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
colours, 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
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.:3
Lichens do not have roots
that absorb water and nutrients as plants do,:2 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
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: arctic
tundra, 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.:2 There are about 20,000 known species of lichens.
Some lichens have lost the ability to reproduce sexually, yet continue
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
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. The long
life-span and slow and regular growth rate of some lichens can be used
to date events (lichenometry).
1 Pronunciation and etymology
2 Growth forms
2.2 Internal structure and growth forms
3.2.1 Miniature ecosystem and holobiont theory
3.2.2 Lichenicolous fungi
3.3 Reaction to water
3.4 Metabolites, metabolite structures and bioactivity
3.5 Growth rate
3.6 Life span
3.7 Response to environmental stress
4 Reproduction and dispersal
4.1 Vegetative reproduction
4.2 Sexual reproduction
5 Taxonomy and classification
5.3 Controversy over classification method and species names
5.5 Identification methods
5.6 Evolution and paleontology
6 Ecology and interactions with environment
6.1 Substrates and habitats
Lichens and soils
6.3 Ecological interactions
6.4 Effects of air pollution
7 Human use
7.4 As dyes
Traditional medicine and research
7.6 Aesthetic appeal
7.7 In literature
10 See also
13 External links
13.1 Identification and classification
13.2 Internal structure
13.5 By region
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
The Greek noun, which literally means "licker", derives from the verb
λείχειν leichein, "to lick".
Lichen growth forms
Lichens grow in a wide range of shapes and forms (morphologies). The
shape of a lichen is usually determined by the organization of the
fungal filaments. The nonreproductive tissues, or vegetative body
parts, is called the thallus.
Lichens are grouped by thallus type,
since the thallus is usually the most visually prominent part of the
Thallus growth forms typically correspond to a few basic
internal structure types. Common names for lichens often come from a
growth form or color that is typical of a lichen genus.
Common groupings of lichen thallus growth forms are:
fruticose – growing like a tuft or multiple-branched
leafless mini-shrub, upright or hanging down, 3-dimensional branches
with nearly round cross section (terete) or flattened
foliose – growing in 2-dimensional, flat, leaf-like lobes
crustose – crust-like, adhering tightly to a surface
(substrate) like a thick coat of paint
squamulose - formed of small leaf-like scales crustose below but
free at the tips
leprose – powdery
gelatinous – jelly like
filamentous – stringy or like matted hair
byssoid – wispy, like teased wool
There are variations in growth types in a single lichen species, grey
areas between the growth type descriptions, and overlapping between
growth types, so some authors might describe lichens using different
growth type descriptions.
When a crustose lichen gets old, the center may start to crack up like
old-dried paint, old-broken asphalt paving, or like the polygonal
"islands" of cracked-up mud in a dried lakebed. This is called being
rimose or areolate, and the "island" pieces separated by the cracks
are called areolas. The areolas appear separated, but are (or
were) connected by an underlying "prothallus" or
"hypothallus". When a crustose lichen grows from a center and
appears to radiate out, it is called crustose placodioid. When the
edges of the areolas lift up from the substrate, it is called
These growth form groups are not precisely defined.
may sometimes branch and appear to be fruticose.
Fruticose lichens may
have flattened branching parts and appear leafy.
may appear where the edges lift up. Gelatinous lichens may appear
leafy when dry.:159 Means of telling them apart in these cases are
in the sections below.
Structures involved in reproduction often appear as discs, bumps, or
squiggly lines on the surface of the thallus.:4 The thallus is not
always the part of the lichen that is most visually noticeable. Some
lichens can grow inside solid rock between the grains (endolithic
lichens), with only the sexual fruiting part visible growing outside
the rock. These may be dramatic in color or appearance. Forms
of these sexual parts are not in the above growth form categories.
The most visually noticeable reproductive parts are often circular,
raised, plate-like or disc-like outgrowths, with crinkly edges, and
are described in sections below.
Lichens come in many colors.:4 Coloration is usually determined by
the photosynthetic component.
Special pigments, such as yellow
usnic acid, give lichens a variety of colors, including reds, oranges,
yellows, and browns, especially in exposed, dry habitats. In the
absence of special pigments, lichens are usually bright green to olive
gray when wet, gray or grayish-green to brown when dry. This is
because moisture causes the surface skin (cortex) to become more
transparent, exposing the green photobiont layer. Different
colored lichens covering large areas of exposed rock surfaces, or
lichens covering or hanging from bark can be a spectacular display
when the patches of diverse colors "come to life" or "glow" in
brilliant displays following rain.
Different colored lichens may inhabit different adjacent sections of a
rock face, depending on the angle of exposure to light. Colonies
of lichens may be spectacular in appearance, dominating much of the
surface of the visual landscape in forests and natural places, such as
the vertical "paint" covering the vast rock faces of Yosemite National
Color is used in identification.:4 Color changes depending on when
a lichen is wet or dry. Color descriptions when used for
identification are based on when the lichen is dry. Dry lichens
with a cyanobacterium as the photosynthetic partner tend to be dark
grey, brown, or black.
The underside of the leaf-like lobes of foliose lichens is a different
color from the top side (dorsiventral), often brown or black,
sometimes white. A fruticose lichen may have flattened "branches",
appearing similar to a foiliose lichen, but the underside of a
leaf-like structure on a fruticose lichen is the same color as the top
side. The leaf-like lobes of a foliose lichen may branch, giving the
appearance of a fruticose lichen, but the underside will be a
different color from the top side.
The sheen on some jelly-like gelatinous lichens is from mucilaginous
Internal structure and growth forms
The cyanobacterium Hyella caespitosa with fungal hyphae in the lichen
A lichen consists of a simple photosynthesizing organism, usually
green algae or cyanobacteria, surrounded by filaments of a fungus.
Generally, most of a lichen's bulk is made of interwoven fungal
filaments, although in filamentous and gelatinous lichens this
is not the case. The fungus is called a mycobiont. The
photosynthesizing organism is called a photobiont.
are called phycobionts.
Cyanobacteria photobionts are called
The part of a lichen that is not involved in reproduction, the "body"
or "vegetative tissue" of a lichen, is called the thallus. The thallus
form is very different from any form where the fungus or alga are
growing separately. The thallus is made up of filaments of the fungus
called hyphae. The filaments grow by branching then rejoining to
create a mesh, which is called being "anastomose". The mesh of fungal
filaments may be dense or loose.
Generally, the fungal mesh surrounds the algal or cyanobacterial
cells, often enclosing them within complex fungal tissues that are
unique to lichen associations. The thallus may or may not have a
protective "skin" of densely packed fungal filaments, often containing
a second fungal species, which is called a cortex. Fruticose
lichens have one cortex layer wrapping around the "branches". Foliose
lichens have an upper cortex on the top side of the "leaf", and a
separate lower cortex on the bottom side.
Crustose and squamulose
lichens have only an upper cortex, with the "inside" of the lichen in
direct contact with the surface they grow on (the substrate). Even if
the edges peel up from the substrate and appear flat and leaf-like,
they lack a lower cortex, unlike foliose lichens. Filamentous,
byssoid, leprose, gelatinous, and other lichens do not have a
cortex, which is called being ecorticate.
Schematic cross section of foliose lichen:
1. The cortex is the outer layer of tightly woven fungus filaments
(hyphae) 2. This photobiont layer has photosynthesizing green algae
3. Loosely packed hyphae in the medulla
4. A tightly woven lower cortex, with anchoring hyphae called rhizines
where the fungus attaches to the substrate.
Fruticose, foliose, crustose, and squamulose lichens generally have up
to three different types of tissue, differentiated by having different
densities of fungal filaments. The top layer, where the lichen
contacts the environment, is called a cortex. The cortex is made
of densely tightly woven, packed, and glued together (agglutinated)
fungal filaments. The dense packing makes the cortex act like a
protective "skin", keeping other organisms out, and reducing the
intensity of sunlight on the layers below. The cortex layer can be
up to several hundred micrometers (μm) in thickness (less than a
millimeter). The cortex may be further topped by an epicortex of
secretions, not cells, 0.6–1 μm thick in some lichens.
This secretion layer may or may not have pores.
Below the cortex layer is a layer called the photobiontic layer or
symbiont layer. The symbiont layer has less densely packed
fungal filaments, with the photosynthetic partner embedded in
them. The less dense packing allows air circulation during
photosynthesis, similar to the anatomy of a leaf. Each cell or
group of cells of the photobiont is usually individually wrapped by
hyphae, and in some cases penetrated by an haustorium. In crustose
and foliose lichens, algae in the photobiontic layer are diffuse among
the fungal filaments, decreasing in gradation into the layer below. In
fruticose lichens, the photobiontic layer is sharply distinct from the
The layer beneath the symbiont layer called is called the medulla. The
medulla is less densely packed with fungal filaments than the layers
above. In foliose lichens, there is usually, as in Peltigera,:159
another densely packed layer of fungal filaments called the lower
cortex. Root-like fungal structures called rhizines
(usually):159 grow from the lower cortex to attach or anchor the
lichen to the substrate.
Fruticose lichens have a single cortex
wrapping all the way around the "stems" and "branches". The
medulla is the lowest layer, and may form a cottony white inner core
for the branchlike thallus, or it may be hollow.:159
squamulose lichens lack a lower cortex, and the medulla is in direct
contact with the substrate that the lichen grows on.
In crustose areolate lichens, the edges of the areolas peel up from
the substrate and appear leafy. In squamulose lichens the part of the
lichen thallus that is not attached to the substrate may also appear
leafy. But these leafy parts lack a lower cortex, which distinguishes
crustose and squamulose lichens from foliose lichens. Conversely,
foliose lichens may appear flattened against the substrate like a
crustose lichen, but most of the leaf-like lobes can be lifted up from
the substrate because it is separated from it by a tightly packed
Gelatinous,:159 byssoid, and leprose lichens lack a cortex (are
ecorticate), and generally have only undifferentiated tissue, similar
to only having a symbiont layer.
In lichens that include both green algal and cyanobacterial symbionts,
the cyanobacteria may be held on the upper or lower surface in small
pustules called cephalodia.
Pruinia is a whitish coating on top of an upper surface. An
epinecral layer is "a layer of horny dead fungal hyphae with
indistinct lumina in or near the cortex above the algal layer".
In August 2016, it was reported that macrolichens have more than one
species of fungus in their tissues.
Symbiosis in lichens
Lichens are fungi that have discovered agriculture" —Trevor
A lichen is a composite organism that emerges from algae or
cyanobacteria living among the filaments (hyphae) of two fungi in a
mutually beneficial symbiotic relationship. The fungi benefit from the
carbohydrates produced by the algae or cyanobacteria via
photosynthesis. The algae or cyanobacteria benefit by being protected
from the environment by the filaments of the fungi, which also gather
moisture and nutrients from the environment, and (usually) provide an
anchor to it. Although some photosynthetic partners in a lichen can
survive outside the lichen, the lichen symbiotic association extends
the ecological range of both partners, whereby most descriptions of
lichen associations describe them as symbiotic. However, while
symbiotic, the relationship is probably not mutualistic, since the
algae give up a disproportionate amount of their sugars (see below).
Both partners gain water and mineral nutrients mainly from the
atmosphere, through rain and dust. The fungal partner protects the
alga by retaining water, serving as a larger capture area for mineral
nutrients and, in some cases, provides minerals obtained from the
substrate. If a cyanobacterium is present, as a primary partner or
another symbiont in addition to a green alga as in certain tripartite
lichens, they can fix atmospheric nitrogen, complementing the
activities of the green alga.
The algal or cyanobacterial cells are photosynthetic and, as in
plants, they reduce atmospheric carbon dioxide into organic carbon
sugars to feed both symbionts. Phycobionts (algae) produce sugar
alcohols (ribitol, sorbitol, and erythritol), which are absorbed by
the mycobiont (fungus). Cyanobionts produce glucose.
Lichenized fungal cells can make the photobiont "leak" out the
products of photosynthesis, where they can then be absorbed by the
The lichen combination of alga or cyanobacterium with a fungus has a
very different form (morphology), physiology, and biochemistry than
the component fungus, alga, or cyanobacterium growing by itself,
naturally or in culture. The body (thallus) of most lichens is
different from those of either the fungus or alga growing separately.
When grown in the laboratory in the absence of its photobiont, a
lichen fungus develops as a structureless, undifferentiated mass of
fungal filaments (hyphae). If combined with its photobiont under
appropriate conditions, its characteristic form associated with the
photobiont emerges, in the process called morphogenesis. In a few
remarkable cases, a single lichen fungus can develop into two very
different lichen forms when associating with either a green algal or a
cyanobacterial symbiont. Quite naturally, these alternative forms were
at first considered to be different species, until they were found
growing in a conjoined manner.
Evidence that lichens are examples of successful symbiosis is the fact
that lichens can be found in almost every habitat and geographic area
on the planet. Two species in two genera of green algae are found
in over 35% of all lichens, but can only rarely be found living on
their own outside of a lichen.
In a case where one fungal partner simultaneously had two green algae
partners that outperform each other in different climates, this might
indicate having more than one photosynthetic partner at the same time
might enable the lichen to exist in a wider range of habitats and
Phycobionts can have a net output of sugars with only water vapor.
The thallus must be saturated with liquid water for cyanobionts to
Algae produce sugars that are absorbed by the fungus by diffusion into
special fungal hyphae called appressoria or haustoria in contact with
the wall of the algal cells. The appressoria or haustoria may
produce a substance that increases permeability of the algal cell
walls, and may penetrate the walls. The algae may lose up to 80%
of their sugar production to the fungus.
Lichen associations may be examples of mutualism, commensalism or even
parasitism, depending on the species. There is evidence to suggest
that the lichen symbiosis is parasitic or commensalistic, rather than
mutualistic. The photosynthetic partner can exist in nature
independently of the fungal partner, but not vice versa. Photobiont
cells are routinely destroyed in the course of nutrient exchange. The
association is able to continue because reproduction of the photobiont
cells matches the rate at which they are destroyed. The fungus
surrounds the algal cells, often enclosing them within complex
fungal tissues unique to lichen associations. In many species the
fungus penetrates the algal cell wall, forming penetration pegs
(haustoria) similar to those produced by fungi that feed on a host
Cyanobacteria in laboratory settings can
grow faster when they are alone rather than when they are part of a
Miniature ecosystem and holobiont theory
Symbiosis in lichens
Symbiosis in lichens is so well-balanced that lichens have been
considered to be relatively self-contained miniature ecosystems in and
of themselves. It is thought that lichens may be even more
complex symbiotic systems that include non-photosynthetic bacterial
communities performing other functions as partners in a
Many lichens are very sensitive to environmental disturbances and can
be used in cheaply assessing air pollution, ozone
depletion, and metal contamination.
Lichens have been used in making
dyes, perfumes, and in traditional medicines. A few lichen species
are eaten by insects or larger animals, such as reindeer.
Lichens are widely used as environmental indicators or bio-indicators.
If air is very badly polluted with sulphur dioxide there may be no
lichens present, just green algae may be found. If the air is clean,
shrubby, hairy and leafy lichens become abundant. A few lichen species
can tolerate quite high levels of pollution and are commonly found on
pavements, walls and tree bark in urban areas. The most sensitive
lichens are shrubby and leafy while the most tolerant lichens are all
crusty in appearance. Since industrialisation many of the shrubby and
leafy lichens such as Ramalina,
Lobaria species have very
limited ranges, often being confined to the parts with the purest air.
Some fungi can only be found living on lichens as obligate parasites.
These are referred to as lichenicolous fungi, and are a different
species from the fungus living inside the lichen; thus they are not
considered to be part of the lichen.
Reaction to water
Moisture makes the cortex become more transparent.:4 This way, the
algae can conduct photosynthesis when moisture is available, and is
protected at other times. When the cortex is more transparent, the
algae show more clearly and the lichen looks greener.
Metabolites, metabolite structures and bioactivity
Lichens can show intense antioxidant activity. Secondary
metabolites are often deposited as crystals in the apoplast.
Secondary metabolites are thought to play a role in preference for
some substrates over others.
Sometimes lichens contain structures made from fungal metabolites, for
example crustose lichens sometimes have a polysaccharide[clarification
needed] layer in the cortex.
Lichens often have a regular but very slow growth rate of less than a
millimeter per year. Different lichen species have been measured
to grow as slowly as 0.5 mm, and as fast as 0.5 meter per
In crustose lichens, the area along the margin is where the most
active growth is taking place.:159 Most crustose lichens grow only
1–2 mm in diameter per year.
Lichens may be long-lived, with some considered to be among the oldest
living organisms. Lifespan is difficult to measure because of
what defines as the "same" individual lichen is not precise.
Lichens grow by vegetatively breaking off a piece, which may or may
not be defined as the "same" lichen, and two lichens can merge, then
becoming the "same" lichen. An
Arctic species called "map lichen"
Rhizocarpon geographicum) has been dated at 8,600 years, apparently
the world's oldest living organism.
Response to environmental stress
Unlike simple dehydration in plants and animals, lichens may
experience a complete loss of body water in dry periods. Lichens
are capable of surviving extremely low levels of water content
(poikilohydric).:5–6 They quickly absorb water when it becomes
available again, becoming soft and fleshy. Reconfiguration of
membranes following a period of dehydration requires several minutes
or more.
In tests, lichen survived and showed remarkable results on the
adaptation capacity of photosynthetic activity within the simulation
time of 34 days under Martian conditions in the Mars Simulation
Laboratory (MSL) maintained by the
German Aerospace Center
German Aerospace Center (DLR).
European Space Agency
European Space Agency has discovered that lichens can survive
unprotected in space. In an experiment led by Leopoldo Sancho from the
Complutense University of Madrid, two species of lichen—Rhizocarpon
Xanthoria elegans—were sealed in a capsule and
launched on a Russian Soyuz rocket 31 May 2005. Once in orbit, the
capsules were opened and the lichens were directly exposed to the
vacuum of space with its widely fluctuating temperatures and cosmic
radiation. After 15 days, the lichens were brought back to earth and
were found to be unchanged in their ability to
Reproduction and dispersal
Xanthoparmelia sp. with dark-colored reproductive structures
(disc-like apothecia) at center, surrounded by a pale coloured
Many lichens reproduce asexually, either by a piece breaking off and
growing on its own (vegetative reproduction) or through the dispersal
of diaspores containing a few algal cells surrounded by fungal
cells. Because of the relative lack of differentiation in the
thallus, the line between diaspore formation and vegetative
reproduction is often blurred.
Fruticose lichens can easily[citation
needed] fragment, and new lichens can grow from the fragment
(vegetative reproduction). Many lichens break up into
fragments when they dry, dispersing themselves by wind action, to
resume growth when moisture returns. Soredia (singular:
"soredium") are small groups of algal cells surrounded by fungal
filaments that form in structures called soralia, from which the
soredia can be dispersed by wind. Isidia (singular: "isidium") are
branched, spiny, elongated, outgrowths from the thallus that break off
for mechanical dispersal.
Lichen propagules (diaspores) typically
contain cells from both partners, although the fungal components of
so-called "fringe species" rely instead on algal cells dispersed by
the "core species".
Disc-like apothecia (left) and thallus (right) on a foliose lichen
Structures involved in reproduction often appear as discs, bumps, or
squiggly lines on the surface of the thallus.:4 Only the fungal
partner in a lichen reproduces sexually. Many lichen fungi
reproduce sexually like other fungi, producing spores formed by
meiosis and fusion of gametes. Following dispersal, such fungal spores
must meet with a compatible algal partner before a functional lichen
Some lichen fungi belong to
Basidiomycetes (basidiolichens) and
produce mushroom-like reproductive structures resembling those of
their nonlichenized relatives.
Most lichen fungi belong to
Ascomycetes (ascolichens). Among the
ascolichens, spores are produced in spore-producing structures called
ascomata. The most common types of ascomata are the apothecium
(plural: apothecia) and perithecium (plural: perithecia).:14
Apothecia are usually cups or plate-like discs located on the top
surface of the lichen thallus. When apothecia are shaped like squiggly
line segments instead of like discs, they are called lirellae.:14
Perithecia are shaped like flasks that are immersed in the lichen
thallus tissue, which has a small hole for the spores to escape the
flask, and appear like black dots on the lichen surface.:14
The three most common spore body types are raised discs called
apothecia (singular: apothecium), bottle-like cups with a small hole
at the top called perithecia (singular: perithecium), and pycnidia
(singular: pycnidium), shaped like perithecia but without asci (an
ascus is the structure that contains and releases the sexual spores in
fungi of the Ascomycota).
The apothecium has a layer of exposed spore-producing cells called
asci (singular: ascus), and is usually a different color from the
thallus tissue.:14 When the apothecium has an outer margin, the
margin is called the exciple.:14 When the exciple has a color
similar to colored thallus tissue the apothecium or lichen is called
lecanorine, meaning similar to members of the genus Lecanora.:14
When the exciple is blackened like carbon it is called lecideine
meaning similar to members of the genus Lecidea.:14 When the margin
is pale or colorless it is called biatorine.:14
Crust-like thallus with pseudopodetia
A "podetium" (plural: podetia) is a lichenized stalk-like structure of
the fruiting body rising from the thallus, associated with some fungi
that produce a fungal apothecium. Since it is part of the
reproductive tissue, podetia are not considered part of the main body
(thallus), but may be visually prominent. The podetium may be
branched, and sometimes cup-like. They usually bear the fungal
pycnidia or apothecia or both. Many lichens have apothecia that
are visible to the naked eye.
Most lichens produce abundant sexual structures. Many species
appear to disperse only by sexual spores. For example, the
crustose lichens Graphis scripta and Ochrolechia parella produce no
symbiotic vegetative propagules. Instead, the lichen-forming fungi of
these species reproduce sexually by self-fertilization (i.e. they are
homothallic). This breeding system may enable successful reproduction
in harsh environments.
Mazaedia (singular: mazaedium) are apothecia shaped like a
dressmaker's pin in (pin lichen)s, where the fruiting body is a brown
or black mass of loose ascospores enclosed by a cup-shaped exciple,
which sits on top of a tiny stalk.:15
Taxonomy and classification
Lichens are classified by the fungal component.
Lichen species are
given the same scientific name (binomial name) as the fungus species
in the lichen.
Lichens are being integrated into the classification
schemes for fungi. The alga bears its own scientific name, which bears
no relationship to that of the lichen or fungus. There are about
13,500–17,000 identified lichen species. Nearly 20% of known
fungal species are associated with lichens.
"Lichenized fungus" may refer to the entire lichen, or to just the
fungus. This may cause confusion without context. A particular fungus
species may form lichens with different algae species, giving rise to
what appear to be different lichen species, but which are still
classified (as of 2014) as the same lichen species.
Formerly, some lichen taxonomists placed lichens in their own
division, the Mycophycophyta, but this practice is no longer accepted
because the components belong to separate lineages. Neither the
ascolichens nor the basidiolichens form monophyletic lineages in their
respective fungal phyla, but they do form several major solely or
primarily lichen-forming groups within each phylum. Even more
unusual than basidiolichens is the fungus
Geosiphon pyriforme, a
member of the
Glomeromycota that is unique in that it encloses a
cyanobacterial symbiont inside its cells.
Geosiphon is not usually
considered to be a lichen, and its peculiar symbiosis was not
recognized for many years. The genus is more closely allied to
Lichens independently emerged from fungi associating with algae and
cyanobacteria multiple times throughout history.
The fungal component of a lichen is called the mycobiont. The
mycobiont may be an
Ascomycete or Basidiomycete. The associated
lichens are called either ascolichens or basidiolichens, respectively.
Living as a symbiont in a lichen appears to be a successful way for a
fungus to derive essential nutrients since about 20% of all fungal
species have acquired this mode of life.
Thalli produced by a given fungal symbiont with its differing partners
may be similar, and the secondary metabolites
identical, indicating that the
fungus has the dominant role in determining the morphology of the
lichen. But the same mycobiont with different photobionts may also
produce very different growth forms.
Lichens are known in which
there is one fungus associated with two or even three algal species.
Although each lichen thallus generally appears homogeneous, some
evidence seems to suggest that the fungal component may consist of
more than one genetic individual of that species.
Two or more fungal species can interact to form the same lichen.
The following table lists the orders and families of fungi that
include lichen-forming species.
Taxonomy of the
Show all lichen genera
Anderson, Heidi L.; Ekman, Stefan (2005). "Disintegration of the
Micareaceae (lichenized Ascomycota): a molecular phylogeny based on
mitochondrial rDNA sequences". Mycological Research. 109 (1): 21–30.
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(2008). "A new lineage of lichenized basidiomycetes inferred from a
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Grube, Martin; Winka, Katarina (2002). "Progress in understanding the
evolution and classification of lichenized ascomycetes". Mycologist.
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Liu, Yajuan J.; Hall, Benjamin D. (2004). "Body plan evolution of
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The photosynthetic partner in a lichen is called a photobiont. The
photobionts in lichens come from a variety of simple prokaryotic and
eukaryotic organisms. In the majority of lichens the photobiont is a
green alga (Chlorophyta) or a cyanobacterium. In some lichens both
types are present.
Algal photobionts are called phycobionts, while
cyanobacterial photobionts are called cyanobionts. According to
one source, about 90% of all known lichens have phycobionts, and about
10% have cyanobionts, while another source states that two thirds
of lichens have green algae as phycobiont, and about one third have a
cyanobiont. Approximately 100 species of photosynthetic partners
from 40 genera and five distinct classes (prokaryotic:
Cyanophyceae; eukaryotic: Trebouxiophyceae, Phaeophyceae,
Chlorophyceae) have been found to associate with the lichen-forming
Common algal photobionts are from the genus Trebouxia, Trentepohlia,
Pseudotrebouxia, or Myrmecia.
Trebouxia is the most common genus of
green algae in lichens, occurring in about 40% of all lichens.
"Trebouxioid" means either a photobiont that is in the genus
Trebouxia, or resembles a member of that genus, and is therefore
presumably a member of the class Trebouxiophyceae. The second most
commonly represented green alga genus is Trentepohlia. Overall,
about 100 species of eukaryotes are known to occur as photobionts in
lichens. All the algae are probably able to exist independently in
nature as well as in the lichen.
A "cyanolichen" is a lichen with a cyanobacterium as its main
photosynthetic component (photobiont). The most commonly occurring
cyanobacterium genus is Nostoc. Other common cyanobacterium
photobionts are from Scytonema. Many cyanolichens are small and
black, and have limestone as the substrate. Another
cyanolichen group, the jelly lichens of the genera
Leptogium are gelatinous and live on moist soils. Another group of
large and foliose species including Peltigera, Lobaria, and Degelia
are grey-blue, especially when dampened or wet. Many of these
characterize the Lobarion communities of higher rainfall areas in
western Britain, e.g., in the Celtic rain forest. Strains of
cyanobacteria found in various cyanolichens are often closely related
to one another. They differ from the most closely related
The lichen association is a close symbiosis. It extends the ecological
range of both partners but is not always obligatory for their growth
and reproduction in natural environments, since many of the algal
symbionts can live independently. A prominent example is the alga
Trentepohlia, which forms orange-coloured populations on tree trunks
and suitable rock faces.
Lichen propagules (diaspores) typically
contain cells from both partners, although the fungal components of
so-called "fringe species" rely instead on algal cells dispersed by
the "core species".
The same cyanobiont species can occur in association with different
fungal species as lichen partners. The same phycobiont species can
occur in association with different fungal species as lichen
partners. More than one phycobiont may be present in a single
Although each lichen thallus generally appears homogeneous, some
evidence seems to suggest that the photobiont component may consist of
more than one genetic individual of that species. A
single lichen may contain several algal genotypes. These
multiple genotypes may better enable response to adaptation to
environmental changes, and enable the lichen to inhabit a wider range
Controversy over classification method and species names
There are about 20,000 known lichen species. But what is meant by
"species" is different from what is meant by biological species in
plants, animals, or fungi, where being the same species implies that
there is a common ancestral lineage. Because lichens are
combinations of members of two or even three different biological
kingdoms, these components must have a different ancestral lineage
from each other. By convention, lichens are still called "species"
anyway, and are classified according to the species of their fungus,
not the species of the algae or cyanobacteria.
Lichens are given the
same scientific name (binomial name) as the fungus in them, which may
cause some confusion. The alga bears its own scientific name, which
has no relationship to the name of the lichen or fungus.
Depending on context, "lichenized fungus" may refer to the entire
lichen, or to the fungus when it is in the lichen, which can be grown
in culture in isolation from the algae or cyanobacteria. Some algae
and cyanobacteria are found naturally living outside of the lichen.
The fungal, algal, or cyanobacterial component of a lichen can be
grown by itself in culture. When growing by themselves, the fungus,
algae, or cyanobacteria have very different properties than those of
Lichen properties such as growth form, physiology, and
biochemistry, are very different from the combination of the
properties of the fungus and the algae or cyanobacteria.
The same fungus growing in combination with different algae or
cyanobacteria, can produce lichens that are very different in most
properties, meeting non-DNA criteria for being different "species".
Historically, these different combinations were classified as
different species. When the fungus is identified as being the same
using modern DNA methods, these apparently different species get
reclassified as the same species under the current (2014) convention
for classification by fungal component. This has led to debate about
this classification convention. These apparently different "species"
have their own independent evolutionary history.
There is also debate as to the appropriateness of giving the same
binomial name to the fungus, and to the lichen that combines that
fungus with an alga or cyanobacterium (synecdoche). This is especially
the case when combining the same fungus with different algae or
cyanobacteria produces dramatically different lichen organisms, which
would be considered different species by any measure other than the
DNA of the fungal component. If the whole lichen produced by the same
fungus growing in association with different algae or cyanobacteria,
were to be classified as different "species", the number of "lichen
species" would be greater.
The largest number of lichenized fungi occur in the Ascomycota, with
about 40% of species forming such an association. Some of these
lichenized fungi occur in orders with nonlichenized fungi that live as
saprotrophs or plant parasites (for example, the Leotiales,
Dothideales, and Pezizales). Other lichen fungi occur in only five
orders in which all members are engaged in this habit (Orders
Graphidales, Gyalectales, Peltigerales, Pertusariales, and
Teloschistales). Lichenized and nonlichenized fungi can even be found
in the same genus or species.
Overall, about 98% of lichens have an ascomycetous mycobiont.[citation
needed] Next to the Ascomycota, the largest number of lichenized fungi
occur in the unassigned fungi imperfecti, a catch-all category for
fungi whose sexual form of reproduction has never been
observed. Comparatively few
lichenized, but these include agarics, such as species of
Lichenomphalia, clavarioid fungi, such as species of Multiclavula, and
corticioid fungi, such as species of Dictyonema.
Lichen identification uses growth form and reactions to chemical
"Pd" refers to the outcome of the
Pd test or is used as an
abbreviation for the chemical used in the test,
para-phenylenediamine. If putting a drop on a lichen turns an area
bright yellow to orange, this helps identify it as belonging to either
Cladonia or Lecanora.
Evolution and paleontology
The fossil record for lichens is poor. The extreme habitats that
lichens dominate, such as tundra, mountains, and deserts, are not
ordinarily conducive to producing fossils. There are
fossilized lichens embedded in amber. The fossilized Anzia is found in
pieces of amber in northern Europe and dates back approximately 40
Lichen fragments are also found in fossil leaf
beds, such as
Lobaria from Trinity County in northern California, USA,
dating back to the early to middle Miocene.
The oldest fossil lichens in which both symbiotic partners have been
recovered date to the Early
Devonian Rhynie chert, about 400 million
years old. The slightly older fossil
Spongiophyton has also been
interpreted as a lichen on morphological and isotopic grounds,
although the isotopic basis is decidedly shaky. It has been
demonstrated that Silurian-
Devonian fossils Nematothallus and
Prototaxites were lichenized. Thus lichenized
Basidiomycota were a component of early Silurian-
The ancestral ecological state of both
Ascomycota and Basidiomycota
was probably saprobism, and independent lichenization events may have
occurred multiple times. In 1995, Gargas and colleagues proposed
that there were at least five independent origins of lichenization;
three in the basidiomycetes and at least two in the Ascomycetes.
However, Lutzoni et al. (2001) indicate that lichenization probably
evolved earlier and was followed by multiple independent losses. Some
non-lichen-forming fungi may have secondarily lost the ability to form
a lichen association. As a result, lichenization has been viewed as a
highly successful nutritional strategy.
Glomeromycota may extend well back into the Precambrian.
Winfrenatia, an early zygomycetous (Glomeromycota) lichen symbiosis
that may have involved controlled parasitism, is
permineralized in the
Rhynie Chert of Scotland, of early Devonian
age. Lichen-like fossils consisting of coccoid cells
(cyanobacteria?) and thin filaments (mucoromycotinan Glomeromycota?)
are permineralized in marine phosphorite of the Doushantuo Formation
in southern China. These fossils are thought to be 551 to 635 million
years old or Ediacaran.
Ediacaran acritarchs also have many
similarities with Glomeromycotan vesicles and spores. It has also
been claimed that
Ediacaran fossils including Dickinsonia, were
lichens, although this claim is controversial. Endosymbiotic
Glomeromycota comparable with living
Geosiphon may extend back into
Proterozoic in the form of 1500 million year old Horodyskia
and 2200 million year old Diskagma. Discovery of these fossils
suggest that fungi developed symbiotic partnerships with
photoautotrophs long before the evolution of vascular plants.
Ecology and interactions with environment
Substrates and habitats
Lichens on a statue made of limestone on a tower of Regensburg
Lichens grow in a wide range of substrates and habitats, including
some of the most extreme conditions on earth. They are abundant
growing on bark, leaves, and hanging from branches "living on thin
air" (epiphytes) in rain forests and in temperate woodland. They grow
on bare rock, walls, gravestones, roofs, exposed soil surfaces. They
can survive in some of the most extreme environments on Earth: arctic
tundra, hot dry deserts, rocky coasts, and toxic slag heaps. They can
even live inside solid rock, growing between the grains, and in the
soil as part of a biological soil crust in arid habitats such as
deserts. Some lichens do not grow on anything, living out their lives
blowing about the environment.
When growing on mineral surfaces, some lichens slowly decompose their
substrate by chemically degrading and physically disrupting the
minerals, contributing to the process of weathering by which rocks are
gradually turned into soil. While this contribution to weathering is
usually benign, it can cause problems for artificial stone structures.
For example, there is an ongoing lichen growth problem on Mount
Rushmore National Memorial that requires the employment of
mountain-climbing conservators to clean the monument.
Lichens are not parasites on the plants they grow on, but only use
them as a substrate to grow on. The fungi of some lichen species may
"take over" the algae of other lichen species.
their own food from their photosynthetic parts and by absorbing
minerals from the environment.
Lichens growing on leaves may have
the appearance of being parasites on the leaves, but they are not.
However, some lichens, notably those of the genus Diploschistes are
known to parasitise other lichens. Diploschistes muscorum starts its
development in the tissue of a host
In the arctic tundra, lichens, together with mosses and liverworts,
make up the majority of the ground cover, which helps insulate the
ground and may provide forage for grazing animals. An example is
Reindeer moss", which is a lichen, not a moss.
A crustose lichen that grows on rock is called a saxicolous
Crustose lichens that grow on the rock are
epilithic, and those that grow immersed inside rock, growing between
the crystals with only their fruiting bodies exposed to the air, are
called endolithic lichens.:159 A crustose lichen that
grows on bark is called a corticolous lichen.:159 A lichen that
grows on wood from which the bark has been stripped is called a
Lichens that grow immersed inside plant
tissues are called endophloidic lichens or endophloidal
Lichens that use leaves as substrates, whether
the leaf is still on the tree or on the ground, are called epiphyllous
or foliicolous. A terricolous lichen grows on the soil as a
substrate. Many squamulous lichens are terricolous.:159
Umbillicate lichens are foliose lichens that are attached to the
substrate at only one point. A vagrant lichen is not attached to a
substrate at all, and lives its life being blown around by the wind.
Lichens and soils
In addition to distinct physical mechanisms by which lichens break
down raw stone, recent studies indicate lichens attack stone
chemically, entering newly chelated minerals into the ecology.
The lichen exudates, which have powerful chelating capacity, the
widespread occurrence of mineral neoformation, particularly metal
oxalates, together with the characteristics of weathered substrates,
all confirm the significance of lichens as chemical weathering
Over time, this activity creates new fertile soil from lifeless stone.
Lichens may be important in contributing nitrogen to soils in some
deserts through being eaten, along with their rock substrate, by
snails, which then defecate, putting the nitrogen into the soils.
Lichens help bind and stabilize soil sand in dunes. In deserts and
semi-arid areas, lichens are part of extensive, living biological soil
crusts, essential for maintaining the soil structure.
a long fossil record in soils dating back 2.2 billion years.
Lichens are pioneer species, among the first living things to grow on
bare rock or areas denuded of life by a disaster.
Lichens may have
to compete with plants for access to sunlight, but because of their
small size and slow growth, they thrive in places where higher plants
have difficulty growing.
Lichens are often the first to settle in
places lacking soil, constituting the sole vegetation in some extreme
environments such as those found at high mountain elevations and at
high latitudes. Some survive in the tough conditions of deserts,
and others on frozen soil of the
A major ecophysiological advantage of lichens is that they are
poikilohydric (poikilo- variable, hydric- relating to water), meaning
that though they have little control over the status of their
hydration, they can tolerate irregular and extended periods of severe
desiccation. Like some mosses, liverworts, ferns, and a few
"resurrection plants", upon desiccation, lichens enter a metabolic
suspension or stasis (known as cryptobiosis) in which the cells of the
lichen symbionts are dehydrated to a degree that halts most
biochemical activity. In this cryptobiotic state, lichens can survive
wider extremes of temperature, radiation and drought in the harsh
environments they often inhabit.
Lichens suppress the growth of mosses and higher plants around them
Lichens do not have roots and do not need to tap continuous reservoirs
of water like most higher plants, thus they can grow in locations
impossible for most plants, such as bare rock, sterile soil or sand,
and various artificial structures such as walls, roofs and monuments.
Many lichens also grow as epiphytes (epi- on the surface, phyte-
plant) on plants, particularly on the trunks and branches of trees.
When growing on plants, lichens are not parasites; they do not consume
any part of the plant nor poison it.
Lichens produce allelopathic
chemicals that inhibit the growth of mosses. Some ground-dwelling
lichens, such as members of the subgenus Cladina (reindeer lichens),
produce allelopathic chemicals that leach into the soil and inhibit
the germination of seeds, spruce and other plants. Stability
(that is, longevity) of their substrate is a major factor of lichen
habitats. Most lichens grow on stable rock surfaces or the bark of old
trees, but many others grow on soil and sand. In these latter cases,
lichens are often an important part of soil stabilization; indeed, in
some desert ecosystems, vascular (higher) plant seeds cannot become
established except in places where lichen crusts stabilize the sand
and help retain water.
Lichens may be eaten by some animals, such as reindeer, living in
arctic regions. The larvae of a number of
Lepidoptera species feed
exclusively on lichens. These include
Common Footman and Marbled
Beauty. However, lichens are very low in protein and high in
carbohydrates, making them unsuitable for some animals.
also used by the
Northern Flying Squirrel
Northern Flying Squirrel for nesting, food, and a
water source during winter.
Effects of air pollution
Some lichens, like the foliose
Lobaria pulmonaria, are sensitive to
If lichens are exposed to air pollutants at all times, without any
deciduous parts, they are unable to avoid the accumulation of
pollutants. Also lacking stomata and a cuticle, lichens may absorb
aerosols and gases over the entire thallus surface from which they may
readily diffuse to the photobiont layer. Because lichens do not
possess roots, their primary source of most elements is the air, and
therefore elemental levels in lichens often reflect the accumulated
composition of ambient air. The processes by which atmospheric
deposition occurs include fog and dew, gaseous absorption, and dry
deposition. Consequently, many environmental studies with lichens
emphasize their feasibility as effective biomonitors of atmospheric
Not all lichens are equally sensitive to air pollutants, so different
lichen species show different levels of sensitivity to specific
atmospheric pollutants. The sensitivity of a lichen to air
pollution is directly related to the energy needs of the mycobiont, so
that the stronger the dependency of the mycobiont on the photobiont,
the more sensitive the lichen is to air pollution. Upon exposure
to air pollution, the photobiont may use metabolic energy for repair
of its cellular structures that would otherwise be used for
maintenance of its photosynthetic activity, therefore leaving less
metabolic energy available for the mycobiont. The alteration of the
balance between the photobiont and mycobiont can lead to the breakdown
of the symbiotic association. Therefore, lichen decline may result not
only from the accumulation of toxic substances, but also from altered
nutrient supplies that favor one symbiont over the other.
This interaction between lichens and air pollution has been used as a
means of monitoring air quality since 1859, with more systematic
methods developed by William Nylander in 1866.
Further information: Ethnolichenology
Iwatake (Umbilicaria esculenta) gathering at Kumano in Kishū, by
Lichens are eaten by many different cultures across the world.
Although some lichens are only eaten in times of famine, others are a
staple food or even a delicacy. Two obstacles are often encountered
when eating lichens: lichen polysaccharides are generally indigestible
to humans, and lichens usually contain mildly toxic secondary
compounds that should be removed before eating. Very few lichens are
poisonous, but those high in vulpinic acid or usnic acid are
toxic. Most poisonous lichens are yellow.
In the past
Iceland moss (Cetraria islandica) was an important human
food in northern Europe, and was cooked as a bread, porridge, pudding,
soup, or salad. Wila (Bryoria fremontii) was an important food in
parts of North America, where it was usually pitcooked. Northern
peoples in North America and Siberia traditionally eat the partially
digested reindeer lichen (Cladina spp.) after they remove it from the
rumen of caribou or reindeer that have been killed. Rock tripe
(Umbilicaria spp. and Lasalia spp.) is a lichen that has frequently
been used as an emergency food in North America, and one species,
Umbilicaria esculenta, is used in a variety of traditional Korean and
Main article: Lichenometry
Lichenometry is a technique used to determine the age of exposed rock
surfaces based on the size of lichen thalli. Introduced by Beschel in
the 1950s, the technique has found many applications. it is used
in archaeology, palaeontology, and geomorphology. It uses the presumed
regular but slow rate of lichen growth to determine the age of exposed
rock.:9 Measuring the diameter (or other size measurement) of
the largest lichen of a species on a rock surface indicates the length
of time since the rock surface was first exposed.
Lichen can be
preserved on old rock faces for up to 10,000 years,
providing the maximum age limit of the technique, though it is most
accurate (within 10% error) when applied to surfaces that have been
exposed for less than 1,000 years.
Lichenometry is especially
useful for dating surfaces less than 500 years old, as radiocarbon
dating techniques are less accurate over this period. The lichens
most commonly used for lichenometry are those of the genera
Rhizocarpon (e.g. the species
Rhizocarpon geographicum) and Xanthoria.
Lichens have been shown to degrade polyester resins, as can be seen in
archaeological sites in the Roman city of
Baelo Claudia in Spain.
Lichens can accumulate several environmental pollutants such as lead,
copper, and radionuclides.
Many lichens produce secondary compounds, including pigments that
reduce harmful amounts of sunlight and powerful toxins that reduce
herbivory or kill bacteria. These compounds are very useful for lichen
identification, and have had economic importance as dyes such as
cudbear or primitive antibiotics.
The pH indicator (indicated acidic or basic) in the litmus test is a
dye extracted from the lichen
Roccella tinctoria by boiling.
In the Highlands of Scotland, traditional dyes for Harris tweed and
other traditional cloths were made from lichens including the orange
Xanthoria parietina and the grey foliaceous
Parmelia saxatilis common
on rocks known as "crottle".
There are reports dating almost 2000 years old of lichens being used
to make purple and red dyes. Of great historical and commercial
significance are lichens belonging to the family Roccellaceae,
commonly called orchella weed or orchil.
Orcein and other lichen dyes
have largely been replaced by synthetic versions.
Traditional medicine and research
Historically in traditional medicine of Europe,
Lobaria pulmonaria was
collected in large quantities as "Lungwort", due to its lung-like
appearance (the doctrine of signatures suggesting that herbs can treat
body parts that they physically resemble). Similarly, Peltigera
leucophlebia was used as a supposed cure for thrush, due to the
resemblance of its cephalodia to the appearance of the disease.
Lichens produce metabolites in research for their potential
therapeutic or diagnostic value. Some metabolites produced by
lichens are structurally and functionally similar to broad-spectrum
antibiotics while few are associated respectively to antiseptic
Usnic acid is the most commonly studied metabolite
produced by lichens. It is also under research as an bactericidal
Escherichia coli and Staphylococcus aureus.
Pine forest with
Cladonia lichen ground-cover
Colonies of lichens may be spectacular in appearance, dominating the
surface of the visual landscape as part of the aesthetic appeal to
paying visitors of
Yosemite National Park
Yosemite National Park and Sequoia National
Park.:2 Orange and yellow lichens add to the ambience of desert
trees, rock faces, tundras, and rocky seashores. Intricate webs of
lichens hanging from tree branches add a mysterious aspect to forests.
Fruticose lichens are used in model railroading and other
modeling hobbies as a material for making miniature trees and shrubs.
Midrashic literature, the Hebrew word "vayilafeth" in Ruth
3:8 is explained as referring to Ruth entwining herself around Boaz
like lichen. The tenth century Arab physician, Al-Tamimi,
mentions lichens dissolved in vinegar and rose water being used in his
day for the treatment of skin diseases and rashes.
The plot of John Wyndham's novel
Trouble with Lichen
Trouble with Lichen revolves around
an anti-aging chemical extracted from a lichen.
"Lichenes" fancifully drawn by
Ernst Haeckel to emphasize his ideas of
symmetry in his Artforms of Nature, 1904
Although lichens had been recognized as organisms for quite some time,
it was not until 1867, when Swiss botanist
Simon Schwendener proposed
his dual theory of lichens, that lichens are a combination of fungi
with algae or cyanobacteria, whereby the true nature of the lichen
association began to emerge. Schwendener's hypothesis, which at
the time lacked experimental evidence, arose from his extensive
analysis of the anatomy and development in lichens, algae, and fungi
using a light microscope. Many of the leading lichenologists at the
time, such as James Crombie and Nylander, rejected Schwendener's
hypothesis because the common consensus was that all living organisms
Other prominent biologists, such as Heinrich Anton de Bary, Albert
Melchior Treub and
Hermann Hellriegel were not so
quick to reject Schwendener's ideas and the concept soon spread into
other areas of study, such as microbial, plant, animal and human
pathogens. When the complex relationships between pathogenic
microorganisms and their hosts were finally identified, Schwendener's
hypothesis began to gain popularity. Further experimental proof of the
dual nature of lichens was obtained when Eugen Thomas published his
results in 1939 on the first successful re-synthesis experiment.
In the 2010s, a new facet of the fungi-algae partnership was
discovered. Toby Spribille and colleagues found that many types of
lichen that were long thought to be ascomycete-algae pairs were
actually ascomycete-basidiomycete-algae trios.
Lobaria pulmonaria, tree lungwort, lung lichen, lung moss; Upper
Cladonia macilenta var. bacillaris 'Lipstick Cladonia'
Usnea australis, a fruticose form, growing on a tree branch
Hypogymnia cf. tubulosa with Bryoria sp. and Tuckermannopsis sp. in
the Canadian Rockies
Letharia sp. with Bryoria sp. on pine branches near Blackpine Lake,
Lobaria oregana, commonly called 'Lettuce lichen', in the Hoh
Rainforest, Washington State
Xanthoparmelia cf. lavicola, a foliose lichen, on basalt.
Map lichen (
Rhizocarpon geographicum) on rock
Physcia millegrana (a foliose lichen), with an unlichenized polypore
fungus (bottom right), on a fallen log.
Reindeer moss (
Crustose lichens on limestone in Alta Murgia-Southern Italy
Cladonia cf. cristatella, a lichen commonly referred to as 'British
Soldiers'. Notice the red tips.
A crusty crustose lichen on a wall
Lichen on a lilac bush
Foliose lichens on rock growing outward and dying in the center. These
lichens are at least several decades old.
Xanthoria sp. lichen on volcanic rock in Craters of the Moon National
Monument (Idaho, USA)
Lecanora cf. muralis lichen on the banks of the Bega canal in
Microscopic view of lichen growing on a piece of concrete dust.[a]
^ This was scraped from a dry, concrete-paved section of a drainage
ditch. This entire image covers a square that is approximately 1.7
millimeters on a side. The numbered ticks on the scale represent
distances of 230 micrometers, or slightly less than 0.25 millimeter.
^ a b c d Spribille, Toby; Tuovinen, Veera; Resl, Philipp; Vanderpool,
Dan; Wolinski, Heimo; Aime, M. Catherine; Schneider, Kevin;
Stabentheiner, Edith; Toome-Heller, Merje (2016-07-21). "Basidiomycete
yeasts in the cortex of ascomycete macrolichens". Science. 353:
488–92. Bibcode:2016Sci...353..488S. doi:10.1126/science.aaf8287.
ISSN 0036-8075. PMID 27445309.
^ a b c d e f g h i j k l m n o p "What is a lichen?". Australian
National Botanic Gardens. Retrieved 10 October 2014.
^ Introduction to
Lichens – An Alliance between Kingdoms. University
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"Lichens". New International Encyclopedia. 1905.
"Lichens". Encyclopædia Britannica (11th ed.). 1911.
"Lichens". Encyclopedia Americana. 1920.