Lichen Cross Section Diagram

A lichen ( , ) is a composite organism that arises from algae or cyanobacteria living among filaments of multiple fungi species in a mutualistic relationship.Introduction to Lichens – An Alliance between Kingdoms
. University of California Museum of Paleontology. Lichens have properties different from those of their component organisms. They come in many colors, sizes, and forms and are sometimes plant-like, but are not plants. They may have tiny, leafless branches (fruticose); flat leaf-like structures (foliose); grow crust-like, adhering tightly to a surface (substrate) like a thick coat of paint (crustose); have a powder-like appearance ( leprose); or other growth forms.

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 they are not closely related to mosses or any plant.Brodo, Irwin M. and Duran Sharnoff, Sylvia (2001) ''Lichens of North America''. . Lichens do not have roots that absorb water and nutrients as plants do,Sharnoff, Stephen (2014) ''Field Guide to California Lichens'', Yale University Press. but like plants, they produce their own nutrition by photosynthesis. When they grow on plants, they do not live as parasites, but instead use the plant's surface as a substrate.

Lichens occur from sea level to high alpine elevations, in many environmental conditions, and can grow on almost any surface. They are abundant growing on bark, leaves, mosses, or other lichens and hanging from branches "living on thin air" (epiphytes) in rainforests and in temperate woodland. They grow on rock, walls, gravestones, roofs, exposed soil surfaces, rubber, bones, and in the soil as part of biological soil crusts. Various 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 6–8% of Earth's land surface is covered by lichens. There are about 20,000 known species. Some lichens have lost the ability to reproduce sexually, yet continue to speciate. They 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. The long life-span and slow and regular growth rate of some species can be used to date events (lichenometry).

Etymology and pronunciation

The English word ''lichen'' derives from the Greek ("tree moss, lichen, lichen-like eruption on skin") via Latin . The Greek noun, which literally means "licker", derives from the verb , "to lick". In American English, "lichen" is pronounced the same as the verb "liken" (). In British English, both this pronunciation and one rhyming with "kitchen" () are used.

Anatomy and morphology

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, are called the ''thallus''. Lichens are grouped by thallus type, since the thallus is usually the most visually prominent part of the lichen. 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
# structureless

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, 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 squamulose.

These growth form groups are not precisely defined. Foliose lichens may sometimes branch and appear to be fruticose. Fruticose lichens may have flattened branching parts and appear leafy. Squamulose lichens may appear where the edges lift up. Gelatinous lichens may appear leafy when dry. 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. 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.Lichen Vocabulary, Lichens of North America Information, Sylvia and Stephen Sharnoff

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.

Color

Lichens come in many colors. 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 Park.

Color is used in identification.Michigan Lichens, Julie Jones Medlin, B. Jain Publishers, 1996, , 9780877370390

The color of a lichen changes depending on whether the lichen is wet or dry. Color descriptions used for identification are based on the color that shows 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 foliose 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 created by mucilaginous secretions.

Internal structure

A lichen consists of a simple photosynthesizing organism, usually a green alga or cyanobacterium, surrounded by filaments of a fungus. Generally, most of a lichen's bulk is made of interwoven fungal filaments,Lichens: More on Morphology, University of California Museum of Paleontology

but this is reversed in filamentous and gelatinous lichens. The fungus is called a ''mycobiont''. The photosynthesizing organism is called a ''photobiont''. Algal photobionts are called ''phycobionts''. Cyanobacteria photobionts are called ''cyanobionts''.

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 " anastomosed". 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