3.7: Lichens
- Page ID
- 35319
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Lichens are not individual organisms, but a single body formed from multiple symbiotic organisms. Lichens contain a fungal partner (the mycobiont) that forms the majority of the lichen body (called a thallus) and one or more photosynthetic partners (the photobiont) that are typically found in a thin layer or in isolated pockets. The mycobiont is typically an ascoymycete, but can very rarely be a basidiomycete (basidiolichen). The photobiont is usually green algae, but can also be cyanobacteria (cyanolichen) or both green algae and cyanobacteria (tripartite lichen). The mycobiont is responsible for maintenance of the lichen thallus, while the photobiont is responsible for producing food for both partners through photosynthesis. A cyanobacterial partner might also be fixing nitrogen for the lichen. As the study of lichens progresses, we are uncovering more partners with as yet unknown roles, such as basidiomycete yeasts and bacteria.
The Lichen Mutualism
![A cross section through a lichen thallus. The layers, a-e, are shown in order from the upper surface to the lower surface of the thallus.](https://bio.libretexts.org/@api/deki/files/30294/1920px-Lichen_cross_section_%25E2%2580%2593_heteromeric_thallus.svg.png?revision=1&size=bestfit&width=677&height=698)
![Lichens and orange free-living algae on a log](https://bio.libretexts.org/@api/deki/files/27426/Lichens_and_trent.jpg?revision=1&size=bestfit&width=657&height=492)
![A lichen growing in a crack with two apothecia labelled](https://bio.libretexts.org/@api/deki/files/27434/Cute.jpg?revision=1&size=bestfit&width=559&height=415)
![A light micrograph of a cross section through the apothecium of a lichen (labeled)](https://bio.libretexts.org/@api/deki/files/29946/Physcia_apothecium_labeled.jpg?revision=1)
Basidiolichens
![A basidiolichen growing on a log, producing a mushroom](https://bio.libretexts.org/@api/deki/files/27427/Lichenomphalia_baby.jpg?revision=1&size=bestfit&width=350&height=467)
![A close up of a basidiolichen mushroom](https://bio.libretexts.org/@api/deki/files/27428/Lichenomphalia.jpg?revision=1&size=bestfit&width=476&height=469)
Cyanolichens
![A foliose cyanolichen growing amongst moss and spruce needles](https://bio.libretexts.org/@api/deki/files/27430/Peltigera_foliose_cyano.jpg?revision=1&size=bestfit&width=544&height=408)
![Another foliose cyanolichen growing amongst moss and another, lighter-colored lichen](https://bio.libretexts.org/@api/deki/files/27431/Nephroma_is_it_a_cyano.jpg?revision=1&size=bestfit&width=466&height=413)
![Brown, amorphous, gelatinous looking blobs on a field of green.](https://bio.libretexts.org/@api/deki/files/30295/Arctic_Kidney_Lichens_Cephalodia_(3815349887).jpg?revision=1)
Additional Features
![Rhizines on the underside of a foliose lichen](https://bio.libretexts.org/@api/deki/files/27438/small_rhizines.jpeg?revision=1&size=bestfit&width=689&height=517)
![Blue-grey foliose lichen with long white projections from the underside.](https://bio.libretexts.org/@api/deki/files/45265/large.jpg?revision=1&size=bestfit&width=610&height=610)
![A lichen with marginal soralia](https://bio.libretexts.org/@api/deki/files/27437/Foliose_with_soralia.jpeg?revision=1)
![A highly branched yellow fruticose lichen with shiny projections emerging from the cortex](https://bio.libretexts.org/@api/deki/files/29947/Labeled_isidia.jpg?revision=1&size=bestfit&width=546&height=431)
Forms
Lichens can generally be classified into three general forms: crustose, foliose, and fruticose. However, there are some lichens that don't really fit into these categories as nicely, such as the "dustose" lichens that form powdery crusts.
Crustose
Figure \(\PageIndex{10}\): This image shows several different crustose lichens (and one foliose lichen) on the bark of a young pear tree. The crustose lichen closest to the camera is in the genus Graphis. It looks as though it has been painted onto the tree bark, but has small raised black lines running through it, giving it the common name "script lichen". These lines are the elongated apothecia. Photo by Maria Morrow, CC-BY.
![A crustose lichen growing on a rock](https://bio.libretexts.org/@api/deki/files/27424/Crustose.jpg?revision=1&size=bestfit&width=400&height=300)
![A crustose lichen growing on wax myrtle bark](https://bio.libretexts.org/@api/deki/files/27425/Unknown_crustose.jpg?revision=1&size=bestfit&width=484&height=645)
Foliose
Foliose lichens have a distinct upper and lower surface, much like a leaf. Sometimes these can grow appressed to a substrate, but unlike crustose lichens, they can usually be separated from it (with some effort).
Fruticose
![A fruticose lichen growing on tree bark](https://bio.libretexts.org/@api/deki/files/27432/Letharia_I_think.jpg?revision=1&size=bestfit&width=630&height=472)
![A fruticose lichen growing on a rock](https://bio.libretexts.org/@api/deki/files/27433/Pilophorus.jpg?revision=1)