Arthonia

The Norwegian lichenized Arthonia species treated here have an indistinct to well-developed thallus that is immersed in the host bark or superficial. The color varies from whitish to pale grey, olive-grey, grey-brown or fawn. The thallus surface is usually matt or weakly glossy. The margin is either not determinate or delimited by a thin brown prothallus line when in contact with other lichens. The photobionts belong to the family Trentepohliaceae or they are a species of chlorococcoid algae.

The apothecia are variously shaped. Most species have maculate apothecia that are often irregularly rounded, elliptical or lobed, or they are clearly elongated, branched or star-shaped. The apothecia are flat or slightly convex, and level with the thallus surface to raised. The color varies from orange-brown to brownish-black or pure black. The apothecia are well-delimited but lack a clearly defined margin. 

A small group of species that includes A. apatetica, A. fusca, A. ligniariella, A. mediella, and A. patellulata have brownish black or black apothecia that are more or less convex, (irregularly) rounded and sitting on the thallus. Outwardly, they may resemble small species of the unrelated lichen genera Micarea or Lecidea s.lat. Other species including A. atra, A. calcarea and A. excipienda have lirellate apothecia with a distinct dark brown to black margin and an exposed to slit-like apothecia disc. They outwardly resemble species of the lichen genera Opegrapha (A. atra, A. calcarea) or Melaspilea (A. excipienda) to which they have been assigned until recently.

The hymenium of the Norwegian Arthonia species is unpigmented to pale brown, pale reddish brown or greenish, or it is orange to red in color.

The epithecium is either not clearly differentiated from the hymenium, or it is pale to dark reddish brown, greenish brown or orange-red in color. It is composed of the branched tips of the paraphysoids that are either more or less perpendicular or extend horizontally along the surface of the hymenium. The tips are unpigmented or pigmented, and they are often enlarged. Distinct pigment caps or plaques are attached to the tips of the paraphysoids in several species.

The hypothecium varies from unpigmented to dark brown or orange-red in color.

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

The spores are oblong to obovoid or slipper-shaped and divided by 1 to several transverse septa. The spores are persistently unpigmented or they have a brown pigmentation and a granular ornamentation when old. The apical spore cell in some species is clearly enlarged.

Pycnidia have been found in several species. They are reddish brown, dark brown or black, 40–100 μm in diameter and are immersed in the thallus to slightly raised. The wall is dark brown, red-brown or olive-brown. The conidia are rod-shaped, 3–9 × 0.5–2 μm large, and straight or slightly curved.

Most Norwegian Arthonia species lack lichen secondary compounds in thallus or apothecia.  Some species contain orange to red anthraquinone pigments in the apothecia (rarely in the thallus) that change color to purple in K+ solution. Skyrin, O-methylhaematommone and unidentified anthraquinones are reported from Arthonia species in Norway.

The hymenial gels react I+ red, I+ blue or I+ red mottled with blue, and KI+ blue. A KI+ blue ring structure in the tholus of the asci is present in most species.

The brown pigments in the apothecia and in the wall of the pycnidia are either unchanged or they change color to olive-brown or greenish in K solution.