Description
Opegrapha species form a crust-like thallus that adheres tightly to bark, rock or even the surface of other lichens. This crust may be paper-thin or rather thick and is often cracked into a mosaic of small areoles . Colours range from chalk white through shades of grey and mauve to dark brown or olive-green, and a narrow dark prothallus sometimes outlines individual colonies. Most members partner with the orange-pigmented green alga Trentepohlia , but a few live parasitically on lichens that employ different algal partners. Powdery reproductive patches (soralia) are uncommon and, when present, remain discrete rather than fusing into large swathes. [2]
The sexual fruit-bodies are characteristically lirellate apothecia—elongated, often branching slits that resemble tiny scribbles in the thallus. In bark-dwelling or lichenicolous forms these slits may round off into short dashes or nearly circular discs. Each apothecium sits flush with, or slightly above, the substrate; it lacks a true rim of thallus tissue, though a thin pseudo-margin may form. A tough black wall ( excipulum ) encloses the fertile layer and usually remains opaque, so the exposed surface looks like a narrow, glossy cleft; when thin sections are treated with the K spot test the internal tissues turn olive or reddish-brown, a useful field test. Inside, a gel-filled layer of branched, cross-walled filaments ( paraphysoids ) weaves among the club-shaped asci. These asci split apart along two walls when the eight ascospores are released—a mechanism called fissitunicate dehiscence—and a faint blue-staining ring at the tip can be seen with iodine staining. The spindle-shaped spores bear multiple cross-walls, start colourless and may develop a reddish-brown, grainy coat as they age. Minute black pycnidia, partially sunk in the crust, exude colourless curved conidia that provide an alternative means of dispersal. Despite the chemical diversity found in many roccellaceous lichens, Opegrapha generally lacks secondary lichen products. [2]
Taken together—crustose thallus, slit-like or rounded black apothecia with a pigmented excipulum, branched paraphysoids, multiseptate spores and fissitunicate asci—these traits distinguish Opegrapha from superficially similar genera such as Graphis (unbranched paraphyses and iodine-positive spores), Enterographa (much narrower, clustered apothecia) and Schismatomma (persistent thalline margin). Conidial size and shape are especially diagnostic at the species level, as is the number of spore septa. Ongoing molecular work shows that traditional morphological boundaries between Opegrapha and allied genera (e.g. Lecanactis , Lecanographa ) need further refinement, but the suite of characters outlined above still provides a practical field definition of the genus. [2]
This page is based on this
Wikipedia article Text is available under the
CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.
