Polymeridium

The genus was originally circumscribed as a section of the genus Arthopyrenia by Swiss lichenologist Johannes Müller Argoviensis in 1883. Richard Harris elevated it to generic status in 1980. In his 1993 revision of the genus, Harris identified 19 species. Utilizing a broad species concept, he acknowledged the presence of variations in chemistry and hamathecial within individual species. André Aptroot and Marcela Cáceres again revised the genus in 2014, using the absence or presence of lichexanthone and the absence or presence of hamathecial inspersion as distinguishing characters for species. The monotypic genus Exiliseptum was incorporated into Polymeridium, as it only differs in one characteristic from several Polymeridium species and does not possess any unique features. As a result of this work, the number of species in the genus increased from 27 to 53. ==Description==

Polymeridium is characterized by a white to yellowish or grey thallus, which is usually ecorticate (i.e., it lacks a ). Its , the fruiting bodies of the lichen, are simple or fused, appearing as black, spherical to structures that emerge from the . The ostiole, or opening of the ascomata, can be apical to lateral, brown to black, or exhibit a red anthraquinone colour. The , which refers to the filamentous tissue within the ascomata, is colourless and may contain hyaline or red oil droplets that turn green when treated with potassium hydroxide (KOH) solution. are the reproductive spores of the lichen and typically occur in groups of 4 to 8 per ascus. They are colourless, ellipsoid to in shape, with rounded to subacute ends, and symmetrically 3 to 13- to . The ascospores are not constricted at the septa and are sometimes surrounded by a thin to 2–5 μm-thick gelatinous sheath. The septa within the ascospores are not thickened, and the are rectangular and slightly rounded, but not diamond-shaped. , or structures that produce (asexual spores), are rather rare in the genus Polymeridium. In terms of secondary chemistry, the thallus occasionally contains lichexanthone, while anthraquinones are rarely present in the ostiole or hamathecium. ==Species==

• Polymeridium albidoreagens • Polymeridium albidovarians • Polymeridium albidum • Polymeridium albocinereum • Polymeridium alboflavescens • Polymeridium albopruinosum • Polymeridium bambusicola – Argentina • Polymeridium bengoanum • Polymeridium brachysporum • Polymeridium catapastoides • Polymeridium catapastum • Polymeridium contendens • Polymeridium corticatum – Brazil • Polymeridium costaricense • Polymeridium endoflavens – Brazil • Polymeridium fernandoi – Sri Lanka • Polymeridium inspersum • Polymeridium jordanii • Polymeridium julelloides – Brazil • Polymeridium longiflavens – Brazil • Polymeridium microsporum • Polymeridium multiforme • Polymeridium multiseptatum • Polymeridium neuwirthii • Polymeridium ocellatum • Polymeridium pyrenastroides • Polymeridium pyrenuloides • Polymeridium quinqueseptatum • Polymeridium refertum • Polymeridium rhodopruinosum – Puerto Rico • Polymeridium siamense • Polymeridium stramineoatrum • Polymeridium submuriforme • Polymeridium subvirescens • Polymeridium suffusum • Polymeridium sulphurescens • Polymeridium tribulationis • Polymeridium xanthoexcentricum – Bolivia • Polymeridium xanthopleurothecium – Panama • Polymeridium xanthoreagens ==References==