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Graphis (lichen)

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Description

Genus Graphis includes crustose lichens, which have a crust-like appearance that can range from being fully embedded in the substrate to sitting on the surface. The lichen's symbiotic partner, or photobiont, is green algae from the genus Trentepohlia. The apothecia (fruiting bodies) of Graphis can also be immersed or superficial. These structures are typically elongated, resembling slits (referred to as lirelliform), and can be simple, branched, or star-shaped. The disc of the apothecium, where spores are released, often remains slit-like or closed. Unlike some lichens, Graphis lacks a thalline exciple, which is a rim of tissue derived from the lichen thallus surrounding the apothecium. Instead, it has a true exciple, which is usually black, opaque, and well-developed. This exciple is sometimes marked by longitudinal grooves.

Inside the apothecium, the hymenium (a spore-bearing layer) is colourless and does not react to staining with iodine (I-). Beneath this layer, the hypothecium can be either pale or dark, and it is relatively thin. The hamathecium, which supports the developing spores, consists of unbranched filaments called paraphyses. The spore-producing structures, the asci, typically contain up to eight spores. These asci are club-shaped to slightly cylindrical and release their spores through an apical split. The spores themselves are divided by transverse walls (septate) or have a muriform structure (multiple divisions), and they turn violet when stained with iodine as they mature. Initially colourless, these spores may darken to brown if they become over-mature. Graphis also produces conidia, which are asexual spores, within flask-shaped structures called pycnidia. The conidia are usually cylindrical to ellipsoidal and remain colourless.

Chemically, some species of Graphis contain compounds known as β-orcinol depsidones. Additionally, older apothecia in certain species might react with potassium hydroxide solution (K+) to produce a purple colouration, indicating the presence of anthraquinones.

Graphis can be distinguished from the genus Phaeographis by its colourless spores, which may become brown with age. Additionally, in the field, Graphis species may resemble Opegrapha species, but the latter can be differentiated by their distinctively structured asci and spore-bearing tissues.

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