Diplotomma venustum

Diplotomma venustum is a species of saxicolous crustose lichen in the family Caliciaceae. It is widely distributed, having been recorded from all continents, where it grows on calcareous rocks.

Taxonomy

The lichen was first formally described as a new species by German lichenologist Gustav Wilhelm Körber in 1860. Körber's Latin 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. He characterised the colourless ascospores as ellipsoidal to spindle-shaped with 4 cross-walls 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.

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