Rinodina exigua
Rinodina exigua is a species of lichen belonging to the family Physciaceae, characterised by its thin, pale grey-green thallus with an irregular, cracked surface. First described by Erik Acharius in 1798, it features distinctive black reproductive structures that measure 300–460 micrometres in diameter. The lichen grows primarily on tree bark, particularly on ash and oak trees in the United Kingdom, while in continental Europe it shows an altitudinal distribution pattern, occurring mainly in mountains in southern regions but across various elevations in Central Europe. It typically grows alongside other nitrogen-tolerant lichen species and can be identified by its response to specific chemical spot tests.
Taxonomy
Rinodina exigua was first described as Lichen exiguus by Erik Acharius in his 1798 work Lichenographiae Suecicae Prodromus. Samuel Frederick Gray later transferred it to the genus Rinodina in his A Natural Arrangement of British Plants.Rinodina exigua belongs to the section Rinodina, characterised by spores with a well-developed septum at maturity. Within this section, it is placed in series Exiguina, distinguished by thick-walled spores with unequally thickened walls and irregular.
Description
Rinodina exigua is characterised by its relatively thin thallus that appears pale grey-green in colour. The thallus has an irregular, cracked structure with an uneven, surface that sometimes develops small scale-like formations, and lacks distinct boundaries, spreading diffusely across its substrate. No clearly defined border is visible between the lichen and surrounding surfaces.The reproductive structures are quite noticeable, measuring 300–460 μm in diameter. These dark fruiting bodies are either attached to the surface or partially embedded within the thallus. They often occur abundantly and may grow so closely together that they touch. Each apothecium is surrounded by a rim that matches the colour of the main thallus, measuring 20–30 μm wide. This rim remains intact throughout the lichen's development, though it may develop slight scalloping with age. The central of the apothecium is black, initially flat but becoming convex as it matures.
The spore-producing layer measures 160–200 μm in height and turns blue when stained with iodine solution. Beneath this lies a pale yellow layer measuring 120–140 μm that does not react with iodine. The spore sacs measure 130–160 by 15–19 μm with uniformly thick inner walls.
The dark brown spores measure 14–17.5 by 8–10 μm, featuring thick walls with rounded to conical compartments. They have a structure typical of the Physcia-type, with the central dividing wall being thickened and darkly pigmented in the middle. The spores lack a gelatinous outer coating. Asexual reproductive structures have not been observed in this species.
Chemical spot test analysis shows that the thallus is K+ and Pd+. These chemical reactions are important diagnostic features for species identification.