A lichen is a composite organism that arises from algae or cyanobacteria living among filaments of multiple fungi species in a mutualistic relationship. Lichens are important actors in nutrient cycling and act as producers which many higher trophic feeders feed on, such as reindeer, gastropods, nematodes, mites, and springtails. Lichens have properties different from those of their component organisms. They come in many colors, sizes, and forms and are sometimes plant-like, but are not plants. They may have tiny, leafless branches (fruticose); flat leaf-like structures (foliose); grow crust-like, adhering tightly to a surface (substrate) like a thick coat of paint (crustose); have a powder-like appearance (leprose); or other growth forms.
In botany, a cortex is an outer layer of a stem or root in a vascular plant, lying below the epidermis but outside of the vascular bundles. The cortex is composed mostly of large thin-walled parenchyma cells of the ground tissue system and shows little to no structural differentiation. The outer cortical cells often acquire irregularly thickened cell walls, and are called collenchyma cells.
Flavoparmelia caperata, the common greenshield lichen, is a foliose lichen that grows on the bark of trees, and occasionally on rock.
Peltigerales is an order of lichen-forming fungi belonging to the class Lecanoromycetes in the division Ascomycota. The taxonomy of the group has seen numerous changes; it was formerly often treated as a suborder of the order Lecanorales. It contains two suborders, eight families and about 45 genera such as Lobaria and Peltigera.
The Collemataceae are a lichenized family of fungi in the order Peltigerales. The family contains ten genera and about 325 species. The family has a widespread distribution.
Acarospora is a genus of mostly lichen-forming fungi in the family Acarosporaceae. Most species in the genus are crustose lichens that grow on rocks in open and arid places all over the world. They may look like a cobblestone road or cracked up old paint, and are commonly called cobblestone lichens or cracked lichens. They usually grow on rock, but some grow on soil (terricolous) or on other lichens. Some species in the genus are fungi that live as parasites on other lichens. Acarospora is a widely distributed genus, with about 128 species according to a 2008 estimate.
Placidium is a genus of crustose to squamulose to almost foliose lichens. The genus is in the family Verrucariaceae. Most members grow on soil, but some grow on rock (saxicolous). The fruiting bodies are perithecia, flask-like structures immersed in the lichen body (thallus) with only the top opening visible, dotting the thallus. Lichen spot tests are all negative. Members of the genus lack rhizines, but otherwise resemble members of the genus Clavascidium.
Crustose lichens are lichens that form a crust which strongly adheres to the substrate, making separation from the substrate impossible without destruction. The basic structure of crustose lichens consists of a cortex layer, an algal layer, and a medulla. The upper cortex layer is differentiated and is usually pigmented. The algal layer lies beneath the cortex. The medulla fastens the lichen to the substrate and is made up of fungal hyphae. The surface of crustose lichens is characterized by branching cracks that periodically close in response to climatic variations such as alternate wetting and drying regimes.
A foliose lichen is a lichen with flat, leaf-like lobes, which are generally not firmly bonded to the substrate on which it grows. It is one of the three most common growth forms of lichens. It typically has distinct upper and lower surfaces, each of which is usually 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, but occasionally white. Foliose lichens are attached to their substrate either by hyphae extending from the cortex or medulla, or by root-like structures called rhizines. 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 holdfast, 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. The term "foliose" derives from the Latin word foliosus, meaning "leafy".
Lichens are symbiotic organisms made up of multiple species: a fungus, one or more photobionts and sometimes a yeast. They are regularly grouped by their external appearance – a characteristic known as their growth form. This form, which is based on the appearance of vegetative part of the lichen, varies depending on the species and the environmental conditions it faces. Those who study lichens (lichenologists) have described a dozen of these forms: areolate, byssoid, calicioid, cladoniform, crustose, filamentous, foliose, fruticose, gelatinous, leprose, placoidioid and squamulose. Traditionally, crustose (flat), foliose (leafy) and fruticose (shrubby) are considered to be the three main forms. In addition to these more formalised, traditional growth types, there are a handful of informal types named for their resemblance to the lichens of specific genera. These include alectorioid, catapyrenioid, cetrarioid, hypogymnioid, parmelioid and usneoid.
Lichen morphology describes the external appearance and structures of a lichen. These can vary considerably from species to species. Lichen growth forms are used to group lichens by "vegetative" thallus types, and forms of "non-vegetative" reproductive parts. Some lichen thalli have the aspect of leaves ; others cover the substrate like a crust, others such as the genus Ramalina adopt shrubby forms, and there are gelatinous lichens such as the genus Collema.
Lepidocollema wainioi is a species of lichen in the family Pannariaceae. It is known from southeast Asia.
Scytinium is a genus of lichen-forming fungi in the family Collemataceae. It has 49 species. These lichens are typically found on basic rocks, soil, and trees, occasionally in association with mosses. Despite the morphological and ecological diversity within Scytinium, its species share similar ascospore features, such as shape and septation, as well as a small to medium-sized thallus with at least a partial cortex.
Lepidocollema marianum is a species of cyanolichen in the family Pannariaceae. It was first scientifically described by Elias Fries in 1825 as Parmelia mariana. Per Magnus Jørgensen transferred it to the genus Lepidocollema in 2014 following a molecular phylogenetics-guided revision of the Pannariaceae.
Fuscopannaria rugosa is a species of corticolous (bark-dwelling), lichen in the family Pannariaceae. It is found in Hubei and Guanxi in China, where it grows in mountainous forests. Fuscopannaria rugosa is identifiable by its foliose-squamulose thallus, which has longitudinal wrinkles on the upper surface. It also has a distinct hypothallus, a relatively thick algal layer, and a cortex made up of thick-walled cells in both the thalline and apothecial structures. It produces simple, hyaline ascospores that have a smooth, pointed tip at their apex.
Fuscopannaria siamensis is a species of lichen in the family Pannariaceae. It is found in the mountainous forests of northern Thailand and in Tamil Nadu, India, where it grows in humid habitats in association with mosses.
Fuscopannaria leucosticta, commonly known as the rimmed shingle lichen, is a species of lichen in the family Pannariaceae. It has a squamulose (scaley) thallus that lacks soredia and isidia, but has abundant apothecia with distinct white rims. Although its main centres of distribution are eastern North America and southeast Asia, where it grows in damp forests, it has been reported from various other high-altitude, humid locations.
Parmotrema lawreyi is a species of corticolous (bark-dwelling), foliose lichen in the family Parmeliaceae. Found on the Galápagos Islands, it was formally described as a new species in 2019 by lichenologists Frank Bungartz and Adriano Spielmann. The type specimen was collected by the first author from the foothills of Media Luna on San Cristóbal Island, where it was found in dry, open woodland growing on the trunk of Bursera graveolens. The species epithet honours the authors' colleague James D. Lawrey, "on the occasion of his 70th birthday".
Opeltiella is a genus of lichen-forming fungi in the family Candelariaceae. The genus, established by Sergey Kondratyuk in 2020, has four species. It is differentiated from the similar genus Candelaria by its unique features such as eight-spored asci and absence of a lower cortical layer and true rhizines. The genus is characterised by its areolate to more or less squamulose or foliose thallus and the unique chemical substances it contains, such as calycin, pulvinic and vulpinic acids, and pulvinic acid lactone.
