Diorygma fuscum

The thallus forms a thin (60–100 μm), pale grey to olive-grey crust that adheres closely to bark. Its surface is uneven to faintly warty and lacks powdery or finger-like propagules. A poorly developed pseudocortex only 5–10 μm thick overlies a 30–40 μm algal layer composed of green Trebouxia-type cells; below this, the medulla is sparse. Apothecia are abundant, appearing as flexuous, often branched slits (lirellae) 1–4 mm long that begin immersed but later burst open and rise slightly above the thallus. Their discs are thickly dusted with a white powder (pruina), and ageing fruit-bodies frequently crack along the disc surface. The lateral wall (exciple) remains pale or only weakly brown and is largely uncarbonised except at the base. The spore-bearing hymenium, 100–180 μm tall, is iodine-positive (bluish violet), and its cap (epithecium) is well developed, consisting of tangled paraphysis tips rich in gelatinous walls. Paraphyses are 1–2 μm wide, repeatedly branch and re-join, and tend to stick together near the ascus tips. Each club-shaped ascus produces eight colourless to pale-brown, densely muriform ascospores with 10–14 transverse and 3–4 longitudinal walls; a thin gelatinous halo surrounds each spore. No asexual pycnidia have been observed. Chemically, the lichen contains stictic acid as a major metabolite together with minor or trace amounts of constictic, hypostictic, and hypoconstictic acids, and it shows no colour reactions in standard spot tests.^[2]

Diorygma fuscum is known to occur only in subtropical south-eastern China, where it inhabits the bark of trees in relatively dry, open pockets of lowland primary forest at 300–540 m elevation. Collections from Wanmulin (Fujian Province) indicate that it co-occurs with various other script lichens such as Graphis hossei and members of Lecanora and the Graphidaceae, suggesting a preference for lightly shaded trunks that experience regular air movement. Until further surveys are conducted in neighbouring provinces and countries, the authors suggest that the species should be regarded as a regional endemic of subtropical East Asia.^[2] Diorygma fuscum is one of ten species of Diorygma known to occur in China.^[3]