Usnea is a genus of mostly pale grayish-green fruticose lichens that grow like leafless mini-shrubs or tassels anchored on bark or twigs. 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. Like other lichens it is a symbiosis of two or three fungi and an alga. 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. A distinguishing test is that the branches of Usnea are somewhat elastic, but the branches of Alectoria snap cleanly off. Systematics The genus Usnea was circumscribed by Michel Adanson in 1763. He used the name designated by Johann Jacob Dillenius, whose earlier published description did not met the rules of valid publication as established by the International Code of Nomenclature for algae, fungi, and plants. However, he did not specify a type specimen; the species Usnea florida, moved to the genus by Friedrich Heinrich Wiggers in 1780, has been designated as the lectotype. Since the establishment of the genus, hundreds of Usnea species have been described. A three-volume series by Józef Motyka published in 1936 and 1947 listed 451 species. By 2006, the genus contained more than 600 species, which made it one of the largest genera within the family Parmeliaceae. However, many former species are now regarded as morphological varieties and adaptations to local circumstances. The number of recognized species in Finland has decreased for this reason, for example, dropping from 34 in 1951 to 25 in 1963 and only 12 in 2000. In addition, some former Usnea species have been moved to other genera; for instance, Usnea longissima was renamed Dolichousnea longissima in 2004. By 2022, the overall number of species assigned to the genus had dropped to 355. The name Usnea is probably derived from the Arabic word Ushnah, meaning moss or lichen, though it may also mean "rope-like". Based on a fossil Usnea found in Baltic amber, the genus is known to date back to at least the late Eocene, about 34 million years ago. Morphology and reproduction 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