{{Short description|Genus of lichens}} {{Use dmy dates|date=June 2025}} {{Use Oxford spelling|date=June 2025}} {{Automatic taxobox | image = | image_caption = | taxon = Leioderma | authority = Nyl. (1888) | type_species = ''Leioderma pycnophorum'' | type_species_authority = Nyl. (1888) | subdivision_ranks = Species | subdivision = ''L. amphibolum''<br /> ''L. applanatum''<br /> ''L. cherokeense''<br /> ''L. duplicatum''<br /> ''L. erythrocarpum''<br /> ''L. glabrum''<br /> ''L. pycnophorum''<br /> ''L. sorediatum''<br /> ''L. spongiosum'' }}
'''''Leioderma''''' is a genus of lichen-forming fungi in the family Pannariaceae. These lichens form small, leaf-like rosettes that are loosely attached to their growing surface and can reach 10–15 cm across. The upper surface is typically grey-blue in colour and may be smooth or have a fine, cobweb-like texture. The genus contains nine species that are mainly found on tree bark in humid forests, with a distribution centred on New Zealand and Australia but extending to South America and North America.
==Taxonomy==
The genus was circumscribed by the Finnish lichenologist William Nylander in 1888. He assigned ''Leioderma pycnophorum'' as the type species. In his original description, Nylander characterised ''Leioderma'' as having a membranaceous, lobed thallus with {{lichengloss|lecanorine}} apothecia (fruiting bodies) that are sparsely distributed across the lobes. He noted that the thallus was thin, with a thickness of about 0.1 millimetre, and described the lobes as having reflexed (bent back) margins measuring 10–15 millimetres wide. Nylander distinguished the genus by its simple spores (paraphyses lacking septa) and the distinctive surface granulation on the upper portions of the thallus. The species was described from specimens collected on destroyed vegetation at Greymouth (New Zealand), a disturbed habitat.<ref name="Nylander 1888"/>
==Description==
''Leioderma'' forms small, leaf-like (foliose) to scale-like (squamulose) rosettes that are usually loosely attached to their substrate and can reach 10–15 cm across. Individual {{lichengloss|lobes}} are rounded, 1–6 mm wide, and may lie flat, arch slightly, or curve inward with age; their edges are often faintly scalloped and can bear flattened outgrowths ({{lichengloss|phyllidia}}) or coarse, powdery reproductive {{lichengloss|granules}} (soralia). The upper surface is most often grey-blue but may appear brownish; it ranges from smooth to faintly roughened or clothed in a fine, cobweb-like felt (an {{lichengloss|arachnoid}} tomentum), giving each species a distinctive texture. Beneath a thin, brick-like ({{lichengloss|paraplectenchymatous}}) {{lichengloss|cortex}} 30–50 μm thick lies a layer of cyanobacteria—usually chains of ''Scytonema'', but compact clusters of ''Nostoc'' in the subgenus ''Fuscoderma''—followed by a loosely woven white medulla 70–100 micrometres (μm) thick. The lower surface is pale to ochre-brown and carries tufts of {{lichengloss|simple}}, often darkening rhizines up to 2–5 mm long that anchor the thallus. Environmental factors such as moisture and light account for much of the observed variation in lobe width, colour, and rhizine length across populations.<ref name="Galloway 1987"/>
Reproduction is mainly through {{lichengloss|laminal}} apothecia that sit flush with the surface, measure up to about 1.5 mm across, and display pale to dark red-brown discs often framed by a thin margin. Internally, the hymenium turns blue in iodine (an amyloid reaction), and the cylindrical eight-spored asci have a distinct amyloid cap. The resulting spores are single-celled, colourless, thick-walled with an ellipsoid shape, typically 13–20 × 6–10 μm, and can be slightly pointed at the ends in some species. Minute black pycnidia, especially along the lobe margins, release straight rod-shaped conidia 3–5 × 1–2 μm. Chemically the genus is almost inert: thin-layer chromatography usually detects no secondary metabolites, although ursolic acid occurs sporadically and rare individuals of ''L. sorediatum'' produce traces of atranorin, skyrin, and related pigments. Such chemical oddities are considered exceptional and of little taxonomic importance.<ref name="Galloway 1987"/>
==Habitat and distribution==
''Leioderma'' species are mainly corticolous (bark-dwelling) epiphytes of damp, shaded forests; they colonise the bark of trees and shrubs where humidity is consistently high. Saxicolous (rock-dwelling) occurrences are uncommon and largely involve ''L. duplicatum'' and the ecologically plastic ''L. sorediatum''. Four taxa (''L. amphibolum'', ''L. applanatum'', ''L. duplicatum'' and ''L. pycnophorum'') are confined to low elevations below about 1,000 m, whereas ''L. erythrocarpum'' and ''L. sorediatum'' span the full range from sea-level mangroves to montane sites at roughly 2,800 m and 2,600 m, respectively.<ref name="Galloway 1987"/>
The genus was long regarded as a Gondwanan group centred on New Zealand and south-east Australia, with disjunct outliers in Chile, Brazil and Tristan da Cunha.<ref name="Galloway 1987"/> Two species described since that assessment—''L. spongiosum'' from Ecuador and ''L. cherokeense'' from the south-eastern United States—extend its distribution into tropical South America and temperate North America.<ref name="Jørgensen & Arvidsson 2004"/><ref name="Jørgensen & Tønsberg 2005"/> A report of ''Leioderma sorediatum'' further suggests that ''Leioderma'' is more widespread than previously recognised.<ref name="Jørgensen 2009"/>
==Species==
{{As of|2025|June}}, Species Fungorum (in the Catalogue of Life) accepts nine species of ''Leioderma'':<ref name="CoL_CDKYF"/> * ''Leioderma amphibolum'' {{au|(C.Knight) P.M.Jørg. & D.J.Galloway (1987)}}<ref name="Galloway 1987"/> * ''Leioderma applanatum'' {{au|D.J.Galloway & P.M.Jørg. (1987)}}<ref name="Galloway 1987"/> * ''Leioderma cherokeense'' {{au|P.M.Jørg. & Tønsberg (2005)}}<ref name="Jørgensen & Tønsberg 2005"/> * ''Leioderma duplicatum'' {{au|(Müll.Arg.) D.J.Galloway & P.M.Jørg. (1987)}}<ref name="Galloway 1987"/> * ''Leioderma erythrocarpum'' {{au|(Delise ex Nyl.) D.J.Galloway & P.M.Jørg. (1987)}}}<ref name="Galloway 1987"/> * ''Leioderma glabrum'' {{au|D.J.Galloway & P.M.Jørg. (1987)}}<ref name="Galloway 1987"/> * ''Leioderma pycnophorum'' {{au|Nyl. (1888)}}<ref name="Nylander 1888"/> * ''Leioderma sorediatum'' {{au|D.J.Galloway & P.M.Jørg. (1987)}}<ref name="Galloway 1987"/> * ''Leioderma spongiosum'' {{au|P.M.Jørg. & Arv. (2004)}}<ref name="Jørgensen & Arvidsson 2004"/>
==References== {{Reflist|refs=
<ref name="CoL_CDKYF">{{Catalogue of Life |id=CDKYF |title=''Leioderma'' |access-date=29 June 2025}}</ref>
<ref name="Galloway 1987">{{cite journal |last1=Galloway |first1=D.J. |last2=Jørgensen |first2=P.M. |year=1987 |title=Studies in the lichen family Pannariaceae II. The genus ''Leioderma'' Nyl. |journal=The Lichenologist |volume=19 |issue=4 |pages=345–400|doi=10.1017/S0024282987000367 |bibcode=1987ThLic..19..345G }}</ref>
<ref name="Jørgensen & Arvidsson 2004">{{cite journal |last1=Jørgensen |first1=P.M. |last2=Arvidsson |first2=L. |year=2004 |title=The lichen family Pannariaceae in Ecuador |journal=Symbolae Botanicae Upsalienses |volume=34 |issue=1 |pages=113–132}}</ref>
<ref name="Jørgensen & Tønsberg 2005">{{cite journal |last1=Jørgensen |first1=Per Magnus |last2=Tønsberg |first2=Tor |year=2005 |title=''Leioderma cherokeense'' (Pannariaceae, Lecanorales) sp. nov. from the Great Smoky Mountains, North Carolina, U.S.A. |journal=The Bryologist |volume=108 |issue=3 |pages=412–414 |jstor=20061120 |doi=10.1639/0007-2745(2005)108[0412:LCPLSN]2.0.CO;2}}</ref>
<ref name="Jørgensen 2009">{{cite journal |last1=Jørgensen |first1=Per M. |last2=Wolseley |first2=Patricia A. |title=''Leioderma sorediatum'' D. J. Galloway & P. M. Jørg. discovered in Thailand, with a note on the world distribution of ''Erioderma mollissimum'' (Sampaio) Du Rietz |journal=The Lichenologist |volume=41 |issue=3 |year=2009 |doi=10.1017/S0024282909008482 |pages=315–316|bibcode=2009ThLic..41..315J }}</ref>
<ref name="Nylander 1888">{{cite book |last=Nylander |first=W. |year=1888 |title=Lichenes Novae Zelandiae |trans-title=Lichens of New Zealand |pages=47–48 |publisher=Paul Schmidt |location=Paris |language=la |url=https://gallica.bnf.fr/ark:/12148/bpt6k96961z/f48.item}}</ref>
}}
{{Taxonbar|from=Q6520051}}
Category:Pannariaceae Category:Lichen genera Category:Peltigerales genera Category:Taxa named by William Nylander (botanist) Category:Taxa described in 1888