Buellia saxorum

Buellia saxorum forms a moderately thick crust-like growth (thallus) on its substrate. The thallus has a distinctive appearance with a network of fine cracks (described as "-cracked" in technical terminology) and displays a white to pale yellow-grey colouration. One of its distinctive features is the black border () that clearly defines its edges, often creating a mosaic pattern where multiple thalli meet. When stained with iodine, the inner layer (medulla) turns blue. The reproductive structures (apothecia) measure between 0.3 and 1 mm in diameter and sit on the surface of the thallus rather than being embedded within it. These apothecia have a flat to slightly convex dark disc surrounded by a prominent true rim (). Under microscopic examination, the upper layer of the apothecia () appears brown and does not react with nitric acid. The spore-producing layer (hymenium) lacks oil droplets. The spores (ascospores) are divided by a single septum (internal partition) and measure 10–18 by 5–9 micrometres (μm). They have walls that are uniformly thickened and smooth. They are of the Physconia-type The asexual reproductive structures (pycnidia) are abundant, producing spindle to cylinder-shaped spores (conidia) that measure 4–7 (–9) by about 1 μm. When subjected to chemical spot tests, the thallus is C+ (red), K+ (yellow), Pd (± yellow), and UV–. These reactions indicate the presence of two secondary metabolites: atranorin and gyrophoric acid. Lecanoric acid is present in trace amounts. ==Similar species==

Buellia saxorum can be distinguished from related species by its characteristic moderately thick, rimose-cracked, white thallus with a prominent black prothallus, amyloid medulla (I+ blue), and chemical profile (C+ red, K+ yellow, containing atranorin and gyrophoric acid). Buellia flavescens differs from B. saxorum in having a yellowish-inspersed medulla and containing placodiolic acid rather than gyrophoric acid. While both species have a similar rimose to areolate thallus appearance and ascospore type (Physconia-type), B. flavescens gives a C+ deep yellow reaction instead of the C+ red seen in B. saxorum. Buellia leptoclinoides can be distinguished by its rimose to bullate thallus, hymenium containing oil droplets, and ascospores with both median and lateral wall thickenings (Callispora-type). It also contains placodiolic acid but differs from B. flavescens in having a non-amyloid medulla (I-). Buellia subdisciformis, with which B. saxorum was historically confused, differs primarily in its chemistry (containing atranorin, norstictic acid, and connorstictic acid) and slightly different spore dimensions. Buellia dispersa has a similar appearance but can be differentiated by its more variable thallus morphology, and the absence of gyrophoric acid. ==Habitat and distribution==

Buellia saxorum grows on more or less vertical faces of siliceous rocks. In Europe, it occurs in Britain, France, Italy, and Spain. ==See also==