Arthoniaceae, English version

The thallus is crust-forming. In most species it is not differentiated into a separate bark layer, photobiont layer and medulla. The thallus of the Norwegian species is indistinct to well-developed and immersed in the substrate to superficial. The margin is either not determinate or delimited by a thin brown line. The photobionts, when present, belong to the family Trentepohliaceae or they are a species of chlorococcoid algae.

The fruitbodies of the Arthoniaceae are apothecia. The apothecia are variously shaped, but often patch-like (maculate) to clearly elongated (lirellate), branched or star-shaped. They are flat or convex, and immersed in the thallus, raised or attached to the thallus surface. The color is a shade of orange-brown, red-brown, brown black or black. In a few species, the surface of the apothecia is covered in a white, orange or red crystal layer (pruina).

Characteristic for the family is the lack of a distinct, well-differentiated apothecial margin. A few exceptions have been discovered in recent years through molecular analyses. In Norway, such species include Arthonia atra, Arthonia calcarea, Arthonia excipienda and Arthonia granitophila, all with a brown-black (carbonized) apothecial margin.

The spore forming layer (hymenium) of the Arthoniaceae is built of branched and netted hyphae (paraphysoids) that are usually embedded in a colorless to light pigmented gel matrix. The tips of the paraphysoids are often widened and pigmented. In most species, they form a distinct layer (epithecium) on top of the hymenium. The tips of the paraphysoids in the epithecium are either more or less perpendicular or they extend horizontally along the surface of the hymenium. The pigmentation is diffuse or aggregated in distinct pigment granules or plaques. The layer below the hymenium (hypothecium) varies from unpigmented to dark brown or orange-red in color.

The spore-producing cells (asci) are of the Arthonia- or the Arthothelium-type. The inner wall-layer is apically thickened (tholus) and usually penetrated by a triangular to rectangular channel (ocular chamber). All parts of the asci remain colorless in I and KI solution (I, KI negative) except for a KI+ blue ring structure in the tholus that is present in most species. The number of spores per ascus varies from 1 to (usually) 8, depending on the species.

The spores of the Arthoniaceae are 2- to multi-celled. They are divided by transverse septa only (transversely septate) or by both transverse and vertical septa (muriform). In both cases, the apical end cell of the spores is enlarged in several species. The spores are persistently unpigmented or they have a brown pigmentation with or without a granular ornamentation when old.

Asexual, mitotic spores (conidia) of the Arthoniaceae in Norway are produced within pycnidia. The conidia are rod-shaped, ellipsoid or thread-like. Additional conidia-forming structures are known from species of the Arthoniaceae elsewhere.

Many of the Norwegian Arthoniaceae lack lichen secondary compounds in thallus or apothecia. A variety of lichen compounds including depsides and anthraquinone pigments are found in other species. The role played by lichen compounds in the lichen symbiosis is not always known, but important functions have been documented including protection against feeding by snails and insects, anti-bacterial and anti-viral activity, and shielding off excess UV radiation. Lichen compounds are also known to prevent lichen thalli from oversaturation with water, thus facilitating exchange of oxygen and carbon dioxide for photosynthesis.

Identification of lichen compounds is done by means of thin layer chromatography (TLC) or thallus spot tests. For spot testing lichen thalli (or apothecia), simple chemical reagents are applied to the lichens and eventual changes in color are noted. Common reagents used for the identification of lichens include

• K: A 10% solution of potassium hydroxide in water 
• C: A solution of calcium or sodium hypochlorite in water; common household bleach is often used
• KC: C applied after K pretreatment
• PD: A solution of paraphenylenediamine in strong ethyl alcohol or clear methylated spirit (freshly prepared)
• I: Lugol’s iodine solution
• KI: I applied after K pretreatment 

Testing for the amyloid (I+ blue) or hemiamyloid (I+ red, KI+ blue) color reaction of the apothecial gels is particularly important for classification within the Arthoniaceae.