Inoderma is a small genus of four species in the northern Hemisphere and tropical East Africa. A single species, I. byssaceum, is known from Norway. The genus is lichenized with a trentepohlioid photobiont.

Description

Thallus

Inoderma species have a thin but typically well-developed and extensive thallus that is continuous to cracked. The color is pale olive grey, pale fawn or whitish. The surface is weakly felty to (sub-)granular or powdery-mealy. The thallus is delimited by a thin marginal zone of whitish hyphae that can be indistinct or absent.

Fruitbodies

The apothecia of Inoderma are rounded to weakly lobed, and colored pale brown to brown below a thick white pruina. They are level with the thallus surface or raised over the thallus, and 0.4–1.0 mm in size. The disc is flat to weakly convex. Apothecia are not reported for the western European I. subabietina, a species not found in Norway.

Internally, the apothecia are delimited by a poorly separated, greyish to pale brownish margin that contains numerous pale granular crystals.

The hymenium is unpigmented to pale yellowish brown and weakly conglutinate. It is overlain by a greyish to brownish epithecium that contains numerous pale granular crystals.

The epithecium is formed by largely free, densely branched and intertwined tips of paraphysoids.

The hypothecium below the hymenium is colorless or dark brown.

The asci are of the Arthonia-type and contain 8 spores.

The spores are narrow obovoid and divided by (1–)2–4 transverse septa. They are persistently colorless or have a pale brown pigmentation with granular ornamentation when they get old. The apical cell is not enlarged or only slightly enlarged.

Anamorph

Pycnidia are found in all species. They are clearly raised above the thallus and covered in a dense white pruina. The pycnidia are 0.2–0.5 mm in size and open with a 0.1–0.4 mm wide rugged and gapping pore that is often eroded. White to pale pinkish conidial masses are sometimes found protruding from the pore. The pycnidia wall is dark red-brown and has a distinct crystal layer at the outer surface. The conidia are rod-shaped and 3–8 × 1–2 μm in size.

Chemistry

Inoderma species produce lepraric acid, confluentic acid, and two compounds called "the byssaceum unknowns". Species with lepraric acid show a distinct K+ lemon yellow color reaction in thallus, apothecia and pycnidia. The hymenial gels react I+ red or blue and KI+ deep blue. KI+ blue ring structures in the tholus of the asci have not been observed.

The brown pigment in the wall of the pycnidia changes to greenish black in K solution.

Ecology

The genus Inoderma is widely distributed in oceanic to suboceanic regions of the Northern Hemisphere and are currently found in temperate Europe, North America, Eastern Siberia, and Japan. A single species, I. afromontana, is known from tropical montane forests in Uganda.

Inoderma species usually grow on deeply fissured to flaky bark of old trees in humid natural to old-growth forests. The species prefer the dry, rain-shaded side of tree trunks, where they can be found on a large variety of deciduous and coniferous tree species with acidic bark. More rarely, Inoderma species can be found on shady, rain-shaded siliceous rock.    

Remarks

Inoderma has been included in Arthonia until recently. Based on molecular systematic studies, it has been accepted as an independent genus by Frisch et al.  (2015). The genus is well-defined by the distinct white pruinose pycnidia sitting on a conspicuous pale olive grey to near whitish thallus, and when present, the densely white pruinose apothecia with colorless, transversely septate spores lacks a clearly enlarged apical cell. The genus has a distinctly southern distribution in Norway. Its only representative, I. byssaceum, is a rare species confined to old oak trees in a narrow area near Skien in Telemark and Vestfold.

Literature

Frisch A, Klepsland J, Palice Z, Bendiksby M, Tønsberg T and Holien H (2020). New and noteworthy lichens and lichenicolous fungi from Norway. Graphis Scripta 32(1): 1–47.

Frisch A, Ohmura Y, Ertz D and Thor G (2015). Inoderma and related genera in Arthoniaceae with elevated white pruinose pycnidia or sporodochia. Lichenologist 47: 233–256.