Physcia is a genus of lichen-forming fungi in the family Physciaceae. The widely distributed genus contains about 80 species. The genus is cosmopolitan, and has been extensively studied in various regions in the past several decades, with significant biodiversity in South America identified as a central diversity hotspot. Physcia species are foliose, lichens that grow with a loose to close appressed habit. Their upper surface is typically whitish, pale greenish, green-grey, or dark grey in colour. The thallus colour remains relatively unchanged when moistened. Physcia lichens typically grow on bark, on wood, or rock, although they have occasionally been recorded dwelling on man-made structures. They thrive in nutrient-rich environments and are expanding rapidly in urban areas of the United Kingdom previously affected by SO2 pollution.
The main characteristics that separate Physcia from similar genera in the same order, including Dirinaria, Heterodermia, Hyperphyscia, Kashiwadia, Phaeophyscia, and Pyxine, are the distinct morphology of its (brown and two-celled), its somewhat cylindrical (asexual reproductive structures), and the presence of the chemical atranorin in the upper . Physcia has been divided into sections based on morphological and chemical , such as the presence or absence of on the thallus margins and K+ (yellow) spot test reaction in the cortex.
The genus Physcia was formally established by André Michaux in 1805, who elevated it from a section within the genus Lichen as originally outlined by Johann Christian Daniel von Schreber in 1791. Over the years, the genus has been divided into various sections based on such as colour, presence of , thallus spotting, and chemical reactions, with significant contributions from taxonomists like Edvard August Vainio in 1890 and Roland Moberg, who in 1977 and later in 1986, refined the infrageneric classification of this diverse genus.
Numerous lichenicolous fungi are known to colonise Physcia species include those with species epithets reflecting their ecological ties to this host, such as Bryostigma epiphyscium and Xanthoriicola physciae. Infections by these fungi can cause distinct physical symptoms useful for identification, such as the gall formations by Syzygospora physciacearum and the orange discolouration by Marchandiomyces auranticus. Additionally, the long cilia of Physcia adscendens, which confer velcro-like attachment capabilities to the thallus of this species, are used by birds in nest building. Some Physcia species have been employed in biomonitoring studies of air quality.
Systematics Historical taxonomy
The first member of the present-day genus Physcia to be formally described was Physcia stellaris. This was one of several dozen lichen species described by the Swedish taxonomist Carl Linnaeus in his 1753 treatise Species Plantarum. The genus was originally circumscribed in 1791 by Johann Christian Daniel von Schreber as a section of the genus Lichen. André Michaux promoted the section to full genus status in 1805. Although he cited the description given by Erik Acharius in his 1798 Prodromus, Acharius himself had treated the taxon as a tribus, meaning that it is not validly published according to the rules of botanical nomenclature. Because Acharius in an earlier 1794 publication cited Schreber 1791, the authorship of the basionym is attributed to him. The type species of the genus was only chosen about 150 years later: in 1963, John Walter Thomson chose Physcia tenella from the list elaborated by Michaux (1803) based on the species cited by Acharius (1798). Physcia is the type genus of the Physciaceae, a family proposed by Alexander Zahlbruckner in 1898.
Infrageneric classification In 1890, Edvard August Vainio, divided the genus into two sections, Euphyscia and Dirinaria, which were characterised by having a colourless or brownish-black hypothecium, respectively. Euphyscia, in turn, was divided into Albida (K+ yellow thallus) and Sordulenta (K−), while the latter was further divided into Brachysperma and Macrosperma based on the morphology of their conidia. Gustaf Einar Du Rietz used Vainio's sectional arrangement in his 1925 treatment of Scandinavian Physcia species. A decade later, Lynge proposed instead a different infrageneric organisation, dividing the genus into the subgenera Macrosperma and Brachysperma, the latter of which was further subdivided into several sections.
The Swedish lichenologist Roland Moberg has authored or co-authored numerous scientific publications on the taxonomy and phytogeography of Physcia and related genera. In a 1977 work, he divided Physcia into four sections: Sect. Physcia (type: P. tenella) Distinguished by the presence of cilia along the edges of the lobes. Sect. Caesiae (type: P. caesia) Identified by a maculated (spotted) thallus surface, lobes that are cilia-free, and both cortex and medulla reacting K+ yellow, with atranorin present along with zeorin. Sect. Fusisporae (type: P. magnussonii) Characterised by their narrowly ellipsoid spores featuring a unique ornamentation on their surface. Sect. Stellares (type: P. dubia) Noted for the absence of marginal cilia and maculation on the surface, as well as a K-negative spot test reaction in the medulla.
In 1986, Moberg proposed an additional section: Sect. Atrae (type: P. atrostriata) Identified by an upper surface without maculation, a cortex and medulla that both react K+ yellow (containing atranorin and zeorin), no cilia, and a black underside.
Naming
The generic name Physcia is derived from the Ancient Greek , physkion, meaning "bladder" or "blister". One source suggests that this name alludes to the lichen's apothecia and soredia found on its foliose thallus, which give the surface a vesiculose ("blistered") aspect when these reproductive structures develop. In an alternative etymological interpretation, John Alan Elix suggests that the name was originally intended to refer to species with inflated or hollow lobes. However, he notes, the name is somewhat of a misnomer, as species like Physcia physodes (now classified as Hypogymnia physodes) that were originally included under this term, no longer fall within the genus.
Species in the genus Physcia are commonly known as "rosette lichens". This common name alludes the centrifugal (rosette-like) growth form characteristic of many of the species.
Species pairs
within a genus are closely related species that are morphologically similar but can be distinguished by subtle differences, often in their chemical compositions or reproductive strategies. However, advanced molecular methods have challenged the traditional concept of species pairs within the genus Physcia, which was previously defined largely through these similarities. Molecular analyses reveal that the phylogenetic relationships among taxa like Physcia aipolia and P. caesia are more complex than previously understood. These findings indicate that distinctions based on morphological and chemical characteristics alone do not necessarily reflect the true genetic diversity and evolutionary relationships within the genus. Consequently, the concept of species pairs, which suggests a straightforward dichotomy between similar species, is considered outdated. This perspective advocates for an integrative approach to species differentiation that combines morphological, chemical, and genetic data to more accurately delineate species within Physcia.
Description
The thallus of Physcia is foliose, meaning it is leaf-like in structure, and , typically forming a roughly circular shape with lobes radiating outward. The thallus is generally loosely attached to its Thallus lobes are narrow, generally less than 3 mm wide, with a colouration that ranges from whitish and bluish-grey to dark grey. The thallus of Physcia grows to about in diameter, although neighbouring thalli can combine to form more extensive growths. The upper surface colour ranges from greenish grey or whitish grey to dark grey. The thallus shows minimal colour change or remains unchanged when wet. The surface can be or slightly glossy and might display minute white spots known as pseudocyphellae (tiny pores that allow gas exchange) visible with magnification. The texture of the surface varies from smooth to wrinkled () to pimpled (), as in the example of P. verrucosa. Sometimes, the surface is covered with white (spots). The lobes may also be covered with a fine, white powdery coating () and can have fringe-like projections () along the edges. Although the value of pruina as a taxonomic character is sometimes considered dubious due to possible environmental influences, a group of species has a constant, fine, and homogeneous pruinosity, mainly on the distal parts of the upper thallus surface: P. atrostriata, P. krogiae, P. phaeocarpa, P. undulata, and P. verrucosa.
The underside of the thallus is typically whitish, pale tan, grey, or pinkish. In contrast, the lower surface of Physcia atrostriata is distinctively , featuring a brown-black colour with striations, which sets it apart from other species in the genus. The thallus underside has rhizines, which are root-like structures that can be simple or branched. They range in colour from white to brownish-black. Many species within this genus develop soralia, structures that produce powdery reproductive propagules, on their upper surface. Physcia species can be distinguished and grouped into distinct taxa based on the position and shape of their soralia, which provides valuable characteristics for identification and classification.
The upper —the protective outer layer of the thallus—is , meaning it consists of tightly packed, roughly equal-sized cells. The lower cortex, in most instances, is —a tissue arrangement in which constituent fungal hyphae are aligned in a particular direction. The lower cortical layer is a layer that is usually 6–8 cells thick (about 20–30 μm) arranged in a fashion. Less frequently, Physcia species have been reported to have an anomalous "paraplectenchymatous" lower cortex. Two groups of these occur: those with a pale lower cortex (such as P. albata and P. halei); and those with a black lower cortex (such as P. crispa and P. sorediosa). Following detailed examination of various Physcia species through electron microscopy, Mason Hale discerned that the atypical lower cortex comprises two distinct layers. The basal layer is paraplectenchymatous, consisting of two to three cells in thickness, while the upper layer is prosoplectenchymatous and varies in thickness, occasionally blending seamlessly into the medulla. The photosynthetic partner in the lichen is —a type of green algae.
Reproductive structures (apothecia) are common; these are in form, meaning they are disc-like and surrounded by a pale . They can be directly attached to the thallus or borne on short stalks, without rhizines beneath them. The of these structures are typically brown to black, often with a white pruina, surrounded by a . The tissue layers within, namely the hymenium and , are colourless. The structural support within the hymenium consists of paraphyses, which are slender and may branch at their tips which are pale brown with a darker cap.