Rhizocarpon geographicum (L.) DC.
Lichen geographicus L., Sp. Pl.: 1140 (1753).
Thallus crustose, orbicular to irregularly spreading or in coalescing patches, to 20 cm diam., prothallus black, often conspicuous at margins and between areolae. Areolae contiguous to dispersed, to 2 mm diam., rarely rounded, ± angular, flat to convex, bright yellow to greenish-yellow, smooth, dull or glossy. Apothecia to 1.5 mm diam., between areolae, rounded to angular, flat, matt, rarely subconvex, margins thin, distinct or disappearing. Hypothecium dense, brown-black 100-300 µm tall. Hymenium 100-180 µm tall colourless or faint brownish. Asci 100-150 × 20-30 µm containing 8 spores. Ascospores muriform with few to many septa 20-40 × 10-22 µm. Chemistry: Medulla K-, C- or ± red, Pd+ yellow. Rhizocarpic acid (UV++ yellow), psoromic and often gyrophoric acids, rarely barbatic acid.
N: Auckland to Wellington. S: Nelson to Fiordland. St: A: C: Very widely distributed from rocks at s.l., to highest exposed rocks of Mt Cook at 3450 m. Most common in alpine areas.
Cosmopolitan
