{{Short description|Single-species lichen genus}} {{Use dmy dates|date=June 2025}} {{Use Oxford spelling|date=June 2025}} {{Speciesbox | image = Ptychographa xylographoides.jpg | image_caption = Botanical illustration of ''Ptychographa xylographoides'' by J.M. Crombie & Blair, 1874 | parent_authority = Nyl. (1874) | taxon = Ptychographa xylographoides | authority = Nyl. (1874) | synonyms_ref = <ref name="Species Fungorum synonymy"/> | synonyms = *''Leptographa'' {{au|(Th.Fr.) M.Choisy (1950)}} *''Placographa'' sect. ''Leptographa'' {{au|Th.Fr. (1874)}} *''Ptychographomyces'' {{au|Cif. & Tomas. (1953)}} }}
'''''Ptychographa''''' is a fungal genus in the family Xylographaceae.<ref name="CoL_CF2BV"/> It is a monospecific genus, containing the single species '''''Ptychographa xylographoides'''''. This inconspicuous lichen grows as a barely visible coating of dark grains on rotting logs and branches in undisturbed old forests. The species is most readily identified by its narrow, elongated black fruiting structures that run parallel to the wood grain.
==Taxonomy==
The genus ''Ptychographa'' was established by the Finnish lichenologist William Nylander in 1874 with the description of the type species ''Ptychographa xylographoides''. In his original Latin {{lichengloss|diagnosis}}, Nylander characterized the genus by its white thallus with black, lance-shaped ({{lichengloss|lanceolate}}) apothecia. He described the apothecia as somewhat prominent above the flattened thallus, with longitudinally folded epithecium and margins the same colour. The asci were noted to contain eight colourless, simple ellipsoidal spores measuring 11–14 micrometres (μm) in length and 6–7 μm in width, with a slightly coloured epithecium and blackish hypothecium. Nylander also observed a reddish-brown gelatinous hymenium.<ref name="Nylander 1874"/>
Nylander collected the type specimen from Scotland, growing on wood of ''Sorbus aucuparia'' (rowan), and noted its occurrence at Creag na Cailleach in Killin. He distinguished ''Ptychographa'' from the related genus ''Xylographa'' by noting that while both genera share similar apothecial characteristics, ''Ptychographa'' has a distinctive arrangement of hymenia that differs from ''Xylographa'' and ''Opegrapha'', along with all other members of the Graphidaceae. Nylander concluded that the subglomerate conidia were absent in this genus.<ref name="Nylander 1874"/>
==Description==
''Ptychographa xylographoides'' grows as an exceedingly thin crust that is often little more than a dusting of dark-brown grains on decaying wood. Each grain is a {{lichengloss|goniocyst}}—a microscopic packet in which a few green algal cells (the {{lichengloss|chlorococcoid}} photobiont) are wrapped by fungal hyphae. These goniocysts measure roughly 20–50 micrometres (μm) across and are sheathed in a uniform {{lichengloss|cortex}} of angular, brown-walled cells, giving the thallus its subdued {{lichengloss|matt}} appearance. Because the vegetative body is so inconspicuous, the lichen is easiest to spot when it develops its slit-like fruit bodies.<ref name="Cannon et al. 2021"/>
The reproductive structures are narrowly elongate apothecia ({{lichengloss|lirellae}}) that run parallel to the grain of the wood. Each lirella is 0.3–1.4 mm long yet only 0.1–0.3 mm wide, with a black fissured {{lichengloss|disc}} that may split into two parallel slits—each over a separate spore layer—separated by dark tissue. The rim surrounding the disc (the {{lichengloss|true exciple}}) is unusually thick, entirely black-brown and friable, being composed of tightly cemented hyphae. Internally, a dark-brown {{lichengloss|hypothecium}} merges into the exciple, while the clear hymenium above it reaches 45–60 μm in height and stains yellow then blue in iodine–potassium iodide tests. Sparse, mostly unbranched paraphyses thread the hymenium, their tips darkened and stuck together to form an {{lichengloss|epithecium}}.<ref name="Cannon et al. 2021"/>
Club-shaped asci contain eight smooth, colourless ascospores, 8.5–13 × 4.5–6.5 μm, that remain aseptate (single-celled) and lack any gelatinous sheath. The ascus apex shows a blue-staining outer coat in iodine, with a non-reactive dome at the very tip—an arrangement characteristic of the ''Trapelia'' type. Asexual propagation occurs through minute, immersed or superficial pycnidia that release rod-shaped conidia about 4–7 × 0.8 μm.<ref name="Cannon et al. 2021"/>
==Ecology==
''Ptychographa xylographoides'' species colonises the upper surfaces of fallen trunks and large branches in undisturbed, often ancient woodlands, where the damp, slowly rotting timber provides the stable microhabitat it requires.<ref name="Cannon et al. 2021"/>
==References== {{Reflist|refs=
<ref name="Cannon et al. 2021">{{cite book |last1=Cannon |first1=P. |last2=Fryday |first2=A. |last3=Spribille |first3=T. |last4=Coppins |first4=B. |last5=Vondrák |first5=J. |last6=Sanderson |first6=N. |last7=Simkin |first7=J. |year=2021 |title=Baeomycetales: Xylographaceae, including the genera ''Lambiella'', ''Lithographa'', ''Ptychographa'' and ''Xylographa'' |series=Revisions of British and Irish Lichens |volume=17 |page=7 |url=https://britishlichensociety.org.uk/sites/default/files/Xylographaceae.pdf}}{{Open access}}</ref>
<ref name="CoL_CF2BV">{{Catalogue of Life |id=CF2BV |title=''Ptychographa'' |access-date=19 June 2025}}</ref>
<ref name="Nylander 1874">{{cite journal |last=Nylander |first=W. |year=1874 |title=Addenda nova ad lichenographiam europaeam. Contin. XVIII |journal=Flora (Regensburg) |volume=57 |pages=305–318 [315] |language=la |url=https://www.biodiversitylibrary.org/page/62240}}</ref>
<ref name="Species Fungorum synonymy">{{cite web |title=Synonymy: ''Ptychographa'' Nyl., Flora, Regensburg 57: 315 (1874) |url=http://www.speciesfungorum.org/Names/SynSpecies.asp?RecordID=4542 |publisher=Species Fungorum |access-date=19 June 2025}}</ref>
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Category:Baeomycetales Category:Lichen genera Category:Baeomycetales genera Category:Taxa named by William Nylander (botanist) Category:Taxa described in 1874