). In British English, both this pronunciation and one rhyming with "kitchen" /ˈlɪtʃən/) are used.
English lichen derives from Greek λειχήν leichēn ("tree moss, lichen, lichen-like eruption on skin") via Latin lichen. The Greek noun, which literally means "licker", derives from the verb λείχειν leichein, "to lick".
Macro vs micro
A macrolichen is a lichen that is either bush-like or leafy; all other lichens are termed microlichens. Here, "macro" and "micro" do not refer to size, but to the growth form. Common names for lichens may contain the word moss (e.g., "reindeer moss", "Iceland moss"), and lichens may superficially look like and grow with mosses, but lichens are not related to mosses or any plant.
Distribution
Lichens occur from sea level to high alpine elevations, in many environmental conditions, and can grow on almost any surface. Lichens are abundant growing on bark, leaves, mosses, on other lichens, and hanging from branches "living on thin air" (epiphytes) in rain forests and in temperate woodland. They grow on rock, walls, gravestones, roofs, exposed soil surfaces, and in the soil as part of a biological soil crust. Different kinds of lichens have adapted to survive in some of the most extreme environments on Earth: Antarctica, hot dry deserts, rocky coasts, and toxic slag heaps. They can even live inside solid rock, growing between the grains.
It is estimated that 6% of Earth's land surface is covered by lichen. There are about 20,000 known species of lichens. Some lichens have lost the ability to reproduce sexually, yet continue to speciate. Lichens can be seen as being relatively self-contained miniature ecosystems, where the fungi, algae, or cyanobacteria have the potential to engage with other microorganisms in a functioning system that may evolve as an even more complex composite organism.
Lichens may be long-lived, with some considered to be among the oldest living things. They are among the first living things to grow on fresh rock exposed after an event such as a landslide.
Human use
As a food
There are records of lichens being used as food by many different human cultures across the world. Lichens are eaten by people in North America, Europe, Asia, and Africa, and perhaps elsewhere. Often lichens are merely famine foods eaten in times of dire needs, but in some cultures lichens are a staple food or even a delicacy. Two problems often encountered with eating lichens is that they usually contain mildly toxic secondary compounds, and that lichen polysaccharides are generally indigestible to humans. Many human cultures have discovered preparation techniques to overcome these problems. Lichens are often thoroughly washed, boiled, or soaked in ash water to help remove secondary compounds.
In the past Iceland moss (Cetraria islandica) was an important human food in northern Europe and Scandinavia, and was cooked in many different ways, such as bread, porridge, pudding, soup, or salad. Wila (Bryoria fremontii) was an important food in parts of North America, where it was usually pitcooked. It is even featured in a Secwepemc story. Reindeer lichen (Cladina spp.) is a staple food of reindeer and caribou in the Arctic. Northern peoples in North America and Siberia traditionally eat the partially digested lichen after they remove it from the rumen of caribou that have been killed. It is often called 'stomach icecream'. Rock tripe (Umbilicaria spp. and Lasalia spp.) is a lichen that has frequently been used as an emergency food in North America. One species of Umbilicaria, Iwa-take (U. esculenta), is used in a variety of traditional Korean and Japanese foods. It is quite expensive, and is collected off the sides of cliffs. In India, and other centers of curry powder production, garam masala sauce contains certain lichens used as bulking agents.
Very few lichens are poisonous. Poisonous lichens include those high in vulpinic acid or usnic acid. Most (but not all) lichens that contain vulpinic acid are yellow, so any yellow lichen should be considered to be potentially poisonous.