Diplotomma venustum

Diplotomma venustum is a species of saxicolous (rock-dwelling) crustose lichen in the family Caliciaceae.^[2] It is widely distributed, having been recorded from all continents, where it grows on calcareous rocks.^[3]

Diplotomma venustum
Scientific classification
Kingdom:	Fungi
Division:	Ascomycota
Class:	Lecanoromycetes
Order:	Caliciales
Family:	Caliciaceae
Genus:	Diplotomma
Species:	D. venustum
Binomial name
Diplotomma venustum Körb. (1860)
Synonyms[1]
List Rhizocarpon alboatrum var. venustum (Körb.) Anzi (1864) Buellia alboatra var. venusta (Körb.) Th.Fr. (1874) Lecidea alboatra var. venusta (Körb.) Vain. (1883) Diplotomma epipolium f. venustum (Körb.) Arnold (1884) Buellia epipolia var. venusta (Körb.) Mong. (1900) Lichen amylaceus var. venusta (Körb.) Mong. (1900) Buellia venusta (Körb.) Lettau (1913) Diplotomma epipolium var. venustum (Körb.) Szatala (1938) Buellia margaritacea var. venusta (Körb.) Räsänen (1943) Diplotomma alboatrum var. venustum Körb. (1858) Lecidea epipolia var. venusta Körb. ex Hepp (1860)

Taxonomy

The lichen was first formally described as a new species by German lichenologist Gustav Wilhelm Körber in 1860. Körber's Latin diagnosis emphasised the firmly attached, granular-powdery thallus with very fine wrinkling, and the mostly solitary apothecia that are immersed in the thallus when young but become convex with a black disc. He characterised the colourless ascospores as ellipsoidal to spindle-shaped with 4 cross-walls (4-septate) and noted they turn sooty-brown with age. The species was described from limestone and dolomite rocks in mountainous regions of central Europe, including collections from Germany, Austria, and the Carpathian Mountains.^[4]

Description

Diplotomma venustum forms a crust-like thallus that is tightly attached to the substrate (crustose) and often broken up by numerous irregular cracks and fissures (rimose). The margin may become weakly lobed. A thin black border (the prothallus) is sometimes visible around the thallus. The upper surface is chalky white, grey, or ochre-tinged and lacks powdery vegetative propagules. The interior (medulla) is white and contains abundant needle-shaped crystals of calcium oxalate; these crystals also generate the fine white pruina that commonly dusts the fruiting discs. Under ultraviolet light the thallus does not fluoresce (UV-), and standard spot tests on the thallus and medulla are negative in North American material (K-, P-, C-); outside North America, some collections react K+ (yellow turning red) and P+ (yellow-orange), reflecting the presence of norstictic and connorstictic acids in the medulla.^[5]