Lichen A lichen ( , ) is a composite organism that arises from algae or cyanobacteria living among filaments of multiple fungi species in a mutualistic relationship.composite Composite or compositing may refer to: Materials * Composite material, a material that is made from several different substances ** Metal matrix composite, composed of metal and other parts ** Cermet, a composite of ceramic and metallic materials ...

organisms made up of multiple species: a fungal partner, one or more

photosynthetic Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's activities. Some of this chemical energy is stored in ...

partners, and sometimes a basidiomycete yeast. They are regularly grouped by their external appearance – a characteristic known as their growth form. Lichenologists have described a dozen of these forms: areolate, byssoid, calicioid, cladoniform, crustose, filamentous, foliose, fruticose, gelatinous, leprose, placoidioid and squamulose. Of these, crustose, foliose and fruticose are the most commonly encountered. With the exception of calicioid lichens, growth forms are based on the appearance of the

thallus Thallus (plural: thalli), from Latinized Greek (), meaning "a green shoot" or "twig", is the vegetative tissue of some organisms in diverse groups such as algae, fungi, some liverworts, lichens, and the Myxogastria. Many of these organisms ...

, which is the vegetative (non-reproductive) part of the lichen. In most species, this form is determined by the lichen's fungal partner, though in a small number, it is instead the

photobiont A lichen ( , ) is a composite organism that arises from algae or cyanobacteria living among filaments of multiple fungi species in a mutualistic relationship.cortex, may be found on all external surfaces, or only on the lichen's upperside. It is not found at all in byssoid, gelatinous or leprose lichens. Lichens with a cortex on at least one surface are called ''corticate'', while those without a cortex are called ''ecorticate''. While most lichens grow on the surface of a substrate, some crustose lichens are instead partially or completely embedded within it. Those growing within the upper layers of rock are called ''endolithic'', while those growing within the upper layers of bark are called ''endophloeodal''. Lichenologists split lichens into macrolichens and microlichens. Fruticose and foliose lichens are classified as macrolichens, while crustose and squamulose lichens are classified as microlichens. This has to do with the size of the lichen's structures rather than the size of the colonies themselves. Structures on microlichens typically need to be magnified (i.e. with a

compound microscope The optical microscope, also referred to as a light microscope, is a type of microscope that commonly uses visible light and a system of lenses to generate magnified images of small objects. Optical microscopes are the oldest design of microsc ...

) in order to be identified. Growth form groupings are not always consistent with lichen

taxonomy Taxonomy is the practice and science of categorization or classification. A taxonomy (or taxonomical classification) is a scheme of classification, especially a hierarchical classification, in which things are organized into groups or types. ...

; lichens with similar growth forms are not necessarily related, and some of those which are related do not have similar growth forms. Not every species can be easily categorized. Some show characteristics of two growth forms, and different authors may place such species in different groups. In general, a particular species shows same overall growth form wherever it is found, but this is not always the case.

Areolate

An areolate lichen is the most common form of

crustose lichen Crustose lichens are lichens that form a crust which strongly adheres to the substrate (soil, rock, tree bark, etc.), making separation from the substrate impossible without destruction. The basic structure of crustose lichens consists of a co ...

. As with all crustose lichens, it has a paint-like appearance, and is inseparable from the substrate on which it grows. However, its thallus is broken into regular

polygonal In geometry, a polygon () is a plane figure that is described by a finite number of straight line segments connected to form a closed ''polygonal chain'' (or ''polygonal circuit''). The bounded plane region, the bounding circuit, or the two to ...

sections, which can look a bit like cracked mud, flaking paint or little islands. These sections, known as areolae, are surrounded by a thin layer of fungal hyphae called a hypothallus. This layer, which is usually dark, generally grows faster than the thallus which rides above it. This growth form is an adaptation which allows the lichen to cope with alternating periods of wet and dry. During wet periods, the lichen can absorb water, its tissues can swell, and the cracks close.

Byssoid

A byssoid lichen has a wispy, cottony or

teased wool Carding is a mechanical process that disentangles, cleans and intermixes fibres to produce a continuous web or sliver suitable for subsequent processing. This is achieved by passing the fibres between differentially moving surfaces covered wit ...

appearance due to the loosely woven hyphae in its thallus. It has no outer cortex. Lichens with this growth type can be split into two types. In one type, the thallus is dominated by fungal hyphae, with a photobiont – typically a coccoid green alga – sprinkled throughout. In the other, the thallus is dominated by photobiont filaments which have a thin fungal coating. Byssoid lichens are not particularly common, but they occur across a range of orders and families. Though they are found in a variety of habitats, they appear to be most common in rainforests. Byssoid lichens typically grow in areas of high humidity, on surfaces where they have no direct contact with rainfall or running water – areas such as cactus spines in fog oases or the underside of branches in rainforests. Their loose thallus structure and lack of a cortex may allow them to absorb water vapor directly from the air. Some lichenologists consider byssoid lichens to be a specialised type of fruticose lichen.

Calicioid

Unlike the other growth forms detailed here, a calicioid lichen is distinguished by its

fruiting bodies The sporocarp (also known as fruiting body, fruit body or fruitbody) of fungi is a multicellular structure on which spore-producing structures, such as basidia or asci, are borne. The fruitbody is part of the sexual phase of a fungal life cyc ...

rather than its thallus. Members of the order

Caliciales Caliciales is an order of mostly lichenized fungi in the class Lecanoromycetes. It consists of two families: Caliciaceae and Physciaceae, which together contain 54 genera and more than 1200 species. The order was circumscribed by American botani ...

, they are commonly known as "stubble lichens" or "pin lichens". In these lichens, mature

spore In biology, a spore is a unit of sexual or asexual reproduction that may be adapted for dispersal and for survival, often for extended periods of time, in unfavourable conditions. Spores form part of the life cycles of many plants, algae, ...

s build up in a thick layer on the surface of the fruiting bodies. This layer, called a , is typically brown or black, and spores are dispersed passively from it. Most calicioid lichens are crustose with tiny stalked fruiting bodies. However, because the fundamental characteristic of a calicioid lichen is the presence of a mazaedium rather than a stalked fruiting body, a handful of

fruticose lichen A fruticose lichen is a form of lichen fungi that is characterized by a coral-like shrubby or bushy growth structure. It is formed from a symbiotic relationship of a photobiont such as green algae or less commonly cyanobacteria and one, two or m ...

s also fall into this category. Calicioid lichens are generally restricted to

old-growth forest An old-growth forestalso termed primary forest, virgin forest, late seral forest, primeval forest, or first-growth forestis a forest that has attained great age without significant disturbance, and thereby exhibits unique ecological feature ...

, and can be used as indicators of the age and quality of such ecosystems.

Cladoniform

A cladoniform lichen is one with a dimorphic thallus. The form is named for the genus '' Cladonia'', as most lichens in this genus show a combination of two growth types:

squamulose A squamulose lichen is a lichen that is composed of small, often overlapping "scales" called squamules. If they are raised from the substrate and appear leafy, the lichen may appear to be a foliose lichen, but the underside does not have a "sk ...

and

fruticose A fruticose lichen is a form of lichen fungi that is characterized by a coral-like shrubby or bushy growth structure. It is formed from a symbiotic relationship of a photobiont such as green algae or less commonly cyanobacteria and one, two ...

. The primary thallus is composed of small, overlapping scales, while the secondary thallus (which supports the lichen's fruiting structures) is fruticose in appearance. These secondary thalli, which are known as '' podetia'', can be branched, spike-like or cup-shaped.

Crustose

A crustose lichen, as its name suggests, is crust-like and two-dimensional, closely and completely bound at nearly all points to the substrate on which it grows. It typically cannot be removed from the substrate without at least partial destruction of one or the other. Some crustose lichens are thick and lumpy, others thin and smooth, and some are almost completely submerged into the substrate with only apothecia emerging to the surface. Crustose lichens lack a lower cortex, though most have an upper cortex. The layer lies just below the upper cortex. Many crustose lichens have a ring of unlichenised fungal hyphae at their edges. This fringe, known as a , may be black, white or the same colour as the rest of the thallus.

Filamentous

Unlike most of the other forms detailed here (the exception being some byssoid lichens), a filamentous lichen's morphology is determined by its algal partner rather than its fungal partner. A thin layer of fungal hyphae surrounds an algal chain, resulting in a thread-like or hair-like structure. Because of their high surface to mass ratio they can quickly absorb moisture, enabling them to take advantage of even short periods of high humidity (such as fog or dew). They are often epiphytic, growing on trees in forested areas, but are also common in some alpine zones. Lichenologists tend to consider filamentous lichens to be a type of fruticose lichen. This is an uncommon growth form, found in only a handful of genera.

Foliose

A foliose lichen has flat, leaf-like lobes that are generally not firmly bonded to the substrate on which it grows. It typically has distinct upper and lower surfaces, each of which is typically covered with a cortex; some, however, lack a lower cortex. The photobiont layer lies just below the upper cortex. Where present, the lower cortex is usually dark (sometimes even black), but occasionally white. Foliose lichens are attached to their substrate either by hyphae extending from the cortex or , or by root-like structures called . The latter, which are found only in foliose lichens, come in a variety of shapes, the specifics of which can aid in species identification. Some foliose lichens attach only at a single stout peg called a , typically located near the lichen's centre. Lichens with this structure are called "umbilicate". In general, medium to large epiphytic foliose lichens are moderately sensitive to air pollution, while smaller or ground-dwelling foliose lichens are more tolerant.

Fruticose

A fruticose lichen is typically shrubby or coral-like in appearance, though some are hair-like or strap-like instead. Some grow upright while others hang. They attach to the substrate only at a single point at their base (or at most a very few points), and can be easily removed. Sometimes, as in the case of

vagrant lichen A vagrant lichen is a lichen that is either not attached to a substrate, or can become unattached then blow around, yet continue to grow and flourish. Some authors reserve the expression "vagrant lichen" for those lichens that never attach, that is, ...

s, they are completely unattached to a substrate. Unlike a foliose lichen, a fruticose lichen does not have a distinct upper and lower surface. Instead, a cortex covers its entire surface, and the photobiont layer lies just below this, on all sides of the lichen's branches. The centre of a fruticose lichen's branches varies depending on the genus involved. In most, the centre is hollow. However, lichens in the genus ''

Usnea ''Usnea'' is a genus of mostly pale grayish-green fruticose lichens that grow like leafless mini-shrubs or tassels anchored on bark or twigs.Field Guide to California Lichens, Stephen Sharnoff, Yale University Press, 2014, The genus is in the f ...

'' have a fairly elastic cord running through the middle. This is the most three-dimensional of the lichen growth forms, and the most sensitive to air pollution.

Gelatinous

A gelatinous lichen, also widely known as a "jelly lichen", is one with a cyanobacterial species ("blue-green alga") as the principal photobiont. Chains of the photobiont, rather than fungal hyphae, make up the bulk of the thallus, which is unlayered (and undifferentiated) as a result. Such lichens lack a cortex. Despite this lack of internal structure, gelatinous lichens usually have external growth forms that resemble those of fungal-dominated lichens. Gelatinous lichens are particularly common in areas with erratic rainfall or periodic inundation (such as

rock pool A tide pool or rock pool is a shallow pool of seawater that forms on the rocky intertidal shore. Many of these pools exist as separate bodies of water only at low tide. Many tide pool habitats are home to especially adaptable animals that ...

s). The presence of ''

Nostoc ''Nostoc'', also known as star jelly, troll’s butter, spit of moon, fallen star, witch's butter (not to be confused with the fungi commonly known as witches' butter), and witch’s jelly, is the most common genus of cyanobacteria found in var ...

'' cyanobacteria allows the lichen to absorb significant amounts of moisture, swelling in the process. It can then remain sufficiently hydrated to allow

photosynthesis Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's activities. Some of this chemical energy is stored in ...

long after the wetting event is over.

Leprose

A leprose lichen, which is typically considered to be a form of crustose lichen, is one with a powdery or dust-like appearance. Its undifferentiated thallus is an irregular mix of fungal hyphae and scattered photobiont cells, lacking a cortex or any definable layers. Morphologically, it is the simplest growth form. The cell walls of leprose lichens contain chemical compounds which make them

hydrophobic In chemistry, hydrophobicity is the physical property of a molecule that is seemingly repelled from a mass of water (known as a hydrophobe). In contrast, hydrophiles are attracted to water. Hydrophobic molecules tend to be nonpolar and, ...

, and thus largely water repellent. However, the lack of a cortex allows them to absorb water directly from humid air. Leprose lichens often grow in damp, shaded places generally untouched by rain. They may be completely covered in

soredia Soredia are common reproductive structures of lichens. Lichens reproduce asexually by employing simple fragmentation and production of soredia and isidia. Soredia are powdery propagules composed of fungal hyphae wrapped around cyanobacteria or ...

– small aggregates of fungal hyphae and photobiont cells which can break off to form new lichen colonies. They have never been found with fruiting bodies.

Placodioid

A placodioid lichen is a form of crustose lichen with lobed margins. These lobed edges, which radiate from the central part of the lichen, are its only growing sections; the central part of the lichen typically contains reproductive structures and does not expand. The growth rates of these lobes can vary – sometimes significantly – which can lead to some lobes being overgrown by others. When this happens, the overgrown lobes stop growing and are ultimately swallowed up by the expanding lichen. As with other crustose lichens, placodioid lichens have a cortex on their upper surface, but not their lower one. This allows them to be distinguished from foliose lichens, which can be similar in appearance but have both an upper and a lower cortex. Crustose lichens may be both placodioid and areolate, as in, for example, '' Caloplaca flavescens''.

Squamulose

A squamulous lichen has a thallus composed of small, scale-like plates – known as squamules – measuring 1–15 mm in diameter. These plates may be attached to the substrate across their entire lower surface, or they may be attached only along one edge, so that they overlap neighbouring plates like

shingles Shingles, also known as zoster or herpes zoster, is a viral disease characterized by a painful skin rash with blisters in a localized area. Typically the rash occurs in a single, wide mark either on the left or right side of the body or fac ...

on a roof. A squamulose lichen typically has no cortex on its lower surface, though a few species do. Some lichenologists consider squamulose lichens to be crustose lichens which peel up at their outer edges. Others consider them to be intermediate between crustose and foliose lichens. Squamulose lichens are particularly common in biological soil crust communities. In the

Intermountain West The Intermountain West, or Intermountain Region, is a geographic and geological region of the Western United States. It is located between the front ranges of the Rocky Mountains on the east and the Cascade Range and Sierra Nevada on the wes ...

of the United States, for example, almost 60% of all soil lichens are squamulose.

The "look-alikes"

In addition to the above forms, lichenologists have named a handful of informal growth forms for their resemblance to members of particular genera. In some cases, it is the overall appearance of the lichen that is similar, while in others, only their

s or fruiting structures share a resemblance.

Alectorioid

Alectorioid lichens are either members of, or resemble members of, the genus '' Alectoria''. They are fruticose, typically with a beard-like thallus that is or clustered; members of the genera '' Bryoria'', ''

Oropogon ''Oropogon'' is a genus of lichenised ascomycetes in the large family Parmeliaceae. It is a genus of roughly 40 currently accepted species. It was previously included in the family Alectoriaceae, but this group has since been subsumed into the Pa ...

'', ''

Pseudephebe ''Pseudephebe'' is a genus of fruticose lichens in the family Parmeliaceae. It contains three species that grow on rocks. Taxonomy ''Pseudephebe'' was circumscribed by French lichenologist Maurice Choisy in 1930. Molecular phylogenetic work ...

'', and ''

Sulcaria ''Sulcaria'' is a genus of three species of fruticose lichens in the family Parmeliaceae. ''Sulcaria'' was circumscribed In geometry, the circumscribed circle or circumcircle of a polygon is a circle that passes through all the vertices of ...

'' also have this growth type.

Catapyrenioid

Catapyrenioid lichens were historically members of the genus '' Catapyrenium''; many have now been moved to other genera within the family Verrucariaceae. They are squamulose and lack algae in their

hymenium The hymenium is the tissue layer on the hymenophore of a fungal fruiting body where the cells develop into basidia or asci, which produce spores. In some species all of the cells of the hymenium develop into basidia or asci, while in others so ...

– the part of the fruiting structure where

s are formed. Their simple

ascospores An ascus (; ) is the sexual spore-bearing cell produced in ascomycete fungi. Each ascus usually contains eight ascospores (or octad), produced by meiosis followed, in most species, by a mitotic cell division. However, asci in some genera or ...

have no partitions.

Cetrarioid

Cetrarioid lichens were historically classified in the genus ''

Cetraria ''Cetraria'' is a genus of fruticose lichens that associate with green algae as photobionts. Most species are found at high latitudes, occurring on sand or heath. Species have a characteristic "strap-like" form, with spiny lobe edges. '' Cetrar ...

''; many have now been moved to other genera within the family

Parmeliaceae The Parmeliaceae is a large and diverse family of Lecanoromycetes. With over 2700 species in 71 genera, it is the largest family of lichen-forming fungi. The most speciose genera in the family are the well-known groups: ''Xanthoparmelia'' ( 82 ...

. They are foliose or subfruticose with erect lobes, and their apothecia and

pycnidia A pycnidium (plural pycnidia) is an asexual fruiting body produced by mitosporic fungi, for instance in the order Sphaeropsidales ( Deuteromycota, Coelomycetes) or order Pleosporales (Ascomycota, Dothideomycetes). It is often spherical or inverse ...

are located on the lobe margins.

Usneoid

Usneoid lichens are either members of, or resemble members of, the genus ''

''. They are fruticose with an elastic in the ; members of the genus '' Dolichousnea'' also have this growth type.

References

Sources

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