Lithographa Opegraphoides

''Lithographa'' is a genus of lichen-forming fungi in the family Xylographaceae. These rock-dwelling lichens form tightly attached crusts that crack into small tile-like patches, typically appearing in shades of grey, brown, or nearly black. The genus includes six species found primarily in cold regions and high mountains, where they grow on hard rock surfaces in harsh environments. They reproduce through distinctive elongated or round fruiting bodies that appear as dark slits or discs embedded in the crusty surface.

Taxonomy

The genus was circumscribed in 1857 by the Finnish lichenologist William Nylander, with ''Lithographa petraea'' assigned as the type species. This species is now known as '' Lithographa tesserata''. Nylander characterized ''Lithographa'' as having a thallus that is evanescent (disappearing) or scarcely visible, with swollen apothecia, a rim-like , thick convex margins, spore sacs containing numerous spores, and very slender, somewhat branched paraphyses.

Rounded, sometimes gyrose apothecia in the related genus '' Lambiella'' set it apart from slit-disc ''Lithographa''. Phylogenetic work places ''Lithographa'' in the same clade as bark-dwelling '' Ptychographa'', and both differ from '' Wadeana'', which has a weaker exciple, reddish apothecia and a filamentous green alga ('' Trentepohlia'') as its .

Description

''Lithographa'' forms a tightly attached crust that cracks into small, tile-like patches (). Each patch is coated by an —a thin film of dead fungal cells that lends a finish—and may be fringed by a barely visible , the pale hyphal growth that first colonises the rock. Thallus colour varies from pale grey through brown to almost black. Internally, the fungal partner houses minute, spherical green algae (a photobiont). Chemical analyses reveal a suite of orcinol-derived compounds, including both depsides and β-orcinol depsidones.

The reproductive bodies are apothecia that range from elongate, slit-like to small angular or round . They sit flush with, or slightly above, the thallus and never bear a ; instead, the visible border is a —a dark, opaque ring of densely fused hyphae that radiate outward from the base. The disc itself is a narrow, black fissure. Inside, the clear hymenium turns blue with iodine staining, a sign of amyloid material in the ascus walls, while a sparse mesh of branched paraphyses threads the spore layer. Asci are club-shaped and always eight-spored; when stained, the sides of the ascus apex turn dark blue, but a broad central plug remains unstained, a pattern shared with the '' Trapelia'' type. Mature ascospores are typically single-celled (aseptate), colourless and ellipsoidal; two southern-hemisphere species develop somewhat spores, although it is suspected that they may not belong to this genus. Many species also produce immersed pycnidia that release slender, rod-shaped conidia, but these structures are absent in the type species, ''L. tesserata''.

Species

*'' Lithographa graphidioides''
*'' Lithographa olivacea''
*'' Lithographa opegraphoides''
*'' Lithographa serpentina''
*'' Lithographa skottsbergii''
*'' Lithographa tesserata''

References

{{Taxonbar, from=Q6648027

Baeomycetales
Baeomycetales genera
Lichen genera
Taxa named by William Nylander (botanist)
Taxa described in 1857