Editing Lichen (section)

=== 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"/>