Atla praetermissa
Atla praetermissa is a species of saxicolous crustose lichen in the family Verrucariaceae. It forms a very thin, mesh-like thallus that varies in colour from grey to dark green, sometimes with a brownish tint. The species is characterised by small, spherical reproductive structures and hyaline spores with a distinctive internal wall structure. First described in 2008, this lichen grows specifically on calcareous rocks near streams and in waterfall spray zones at elevations between 165 and 955 metres. Although known only from Scandinavia, experts suggest it may have a wider distribution but remains overlooked due to its inconspicuous nature and specialised habitat requirements.
Taxonomy
The lichen was formally described as a new species in 2008 by Sanja Savić and Leif Tibell. The type specimen was collected in 2006 by Savić from Anderssjöåfallet, located 1.5 kilometres northwest of Hamra in the Funäsdalen parish, Härjedalen province, Sweden. This holotype is preserved in the herbarium of Uppsala University.Molecular analysis published in 2016 suggests that Atla recondita is the closest relative to A. praetermissa.
Description
Atla praetermissa forms a superficial, crustose thallus that is very thin and has a mesh-like appearance, sometimes becoming more confluent around the reproductive structures. The thallus varies in colour from grey to dark green, occasionally showing a brownish tint. The is an unidentified green alga. The reproductive structures, called perithecia, are relatively small, averaging 0.36 mm in diameter, and are almost spherical in shape. These structures are at the base but lack a thalline cover.The internal anatomy of A. praetermissa includes a well-developed that is about 35–55 μm thick. This structure extends almost to the base of the perithecium, where it diverges from the . The outermost layer of the involucrellum is 18–25 μm thick and black in colour, while the interior portion is dark brown and composed of irregularly intertwined, heavily sclerotised cells.
The excipulum itself is uniformly brown and consists of narrow cells measuring 11–15 by 2–4 μm, arranged in a concentric pattern. The lacks hyphal elements except for that form below the ostiolum. These pseudoparaphyses are slender, measuring 1.5–2 μm in diameter, septate, and branch at wide angles.
The mature asci lack apical thickening, measure 113–133 by 39–55 μm, and have an ellipsoidal to club-shaped form. Each ascus contains eight spores. The ascospores are narrowly ellipsoidal, often asymmetrical with one end slightly widened, and measure 44.7–49.2 by 19.0-22.4 μm. When mature, they are hyaline with a structure. They typically have 8–11 transverse walls that reach the periphery along one side of the spores when viewed in a median optical section, and 2–4 longitudinal walls in the central part.