Arthonia anglica

Arthonia anglica was described as new to science by Brian Coppins, based on British material collected on beech bark in southern England. In its irregularly star-shaped apothecia and spore form, it resembles Arthonia astroidestra and Arthonia stellaris, but it is set apart by the presence of gyrophoric acid restricted to the apothecia (seen as a C+ red reaction in section), and by lacking the UV fluorescence associated with lichexanthone in A. astroidestra. It also differs from A. astroidestra in having apothecia that are not pruinose (i.e., lacking a powdery coating).^[2]

The type material bears the label "St. Leonard's, Jan. 1806" and is likely to have come from the former St Leonard's Forest near Horsham (West Sussex), an area closely associated with the botanist William Borrer, who was probably the collector of the specimen used for the description.^[2] In North America, material now referred to A. anglica was first recognised in the Ozark Highlands and circulated under the manuscript name Arthonia "dryadum". After the species was found in comparable habitats in the southern Appalachians and across the coastal plain and piedmont of the southeastern United States, a specimen was sent to Brian Coppins for comparison with British material; he confirmed that the North American collections were the same taxon as A. anglica, leading to the first published report of the species from the continent.^[3] Molecular work on Arthoniaceae has also shown that Arthonia in the broad sense is heterogeneous, and an mtSSU phylogeny including A. anglica places it in a lineage separate from the core Arthonia group around A. radiata.^[4]

The thallus is mainly embedded in the outer bark (endophloeodal), grey-white to pale fawn, and forms circular to elliptic patches at least 6×3 cm, each bounded by a dark brown hypothalline line. In microscopic preparations the hyphae stain blue with iodine. Standard spot tests on the thallus are negative (K−, C−, KC−, PD−, UV−).^[2]

The apothecia are numerous and often crowded, forming elongated, slit-like structures (lirellae) that are usually irregularly star-shaped (substellate), only occasionally neatly stellate, and reaching up to 12 mm across in the largest aggregates. The individual "arms" are narrow (about 0.08–0.16 mm wide) and pale to dark reddish-brown, without pruina. Microscopically, the apothecia are 80–130 μm tall. The epithecium is reddish-brown and turns olivaceous with K, while the hymenium is hyaline and iodine-positive (I+ blue). The asci are clavate and eight-spored, with only a minute amyloid ring at the apex, and the ascospores are typically 17–24×6–8 μm, 3–4-septate, and macrocephalic (with the upper cell larger than the lower).^[2] North American collections have been reported with slightly smaller spores, about 15–22 × 5–7 μm, while retaining the same general spore form and chemistry. Spores are hyaline and smooth when young, but older spores develop brown, granular warting. Pycnidia have not been observed. The species is lichenized with the green alga Trentepohlia as its photobiont.^[2]

Chemically, the diagnostic feature is gyrophoric acid confined to the apothecia, detectable as a C+ (red) reaction in apothecial sections.^[2]