Polycauliona phlogina

The species forms a thin, crust-like thallus (vegetative body) 0.3–3 cm across, which may be greyish, pale yellow, or yellow. The thallus surface may be broken into small tile-like patches (areoles), embedded within bark or rock, but the areoles are usually almost or completely broken down into powdery reproductive granules (soredia). The soredia are typically thin and loose rather than forming a cracked surface, and the outer layer (cortex) is usually poorly developed. The soralia (patches of soredia) begin as small dot-like spots and later merge into a more continuous, mottled sorediate mass.

Apothecia (disc-shaped fruiting bodies) are usually present, though often scattered, with yellow to orange-yellow discs that may carry a thin white powdery coating (pruina). They are zeorine in form, meaning they have a visible rim derived from the thallus, with a paraplectenchymatous thalline exciple (outer rim of rounded cells) and a prosoplectenchymatous true exciple (inner rim of elongated cells). Scythioria phlogina occurs in two main colour forms. One has yellow soralia coloured by anthraquinones (orange pigments typical of the family), while the other has whitish to pale green soralia that lack anthraquinone pigmentation. Apart from soralia colour, the two forms are very similar in anatomy and other examined characters, including their apothecia and spores. …

Scythioria phlogina is almost entirely bark-dwelling (corticolous), growing chiefly on bark and more occasionally on wood. Although recorded from many tree genera, the species shows a strong preference for high-pH, nutrient-rich bark, such as that of Ulmus, Fraxinus, and Acer. Kondratyuk and coauthors summarized the habitat more broadly as bark of broad-leaved trees in forested areas and, in drier steppe habitats, plant debris, soil, and small shrubs, only rarely rock. In steppe habitats it was reported growing together with Caloplaca chlorina and Gallowayella coppinsii. Black Sea populations are strongly maritime and grow mainly on twigs of salt marsh shrubs such as Halocnemum strobilaceum and Limonium suffruticosum, as well as on plant debris and loess soil. The species is widespread but tends to favour maritime regions with a temperate climate. In the Nordic countries the species was recorded mainly from Denmark and southern Sweden, with only single known collections from Norway and Finland; its centre of occurrence is in southern Scandinavia, where it appears to be chiefly inland rather than strongly maritime. The species occurs across large parts of Europe. In the Netherlands, it occurs throughout the country on nutrient-rich bark, particularly on clay soils, and is most common in coastal areas. …