Xyloschistes platytropa

Xyloschistes platytropa grows on wood, usually on bark or decorticated branches. The thallus is poorly developed—a vague, whitish patch that may lie beneath the bark—and is reported with a green alga of the Palmella-type as its photosynthetic partner (photobiont). The apothecia (disc-like fruiting bodies) are at first immersed in the substrate and then break through, changing from urn-shaped to nearly flat with a thin, slightly raised margin. They are black, round to elongate, and about 0.5 mm across. The hymenium (spore-bearing layer) is about 40 μm thick; the upper and lower tissues around it (epithecium and hypothecium) are brownish, and the exciple is darker, almost black. The supporting threads (paraphyses) are rather loosely coherent and relatively thick, with somewhat darkened tips. In iodine reagents the hymenial gel shows a weak bluish staining reaction that soon shifts to yellowish or wine-red tones. The asci usually contain a single ascospore (rarely two). The spores are long and narrow, initially with several cross-walls and eventually becoming muriform—that is, divided into many cells by both transverse and longitudinal walls; they mature to brownish-black. Reported measurements place them at roughly 22–36 μm long by 6–8 μm wide.

Early accounts agree that Xyloschistes platytropa is a wood-dwelling lichen. It grows on timber and on debarked twigs (especially pine), and is often subcortical: the thallus develops beneath the bark and the fruiting bodies later break through. Reported microhabitats include fence wood and dead, barkless pine branches. All cited nineteenth-century records are from Finland. Nylander listed material from northern Karelia (near Lieksa) and from the municipality of Inari, Finland (near Veskoniemi), while Zahlbruckner noted a specimen from Evois documented by Johan Petter Norrlin in 1866. The 2021 Finnish national lichen checklist confirms the species for Finland and repeats the wood-dwelling type citation, matching these nineteenth-century accounts. It has also been reported from Spain. In Finland, the species is assessed as Endangered (EN) in the 2019 national Red List under criteria C1 and D, indicating a small population with inferred or observed decline and a very small or restricted population; the assessment places it in the forest wood-habitat group. In 2010 it had been listed as Data Deficient. Ancestral-state analyses interpret Xyloschistes as saprotrophic, with Ingvariella as a nearby lichenised relative.

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