Usnea

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Usnea is a genus of mostly pale grayish-green fruticose lichens that grow like leafless mini-shrubs or tassels anchored on bark or twigs.^[1]: 203 The genus is in the family Parmeliaceae. It grows all over the world. Members of the genus are commonly called old man's beard, beard lichen, or beard moss.^[1]: 203

Like other lichens it is a symbiosis of two or three fungi and an alga.^[2] In Usnea, the fungus belongs to the division Ascomycota, while the alga is a member of the division Chlorophyta.

Members of the genus are similar to those of the genus Alectoria.^[1]: 148 A distinguishing test is that the branches of Usnea are somewhat elastic, but the branches of Alectoria snap cleanly off.^[1]: 148

As a fruticose lichen, Usnea appears as a shrub-like growth on host trees. Unlike other similar-looking fruticose lichens, species in this genus have an elastic chord or axis running through the middle of the thallus that can be revealed by gently pulling a filament apart from either end.^[3] It reproduces via vegetative means through fragmentation, asexual means through soredia, or sexual means through ascogonium and spermatogonium.^[4] The growth rate of lichens in nature is slow, but the growth rate has been sped up in laboratory conditions where Usnea is being cultured.^[5]Usnea looks very similar to the plant Spanish moss, so much so that the latter's Latin name is derived from it (Tillandsia usneoides, the 'Usnea-like Tillandsia').

Many species have been described. A three-volume series by Józef Motyka published in 1936 and 1947 distinguished 451 species.^[6] Many of these are now regarded as morphological varieties and adaptations to local circumstances. The taxonomic categorization of many members of this genus remains uncertain. The number of recognized species in Finland is decreasing for this reason, from 34 in 1951 to 25 in 1963 and only 12 in 2000.^[7] It is now noted as including more than 600 species and being one of the largest genera within the Parmeliaceae.^[8] The species Usnea longissima was renamed Dolichousnea longissima in 2004.^[9]

Based on a fossil Usnea found in Baltic amber, it is known that the genus dates back to at least the late Eocene, about 34 million years ago. The elastic central cord that keeps the branch of the lichen intact even when its cortex is split is an adaptation that has helped allow the genus to thrive in many ecosystems and climates.^[10]

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