Knudsen K, Reeb V, Westberg M, Srikantha R, and Bhattacharya D. 2010: Acarospora rosulata in Europe, North America and Asia. - Lichenologist 42(3): 291-296.
Thallus areolate, contiguous to dispersed, forming indeterminate to determinate patches to 7 cm, often confluent. Areoles round to angular, up to 3 mm diam. And 1·5 mm thick, broadly attached (over half of diameter), becoming subsquamulose, the mycelial base thickening, not forming a stipe, elevating the areoles (becoming gomphate), the areoles often lobulate, especially on the margin of thallus but also in the centre, the lobes generally round, 2 to 3 per areole usually less than 1 mm long and less than 1 mm wide. Surface pale yellow brown to dark brown, usually shiny, smooth, epruinose. Upper and lateral cortices paraplectenchymatous to subparaplectenchymatous, 30–80 μm thick, cells 3–6 × 2·5–6 μm, globose to elongate, syncortex (sensu Knudsen 2007) 4–30 μm thick rarely with visible periclinal hyphae; eucortex with reddish brown upper layer to 20 μm thick, and thicker, hyaline lower layer. Lower surface usually ecorticate and white or rarely brown. Photobiont chlorococcoid green alga up to 10 μm diam., forming a continuous stratum 60–100 μm thick, the algal cells sometimes arranged in distinct vertical columns, often thinning beneath apothecia. Medulla prosoplectenchymatous, up to 600 μm thick, continuous with attaching hyphae, hyaline, mostly 3–4 μm diam., cells mostly 4–5 μm long.
Apothecia one to many per areole, immersed and punctiform, sometimes expanding to 1 mm diam., usually remaining immersed, occurring also on marginal areoles, rarely with a thin parathecial ring visible around the disc or elevated with thalline margin. Disc dark brown, epruinose, rough, round to irregular, rarely with umbo of sterile plectenchyma, sometimes reddish when wet. True exciple of radiating hyphae, 20–40 μm,expanding slightly around the surface of the disc, rarely forming parathecial crown. Hymenium 80–120 μm tall, epihymenium dark brown to reddish brown with diffused pigment, conglutinated, 10–20 μm thick; paraphyses lax in water, 1·5–2·5 μm diam.,septate, sometimes with oil drops, apices often expanded, 2·5 – 4·0 μm wide, usually in brown pigmented caps. Subhymenium to 50 μm thick. Hypothecium 15–20 μm thick. Asci clavate, 60–100 × 15–28 μm, ascospores 100–200 per asci. Ascospores simple, hyaline, mostly 4–5 × 1·5–2 μm.
Chemistry. Spot tests: cortex, C+, KC+pinkish red (best seen in mount); gyrophoric acid.
Ecology and substratum. On granite and siliceous rocks, schist, sandstone, rarely on limestone or calcareous rocks.
Distribution. Europe (Oppland, Norway), western North America and Asia (Mongolia).
Discussion. The thallus of Acarospora rosulata can be determinate, especially in hot arid locations in the desert or on smooth hard rock surfaces, but the outer and inner areoles can be lobulate and its areoles regularly become elevated and subsquamulose. The surface is shiny and always smooth. Acarospora rosulata was recognized as a species by Magnusson from a small area of Norway in Oppland. Specimens of A. rosulata from California were included in A. bullata by Magnusson (1929) and Knudsen (2007). We do not recognize A. bullata as occurring in North America.The discs of Mongolian specimens were very rough owing to an uneven build-up of conglutinated pigment and could, superficially, be mistaken for the rugulose discs of Acarospora bullata. Specimens from Norway often produce many apothecia on a single areole, which subsequently begins the process of vegetative division into separate areoles. Western North American specimens often produce the largest areoles usually with a single apothecium.