Flavoparmelia caperata - lichen - Caperatflechte

Lichen morphology describes the external appearance and structures of a

lichen A lichen ( , ) is a composite organism that arises from algae or cyanobacteria living among filaments of multiple fungi species in a mutualistic relationship.Lichen growth forms are used to group lichens by "vegetative"

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 wer ...

types, and forms of "non-vegetative" reproductive parts. Some lichen thalli have the aspect of leaves (foliose lichens); others cover the substrate like a crust (crustose lichens) (''illustration, right''), others such as the genus ''

Ramalina ''Ramalina'' is a genus of greenish fruticose lichens that grow in the form of flattened, strap-like branches. Members of the genus are commonly called strap lichensField Guide to California Lichens, Stephen Sharnoff, Yale University Press, 20 ...

'' adopt shrubby forms (fruticose lichens), and there are gelatinous lichens such as the genus ''

Collema ''Collema'' (jelly lichen) is a genus of lichens in the family Collemataceae. The photobiont is the cyanobacterium genus ''Nostoc''.Dobson, F.S. (2000) Lichens, an illustrated guide to the British and Irish species. 4th edition. Richmond publish ...

''. Although the form of a lichen is determined by the

genetic material Nucleic acids are biopolymers, macromolecules, essential to all known forms of life. They are composed of nucleotides, which are the monomers made of three components: a 5-carbon sugar, a phosphate group and a nitrogenous base. The two main cla ...

of the fungal partner, association with a photobiont is required for the development of that form. When grown in the laboratory in the absence of its photobiont, a lichen fungus develops as an undifferentiated mass of

hypha A hypha (; ) is a long, branching, filamentous structure of a fungus, oomycete, or actinobacterium. In most fungi, hyphae are the main mode of vegetative growth, and are collectively called a mycelium. Structure A hypha consists of one or ...

e. If combined with its photobiont under appropriate conditions, its characteristic form emerges, in the process called

morphogenesis Morphogenesis (from the Greek ''morphê'' shape and ''genesis'' creation, literally "the generation of form") is the biological process that causes a cell, tissue or organism to develop its shape. It is one of three fundamental aspects of devel ...

.Brodo, Sharnoff & Sharnoff, 2001 In a few remarkable cases, a single lichen fungus can develop into two very different lichen forms when associating with either a green algal or a cyanobacterial symbiont. Quite naturally, these alternative forms were at first considered to be different species, until they were found growing in a conjoined manner. Under magnification, a section through a typical foliose lichen

reveals four layers of interlaced fungal filaments. The uppermost layer is formed by densely agglutinated fungal hyphae building a protective outer layer called the

cortex Cortex or cortical may refer to: Biology * Cortex (anatomy), the outermost layer of an organ ** Cerebral cortex, the outer layer of the vertebrate cerebrum, part of which is the ''forebrain'' *** Motor cortex, the regions of the cerebral cortex i ...

, which can reach several hundred μm in thickness. This cortex may be further topped by an epicortex 0.6-1μm thick in some Parmeliaceae, which may be with or without pores, and is secreted by cells—it is not itself cellular. 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. Beneath the upper cortex is an algal layer composed of algal cells embedded in rather densely interwoven fungal hyphae. Each cell or group of cells of the photobiont is usually individually wrapped by hyphae, and in some cases penetrated by an

haustorium In botany and mycology, a haustorium (plural haustoria) is a rootlike structure that grows into or around another structure to absorb water or nutrients. For example, in mistletoe or members of the broomrape family, the structure penetrates t ...

. Beneath this algal layer is a third layer of loosely interwoven fungal hyphae without algal cells. This layer is called the

medulla Medulla or Medullary may refer to: Science * Medulla oblongata, a part of the brain stem * Renal medulla, a part of the kidney * Adrenal medulla, a part of the adrenal gland * Medulla of ovary, a stroma in the center of the ovary * Medulla of t ...

. Beneath the medulla, the bottom surface resembles the upper surface and is called the lower cortex, again consisting of densely packed fungal hyphae. The lower cortex of

foliose Foliose lichen is one of the morphological classes of lichens, which are complex organisms that arise from the symbiotic relationship between fungi and a photosynthetic partner, typically algae. This partnership allows lichen to live in diverse ...

lichens often bears rootlike fungal structures known as

rhizines In lichens, rhizines are multicellular root-like structures, arising mostly from the lower surface. A lichen with rhizines is termed rhizinate, while a lichen lacking rhizines is termed erhizinate. Rhizines serve only to anchor the lichen to their ...

, which serve to attach the thallus to the substrate on which it grows. Lichens also sometimes contain structures made from fungal

metabolite In biochemistry, a metabolite is an intermediate or end product of metabolism. The term is usually used for small molecules. Metabolites have various functions, including fuel, structure, signaling, stimulatory and inhibitory effects on enzymes, c ...

s, for example crustose lichens sometimes have a

polysaccharide Polysaccharides (), or polycarbohydrates, are the most abundant carbohydrates found in food. They are long chain polymeric carbohydrates composed of monosaccharide units bound together by glycosidic linkages. This carbohydrate can react with wa ...

layer in the cortex. Although each lichen thallus generally appears homogeneous, some evidence seems to suggest that the fungal component may consist of more than one genetic individual of that species. This seems to also be true of the photobiont species involved.

Reproductive structures

podetium A podetium (plural: podetia) is the upright secondary thallus in '' Cladonia'' lichens. It is a hollow stalk extending from the primary thallus. Podetia can be pointed stalks, club like, cupped, or branched in shape and may or may not contain the ...

(plural

podetia A podetium (plural: podetia) is the upright secondary thallus in ''Cladonia'' lichens. It is a hollow stalk extending from the primary thallus. Podetia can be pointed stalks, club like, cupped, or branched in shape and may or may not contain the ...

) is a lichenized stem-like structure of an

apothecium An ascocarp, or ascoma (), is the fruiting body ( sporocarp) of an ascomycete phylum fungus. It consists of very tightly interwoven hyphae and millions of embedded asci, each of which typically contains four to eight ascospores. Ascocarps are mos ...

rising from the primary body of the thallus.Alan Silverside's Lichen Glossary (p-z), Alan Silverside
/ref> Since it is part of the reproductive tissue, it is not considered part of the thallus. The podetium may be branched, and sometimes cup-like. It usually bears the

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 inve ...

apothecia An ascocarp, or ascoma (), is the fruiting body ( sporocarp) of an ascomycete phylum fungus. It consists of very tightly interwoven hyphae and millions of embedded asci, each of which typically contains four to eight ascospores. Ascocarps are mos ...

or both.

References

{{Reflist Lichenology Morphology (biology)