Polymeridium multiseptatum

{{Short description|Species of lichen}}

{{Use dmy dates|date=March 2023}}

{{Speciesbox

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| taxon = Polymeridium multiseptatum

| authority = Aptroot, A.A.Menezes & M.Cáceres (2013)

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Polymeridium multiseptatum is a species of corticolous (bark-dwelling) lichen in the family Trypetheliaceae. Found in Brazil, it was described by lichenologists André Aptroot, Aline Anjos Menezes, and Marcela Cáceres in 2013. The thallus of P. multiseptatum is {{lichengloss|ecorticate}} and whitish-grey in color, with the distinctive feature of fluorescing yellow under ultraviolet light. This lichen closely resembles Polymeridium quinqueseptatum, with the primary distinguishing characteristic being the UV+ (yellow) thallus. The ascomata are 0.2–0.4 mm in diameter, featuring a solitary, spherical {{lichengloss|centrum}} that is {{lichengloss|erumpent}}. The ostiole is apical, and the {{lichengloss|hamathecium}} is interspersed with oil droplets, with filaments profusely anastomosing. There are eight {{lichengloss|ascospores}} per ascus, which are iodine-negative, 4–7-septate, and measure 18–28 by 5–7 μm. The ascospores are not ornamented, and the wall is not thickened. The lichen's chemistry reveals the presence of lichexanthone. The type specimens were collected in Chapada do Araripe, Ceará, at an altitude of {{convert|800|m|ft|abbr=on}}, where it was found growing on tree bark.