Inoderma

The name Inoderma first appears in Acharius's Lichenographia Universalis (1810),^[2] where he treated it within Verrucaria as a group of "doubtful species" characterized by a cobwebby, floccose to somewhat spongy thallus (or basal felt).^[4] The name Inoderma was published at genus rank by Samuel Frederick Gray in 1821. In Gray's treatment of Inoderma (attributed to Acharius) in 1821, the genus was diagnosed as a lichen with a soft, tow-like thallus that is closely attached to the substrate and may be spongy or thin and cobwebby. He also placed weight on the form of the fruiting bodies, describing them as very small and nearly spherical, with the fertile tissue sitting low in the thallus and surrounded by a two-layered wall: an outer, thick black layer that splits above into a nipple-like opening ("mouth"), and an inner, very thin membranous layer enclosing the central spore-bearing mass.^[5]

Gray included two species, Inoderma epigea and I. byssacea, and for each he pointed back to earlier placements under Sphaeria (Persoon) and Verrucaria (Acharius). He described I. epigea as forming a thin, uneven, pale yellowish thallus with sunken, globular fruiting bodies and a prominent dark opening, occurring "on muddy ground", while I. byssacea was said to have a dirty-white, cobwebby, somewhat powdery thallus and similarly small, half-sunken fruiting bodies with a black interior, growing "on the trunks of trees".^[5] In subsequent literature, the same group was also treated under other names (for example as Verrucaria section Inoderma, or under the genera Thrombium and Phaeothrombis), which are now regarded as synonyms of Inoderma.^[2]

It was resurrected for use in 2015 for a small group of species with the following features: elevated, white pruinose pycnidia, immersed to adnate white pruinose apothecia, and a weakly gelatinized hymenium. Inoderma byssaceum was assigned as the type species for the genus.^[6] In 2024, Gintaras Kantvilas reported Inoderma from Australasia for the first time and recognised two regional species: I. applanatum (described as new) and a new combination I. platygraphellum. Species in the genus are separated using a combination of often subtle differences in apothecial and pycnidial morphology, ascospore size and septation, and thallus secondary chemistry.^[7]

Inoderma is a genus of lichens that typically forms broad, pale, whitish to light olive-grey patches on its substrate. These patches (the thallus) can be continuous but often develop a network of fine cracks or appear as tiny, powdery granules. The surface is usually not covered by a protective "skin" (cortex) and can feel slightly soft or powdery. The lichen's green partner cells (the photobiont) belong to Trentepohlia, a genus of green algae.^[8]

Some Inoderma species produce apothecia—small, often rounded, and slightly raised structures where spores are formed. These may lie flush with the lichen's surface or sit just above it, and they sometimes carry a thin to thick white powdery coating (pruina). The boundary between the apothecium and the main lichen body (the true exciple) is not clearly defined. Inside the apothecium, the spore-producing tissue (the hymenium) is colourless to pale yellowish-brown, with tiny pale crystals in its upper layer (the epithecium). Beneath this, the hypothecium is made of tangled fungal filaments within a jelly-like matrix, ranging in colour from pale to dark brown. Thin, thread-like structures called paraphysoids run through this layer, and their tips do not widen or darken. The asci, which are the spore-containing "sacs", follow the Arthonia-type structure.^[8] The hymenium stains blue in iodine (KI+), but the asci lack KI+ (blue) structures in the tholus.^[7] The spores are typically elongated, sometimes slightly egg-shaped (cylindric-obovoid or soleiform), and divided into two to four compartments by internal walls (septa). They remain colourless and do not narrow at the dividing lines.^[8]

In addition to apothecia, Inoderma may also form conidiomata—tiny, elevated, spore-producing bumps called pycnidia. These pycnidia have dark brown to black walls and are often dusted with a thick layer of white powder. They release small, rod-shaped spores known as conidia.^[8]

From a chemical perspective, Inoderma species may contain substances such as lepraric acid and confluentic acid, along with some compounds not yet fully identified ("byssaceum unknowns"). When tested with chemicals, the dark brown pigments in the apothecia and pycnidia turn greenish-black in the K spot test and slowly shift to an orange-brown colour when treated with nitric acid. These reactions, along with the lichen's distinct physical features and internal structures, help distinguish Inoderma from related genera.^[8]