Editing Lichen (section)

== Anatomy and morphology ==
=== Growth forms ===
{{Main|Lichen growth forms}}
Lichens grow in a wide range of shapes and forms; this external appearance is known as their [[morphology (biology)|morphology]]. The shape of a lichen is usually determined by the organization of the fungal filaments.<ref name=MSULB/> 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. [[List of common names of lichen genera|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 lichen|fruticose]]<ref name="LNAILV" /><ref name=ASLGPZ/><ref name=Dobson/> – 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 lichen|foliose]]<ref name="LNAILV" /><ref name=ASLGPZ/> – growing in 2-dimensional, flat, leaf-like lobes
# [[crustose lichen|crustose]]<ref name=ANBGLG>{{cite web |url=http://www.anbg.gov.au/glossary/webpubl/lichglos.htm |title=Lichen Glossary |last=Galloway |first=D.J. |date=13 May 1999 |archive-url=https://web.archive.org/web/20141206070827/http://www.anbg.gov.au/glossary/webpubl/lichglos.htm |archive-date=6 December 2014 |publisher=Australian National Botanic Gardens}}</ref><ref name="LNAILV" /><ref name=ASLGPZ/> – crust-like, adhering tightly to a surface ([[substrate (biology)|substrate]]) like a thick coat of paint
# [[Squamulose lichen|squamulose]]<ref name=Dobson/> – formed of small leaf-like scales crustose below but free at the tips
# [[Leprose lichen|leprose]]<ref name=ASLTT/> – powdery
# [[Gelatinous lichen|gelatinous]] – jelly-like
# [[filamentous lichen|filamentous]] – stringy or like matted hair
# [[Byssoid lichen|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]] or [[areolate]], and the "island" pieces separated by the cracks are called areolas.<ref name="LNAILV" /> The areolas appear separated, but are (or were){{citation needed|date=October 2014}} connected by an underlying [[prothallus#In lichens|prothallus]] or [[hypothallus]].<ref name=ASLTT/> 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]].<ref name=VMBMLF/>{{rp|159}}<ref name=Dobson/>

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.<ref name=VMBMLF/>{{rp|159}}

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 lichen]]s), with only the sexual fruiting part visible growing outside the rock.<ref name="LNAILV" /> These may be dramatic in color or appearance.<ref name="LNAILV" /> Forms of these sexual parts are not in the above growth form categories.<ref name="LNAILV">Lichen Vocabulary, Lichens of North America Information, Sylvia and Stephen Sharnoff, [http://www.lichen.com/vocabulary.html] {{Webarchive|url=https://web.archive.org/web/20150120063843/http://www.lichen.com/vocabulary.html|date=20 January 2015}}</ref> 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 ===
[[File:Bipartite and tripartite cyanolichens (10.3897-mycokeys.6.3869) Figure 1.jpg|thumb|Lichens]]
Lichens come in many colors.<ref name=SSFGCL/>{{rp|4}} Coloration is usually determined by the photosynthetic component.<ref name=MSULB>{{cite web|url=https://www.msu.edu/course/bot/423/Plntlist8mosslichen.html|title=Lichens and Bryophytes, Michigan State University, 10-25-99|access-date=10 October 2014|archive-url=https://web.archive.org/web/20111005224410/https://www.msu.edu/course/bot/423/Plntlist8mosslichen.html|archive-date=5 October 2011|url-status=dead}}</ref> Special pigments, such as yellow [[usnic acid]], give lichens a variety of colors, including reds, oranges, yellows, and browns, especially in exposed, dry habitats.<ref name=SJCL>{{cite web|url=http://www.saguaro-juniper.com/i_and_i/lichens/lichens.html|title=Lichens, Saguaro-Juniper Corporation|access-date=10 October 2014|archive-url=https://web.archive.org/web/20150510220114/http://www.saguaro-juniper.com/i_and_i/lichens/lichens.html|archive-date=10 May 2015|url-status=dead}}</ref> In the absence of special pigments, lichens are usually bright green to olive gray when wet, gray or grayish-green to brown when dry.<ref name=SJCL/> This is because moisture causes the surface skin ([[cortex (lichen)|cortex]]) to become more transparent, exposing the green photobiont layer.<ref name=SJCL/> 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.<ref name=SJCL/> 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 [[Lichens of the Sierra Nevada (U.S.)#On rock|vertical "paint"]] covering the vast rock faces of [[Yosemite National Park]].<ref name="LSN">{{Cite journal|last1=McCune|first1=B.|last2=Grenon|first2=J.|last3=Martin|first3=E.|last4=Mutch|first4=L.S.|last5=Martin|first5=E.P.|date=Mar 2007|title=Lichens in relation to management issues in the Sierra Nevada national parks|journal=North American Fungi|volume=2|pages=1–39|doi=10.2509/pnwf.2007.002.003|doi-broken-date=1 November 2024 |doi-access=free}}</ref>

Color is used in identification.<ref name="JJMML">Michigan Lichens, Julie Jones Medlin, B. Jain Publishers, 1996, {{ISBN|0877370397}}, 9780877370390, [https://books.google.com/books?id=-Jo0yP2-69UC&pg=PA8] {{Webarchive|url=https://web.archive.org/web/20161124144607/https://books.google.com/books?id=-Jo0yP2-69UC&pg=PA8|date=24 November 2016}}</ref>{{rp|4}} The color of a lichen changes depending on whether the lichen is wet or dry.<ref name=JJMML/> Color descriptions used for identification are based on the color that shows when the lichen is dry.<ref name=JJMML/> Dry lichens with a cyanobacterium as the photosynthetic partner tend to be dark grey, brown, or black.<ref name=JJMML/>

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.<ref name=ASLTT/>

The sheen on some jelly-like gelatinous lichens is created by [[mucilaginous]] secretions.<ref name=MSULB/>

=== Internal structure ===
[[File:Lichen cross section – heteromeric thallus.svg|thumb|right|Schematic cross section of [[foliose lichen]]:
<br/>(a) The [[cortex (lichen)|cortex]] is the outer layer of tightly woven fungus filaments ([[hyphae]])
<br/>(b) This photobiont layer has photosynthesizing [[green algae]]
<br/>(c) Loosely packed hyphae in the medulla
<br/>(d) A tightly woven lower cortex
<br/>(e) Anchoring hyphae called [[rhizines]] where the fungus attaches to the substrate]]
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,<ref name="LMM">Lichens: More on Morphology, University of California Museum of Paleontology, [http://www.ucmp.berkeley.edu/fungi/lichens/lichenmm.html] {{Webarchive|url=https://web.archive.org/web/20150228172027/http://www.ucmp.berkeley.edu/fungi/lichens/lichenmm.html|date=28 February 2015}}</ref> but this is reversed in filamentous and gelatinous lichens.<ref name=MSULB/> The fungus is called a ''mycobiont''. The photosynthesizing organism is called a ''photobiont''. Algal photobionts are called ''phycobionts''.<ref name=UNOLP>{{Cite web|url=http://www.unomaha.edu/lichens/Bio%204350%20PDF/Photobionts.pdf|archiveurl=https://web.archive.org/web/20141006145803/http://www.unomaha.edu/lichens/Bio%204350%20PDF/Photobionts.pdf|url-status=dead|title=Lichen Photobionts, University of Nebraska Omaha|archive-date=6 October 2014}}</ref> Cyanobacteria photobionts are called ''cyanobionts''.<ref name=UNOLP/>

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 "[[anastomosis|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,<ref name="pmid_27445309" /> 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,<ref name=ASLTT/> gelatinous, and other lichens do not have a cortex; in other words, they are [[ecorticate]].<ref name=ASLGGO/>

Fruticose, foliose, crustose, and squamulose lichens generally have up to three different types of tissue, [[Cellular differentiation|differentiated]] by having different densities of fungal filaments.<ref name=LMM/> The top layer, where the lichen contacts the environment, is called a ''cortex''.<ref name=LMM/> The cortex is made of densely tightly woven, packed, and glued together ([[Agglutination (biology)|agglutinated]]) fungal filaments.<ref name=LMM/> 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.<ref name=LMM/> The cortex layer can be up to several hundred micrometers (μm) in thickness (less than a millimeter).<ref name=Budel1996/> The cortex may be further topped by an epicortex of secretions, not cells, 0.6–1&nbsp;μm thick in [[Parmeliaceae|some lichens]].<ref name=Budel1996/> This secretion layer may or may not have pores.<ref name=Budel1996/>

Below the cortex layer is a layer called the ''photobiontic layer'' or ''symbiont layer''.<ref name=ASLGPZ/><ref name=LMM/> The symbiont layer has less densely packed fungal filaments, with the photosynthetic partner embedded in them.<ref name=LMM/> The less dense packing allows air circulation during photosynthesis, similar to the anatomy of a leaf.<ref name=LMM/> Each cell or group of cells of the photobiont is usually individually wrapped by hyphae, and in some cases penetrated by a [[haustorium]].<ref name=MSULB/> 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 layer below.<ref name=MSULB/>

The layer beneath the symbiont layer is called the ''[[medulla (lichenology)|medulla]]''. The medulla is less densely packed with fungal filaments than the layers above. In foliose lichens, as in ''[[Peltigera]]'',<ref name=VMBMLF/>{{rp|159}} there is usually another densely packed layer of fungal filaments called the lower cortex.<ref name=ASLTT/><ref name=LMM/> Root-like fungal structures called ''rhizines'' ([[Hypogymnia|usually]])<ref name=VMBMLF/>{{rp|159}} grow from the lower cortex to attach or anchor the lichen to the substrate.<ref name=WIL/><ref name=ASLTT/> Fruticose lichens have a single cortex wrapping all the way around the "stems" and "branches".<ref name=VMBMLF/> The medulla is the lowest layer, and may form a cottony white inner core for the branchlike thallus, or it may be hollow.<ref name=VMBMLF/>{{rp|159}} Crustose and squamulose lichens lack a lower cortex, and the medulla is in direct contact with the [[substrate (biology)|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.<ref name=LMM/> 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 lower cortex.<ref name=ASLTT>{{cite web|url=http://www.lichens.lastdragon.org/faq/lichenthallustypes.html#foliose|title=Foliose lichens, Lichen Thallus Types, Allan Silverside|access-date=10 October 2014|archive-url=https://web.archive.org/web/20141019112429/http://www.lichens.lastdragon.org/faq/lichenthallustypes.html#foliose|archive-date=19 October 2014|url-status=live}}</ref>

Gelatinous,<ref name=VMBMLF/>{{rp|159}} byssoid, and leprose lichens lack a cortex (are [[ecorticate]]), and generally have only undifferentiated tissue, similar to only having a symbiont layer.{{citation needed|date=September 2014}}

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 ''[[cephalodium|cephalodia]]''.

''[[Pruinia]]'' is a whitish coating on top of an upper surface.<ref name=HPTCLGD/> An ''[[epinecral layer]]'' is "a layer of horny dead fungal hyphae with indistinct [[lumen (anatomy)|lumina]] in or near the cortex above the algal layer".<ref name=HPTCLGD>{{cite journal|doi=10.2307/3244302 |jstor=3244302 |title=Pruina as a Taxonomic Character in the Lichen Genus ''Dermatocarpon'' |journal=The Bryologist |volume=99 |issue=3 |pages=315–320 |year=1996 |last1=Heiđmarsson |first1=Starri |last2=Heidmarsson |first2=Starri }}</ref>

In August 2016, it was reported that some macrolichens have more than one species of fungus in their tissues.<ref name="pmid_27445309"/>