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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 ,  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. Lichens occur from sea level to high alpine elevations, in many environmental conditions, and can grow on almost any surface.
Section 4 2 What Shapes An Ecosystem Worksheet Answers
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. Lichens may be long-lived , with some considered to be among the oldest living things. The long life-span and slow and regular growth rate of some lichens can be used to date events lichenometry. Like the word moss , the word lichen is also used as an uncountable noun , as in, "Lichen grows on rocks".
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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. 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. 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. 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.
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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. Structures involved in reproduction often appear as discs, bumps, or squiggly lines on the surface of the thallus. Some lichens can grow inside solid rock between the grains endolithic lichens , with only the sexual fruiting part visible growing outside the rock.
Lichens come in many colors. Different colored lichens may inhabit different adjacent sections of a rock face, depending on the angle of exposure to light. Color is used in identification. 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 from mucilaginous secretions. 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,  although in filamentous and gelatinous lichens  this is not the case.
The fungus is called a mycobiont. The photosynthesizing organism is called a photobiont. Algal photobionts are called phycobionts.
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. Fruticose, foliose, crustose, and squamulose lichens generally have up to three different types of tissue, differentiated by having different densities of fungal filaments.
Below the cortex layer is a layer called the photobiontic layer or symbiont layer. In fruticose lichens, the photobiontic layer is sharply distinct from the layer below.
The layer beneath the symbiont layer called is called the medulla.
Chapter 4-2 What Shapes an Ecosystem?
The medulla is less densely packed with fungal filaments than the layers above. In foliose lichens, there is usually, as in Peltigera ,  : another densely packed layer of fungal filaments called the lower cortex. 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. Gelatinous,  : byssoid, and leprose lichens lack a cortex are ecorticate , and generally have only undifferentiated tissue, similar to only having a symbiont layer.
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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. In August , it was reported that macrolichens have more than one species of fungus in their tissues.
A lichen is a composite organism that emerges from algae or cyanobacteria living among the filaments hyphae of the 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. In three different lineages the fungal partner has independently lost the mitochondrial gene atp9, which has key functions in mitochondrial energy production. The loss makes the fungi completely dependent on their symbionts. 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. It appears many, probably the majority, of lichen also live in a symbiotic relationship with an order of basidiomycete yeasts called Cyphobasidiales. The absence of this third partner could explain the difficulties of growing lichen in the laboratory.
The yeast cells are responsible for the formation of the characteristic cortex of the lichen thallus, and could also be important for its shape. 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.
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.
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 geographic locations. At least one form of lichen, the North American beard-like lichens, are constituted of not two but three symbiotic partners: an ascomycetous fungus, a photosynthetic alga, and, unexpectedly, a basidiomycetous yeast.
Phycobionts can have a net output of sugars with only water vapor. 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. 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. 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. In many species the fungus penetrates the algal cell wall,  forming penetration pegs haustoria similar to those produced by pathogenic fungi that feed on a host.
Symbiosis in lichens is so well-balanced that lichens have been considered to be relatively self-contained miniature ecosystems in and of themselves.
Many lichens are very sensitive to environmental disturbances and can be used to cheaply  assess 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. If air is very badly polluted with sulphur dioxide there may be no lichens present, just green algae may be found.