# Arthonia

Arthonia is a genus of lichens in the family Arthoniaceae. It was circumscribed by Swedish botanist Erik Acharius in 1806.
It is a genus of thin crustose lichen of widely varying forms, commonly called comma lichens.$^{:222}$

## Description

Arthonia species are typically crustose lichens, forming thin, often inconspicuous patches that can be either flush with the surface (immersed) or sitting on top (superficial). The thallus—the main body of the lichen—may spread without clear boundaries or sometimes be neatly outlined by thin brown lines. In some species, the thallus may be absent altogether. Under chemical tests, the fungal filaments (hyphae) in the thallus can show distinctive reactions, turning red or pale blue when stained with iodine (I+) and then blue with a potassium iodine (K/I) solution.
The photosynthetic partner (photobiont) is most commonly a green alga belonging to the genus Trentepohlia. In some cases, however, a less well-known type of green alga is involved, and a few species are only weakly lichenised or even live partly by breaking down dead organic matter (saprophytic) or parasitising other lichens (lichenicolous).
The reproductive structures of Arthonia are ascomata, which are often similar to apothecia (open, disc-like fruiting bodies) but can have a wide range of shapes. They may be round, elongated, linear, or even star-shaped, and on bark they often develop a subtle, thin rim of tissue that includes both lichen and host bark cells. The exposed surface of these structures (the disc) can be reddish-brown to black, and sometimes appears dusted with pale or white powder (pruina). Unlike many lichens, Arthonia usually lacks a thalline exciple (a protective rim made of thallus tissue) and often also lacks a true exciple (a layer of fungal tissue), though a few species develop a well-defined, carbonised boundary.
Internally, the top layer (epithecium) can range from colourless to reddish or dark brown. The main spore-producing region (hymenium) typically reacts to the same chemical tests as the thallus hyphae, and these reactions help distinguish Arthonia from other lichens. Beneath the hymenium, a distinct hypothecium (a supporting layer) may be absent or difficult to separate from the hymenium. Thread-like fungal elements (paraphysoids) weave through a gel-like matrix, becoming more intricately branched and pigmented near their tips, where they often form small, dark "caps".
The asci, or spore-bearing cells, usually contain eight spores and have a characteristic Arthonia-type structure. They are often semi-fissitunicate (splitting in a controlled way when releasing spores) and possess a large, domed apex with a specialised "ocular chamber" that may show subtle colour changes with chemical tests. The ascospores themselves are typically oval or elongated, divided into one to seven segments (septa), and start off colourless and smooth. As they mature and age, they may darken and become slightly roughened. Initially, the spores can have a very thin, colourless outer layer (epispore).
In addition to sexual reproduction via ascospores, Arthonia often produces tiny, inconspicuous, flask-shaped structures called pycnidia. These generate small, colourless, single-celled spores (conidia) that are usually rod-shaped but occasionally ellipsoidal or thread-like.
From a chemical standpoint, many Arthonia species do not contain distinct lichen products, but others produce a variety of substances, including xanthones and certain anthraquinones. These chemical differences, along with the various structural features, help distinguish different species within the genus.

## Gallery

* Arthonia caesia
* Photograph of a cross section of an apothecium of A. caesia taken through a compound microscope, x 400.
* Photograph of two spores (3-septate, 4-celled) from Arthonia caesia taken through a compound microscope, x 1000. (spores measure 21 x 5 micrometres)

## Selectes species

* Arthonia didyma
* Arthonia ilicinella
* Arthonia isidiata
* Arthonia stereocaulina
* Arthonia vinosa