Ramalina usnea is a species of corticolous (bark-dwelling), fruticose lichen in the family Ramalinaceae. It grows on tree bark across the Americas and parts of East Africa. Growing up to 30 cm (12 in) long, it forms pale greenish-grey branching strands that darken when dry. The species is highly adaptable: it forms coarse, flattened branches in foggy environments and develops finer, more delicate forms in humid coastal areas. First described by Carl Linnaeus in 1767, it underwent a few taxonomic revisions until Reginald Heber Howe, Jr. established its current name in 1914. Unlike many lichens that grow primarily at their tips, R. usnea expands along its entire length by continuously rebuilding its cell walls. It serves as an important component of some ecosystems, particularly in the Galápagos Islands' transition zones, where it forms characteristic draperies on trees and is used as nesting material by certain birds.
Taxonomy The lichen was first scientifically described by Carl Linnaeus in 1767 as Lichen usnea. In his protologue, Linnaeus characterised it as a filamentous, pendulous (hanging) lichen with a smooth, compressed thallus. He described the species as thread-like and branching, reaching up to a foot in length, with obtuse branch angles and a thallus that could separate into two layers. He noted that specimens were whitish when fresh but became grey-brown when dry. Linnaeus based part of his description on observations by the Dutch-born botanist Nikolaus Joseph von Jacquin, who had collected extensively in the Caribbean and documented the species growing on trees in Martinique, as well as reports of its presence in the East Indies, St. Helena, and Madagascar. It underwent several taxonomic changes over time. Erik Acharius reclassified it as Parmelia usneoides in 1803, and again in 1810 as Alectoria usneoides. Reginald Heber Howe, Jr. finally established its current name, Ramalina usnea, in 1914. The lectotype specimen (Linnaean Herbarium 1273-278), collected from Martinique, was designated by Henry Imshaug in 1972. Chemical analysis revealed that the type specimen contains usnic and divaricatic acids—a variant differing from earlier reports that identified only ramalinolic and sekikaic acids. It is also notable as the only confirmed historical collection of R. usnea from the Lesser Antilles, where the related species R. peranceps is now more commonly found. A 1978 study revealed significant insights into the taxonomy of the R. usnea species complex. Three distinct chemical races were identified within R. usnea, each characterised by different substances in the medulla: a sekikaic acid race, a divaricatic acid race, and a race lacking diagnostic medullary compounds. This chemotaxonomic approach has been important in understanding the species' diversity. Research has shown that R. subanceps and R. bogotensis are synonyms of R. usnea, representing different morphological expressions within the species' range of variation. The R. usnea complex has been found to comprise four distinct species: R. usnea, Ramalina anceps, Ramalina chilensis, and a newly described species, Ramalina sharpii. Some researchers have questioned the distinction between Ramalina usnea and R. anceps, suggesting that R. anceps may represent a chemotype of R. usnea. The two taxa are morphologically similar, with R. anceps appearing thinner and less contorted, but these characteristics are considered unreliable for species delimitation.
Description
The thallus is pendulous, reaching up to 30 cm (12 in) in length, with irregular branching and a solid (non-hollow) structure. Branches are typically flattened (though occasionally slightly cylindrical), often contorted, with apices that taper to a slender or rounded form, and measure 0.5–2.5 mm wide. Soredia are absent. Pseudocyphellae are linear in form, occurring both marginally and laminally. The cortex is more or less distinct, about 10 μm thick; chondroid tissue clearly and heavily cracked. Apothecia are common, marginal or laminal; the disc is flat, becoming convex with age; the thalline exciple is entire, without pseudocyphellae; hymenium 40–45 μm; subhymenium 20–30 μm; the proper exciple is 50–70 μm thick. Ascospores are long-fusiform, 2-celled, often with 3–6 additional septa, typically 18–22 by 3–4 μm.
Similar species Ramalina usnea can be distinguished from Usnea (bear lichen) species by its flattened, two-sided branches that lack a central cord of supporting tissue Several closely related species share morphological features with Ramalina usnea, forming what has been called the R. usnea complex. R. anceps is primarily found in the West Indies, where it grows in moist forests up to 900 m elevation. While it can be difficult to distinguish from finely-branched forms of R. usnea, it can usually be identified by its very shiny cortex, more terete (rounded) branches, general absence of striations, and more rigid, less twisted branch pattern. It also has distinctive chemistry, containing norstictic acid with salazinic acid. A variant, R. anceps var. peranceps, has somewhat more flattened and striated branches and contains only salazinic acid in its medulla. Ramalina sharpii is restricted to pine-oak forests above 2000 m in the mountains of central Mexico. It is distinguished from R. usnea by its much broader branches, which can reach 2–30 mm in width, very prominent white striations, and longer thalli that can grow up to 100 cm. The species also differs in its medullary chemistry, containing norstictic acid with or without salazinic acid. Ramalina chilensis is an endemic species of central and northern Chile, found in coastal habitats and on seaward slopes of coastal mountains where it is influenced by sea fogs. It differs from R. usnea in its shorter length, rarely exceeding 20 cm, and its consistently broad, flat branches with prominent white striations. The species has an unusual chemistry combining sekikaic acid and norstictic acid, and is restricted to Mediterranean climate areas of Chile. Ramalina menziesii from California has sometimes been confused with R. usnea but can be readily distinguished by its distinctive net-like reticulate branching pattern. Additionally, R. menziesii is restricted to the Pacific Coast north of Sinaloa, Mexico, while R. usnea is not found in this region.
Development
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