Punctelia borreri is a species of foliose lichen in the family Parmeliaceae. It is a common and widely distributed species, occurring in tropical, subtropical, and temperate regions of Africa, Asia, Europe, North America, Oceania, and South America. The lichen typically grows on bark of deciduous trees, and less commonly on rock. Some European countries have reported increases in the geographic range or regional frequency of the lichen in recent decades, attributed alternatively to a reduction of atmospheric sulphur dioxide levels or an increase in temperatures resulting from climate change.
The lichen is characterised by a greenish-grey to bluish-grey upper thallus surface, a black lower surface, pseudocyphellae on the surface of the thallus (tiny pores that facilitate gas exchange), and chemically, by the presence of gyrophoric acid in the medulla. There are several lookalike Punctelia species, including P. subrudecta, P. perreticulata, and P. reddenda. These can be distinguished from P. borreri by differences in chemistry, in the nature of the vegetative reproductive structures present on the thallus, or the colour of the thallus underside.
Punctelia borreri is named after William Borrer, a botanist who made the first scientific collections of the lichen in early 19th-century England. James Edward Smith was the first to publish a scientific description of the species in 1807, followed independently a year later by Dawson Turner. It was later assigned as the type species of Punctelia by Hildur Krog when she circumscribed the new genus in 1982. Punctelia borreri is used as a component of traditional Chinese medicine. The lichen has been shown to harbour other organisms, including endophytic fungi and lichenicolous fungi.
Systematics
The lichen was first formally described as Lichen borreri by English botanist James Edward Smith in the 25th volume of his work English Botany, published in 1807. The first scientific collections of the lichen were made by another English botanist, William Borrer, whose name is honoured in the species epithet borreri. The type locality was Sussex, where Borrer noted the lichen to be "not uncommon on the trunks of trees, especially on fruit trees". Smith described the specific characteristics of the new species as follows: "Leafy, slightly imbricated, tawny grey, even, sprinkled with white powdery warts; its segments sinuated, rounded at the ends: brown and fibrous beneath. Shields bright chesnut." Smith suggested that it had been overlooked by botanists until that time, who might have mistaken it for the common and widespread Parmelia saxatilis. The type specimen collected by Borrer is now part of the lichen collection at the Natural History Museum in London.
The species had been independently named and described as Parmelia borreri a year earlier in 1806 by Dawson Turner, who, like Smith, had examined specimens sent to him by Borrer. Turner presented his findings to the Linnean Society of London on 3 June 1806, and in a subsequent publication two years later, he gave the lichen a more thorough description in both English and Latin. Turner mentions that he had sent samples to prominent Swedish botanists Erik Acharius and Olof Swartz, "both of whom acknowledged it altogether new to them, and a very distinct species". Similar to Smith's account, Turner's description of the lichen also compares and contrasts it with Parmelia saxatilis; in Turner's opinion, two main characteristics distinguish the two species:
Other morphologically similar Parmelia species with point-like pseudocyphellae and simple rhizines were referred to as the Parmelia borreri group, and within the genus Parmelia, they were classified in section Parmelia, subsection Simplices, as defined by Mason Hale and Syo Kurokawa in 1965. In 1982, Hildur Krog circumscribed the new genus Punctelia, a segregate of Parmelia. The main characteristics distinguishing the new genus from Parmelia are the development of the pseudocyphellae, the secondary chemistry of the medulla, and phytogeography. Krog transferred twenty-two species to Punctelia from Parmelia. As the earliest-published member of this group, Punctelia borreri was designated the type species of the new genus.