Rhizocarpon geographicum (L.) DC. s.l.

    Crustose thallus up to 15 cm at its greatest length, areolate, somewhat mosaic-shaped, areoles angular, 0.2-2 mm, flat to slightly convex, smooth, yellowish, greenish-yellow, rarely with an orange reflection, hypothallus black clearly visible at the margin or between the areoles. Innate apothecia, not very regular, reaching 1-1.5 mm, buried in the thallus, round or angular, flat to slightly convex disc, not pruinose, black, variable margin: quite thick at first becoming indistinct at maturity. Spores brown-black, broadly elliptical, muriform with 5-22 cells in optical section, 20-50 x 10-20 µm. Photobiont: chlorococcoid. Medulla: C- or C+/- red depending on the concentration of gyrophoric acid, K-, KC-, P- or P+/- yellow-orange. Very polymorphic species, not particularly maritime, frequent on horizontal or slightly inclined acidic rocks, enriched in nutrients, dry and sunny where it can form large colonies, sometimes mosaics. This species is most often mixed with other crustacean species with duller colors: blackish, brownish, etc. it is then the only one that is relatively easy to identify by its yellow color visible from afar.

N.B. It is preferable to speak of a "group" of Rhizocarpon geographicum because the extreme variability of this species has led to the description of numerous taxa whose taxonomic value is unknown because these descriptions are solely based on unreliable macro and microscopic characters and molecular biology studies are non-existent.

We can cite :

_ Rhizocarpon geographicum (L.) DC. subsp. geographicum:

    Taxon with a thallus subdivided into angular and flat areoles, more or less bright yellow and with often numerous angular apothecia, coming on or between the areoles. Muriform spores with 5-10 visible cells, 20-34 x 9-16 µm. C+/- red or C-, K-, KC-, P- or P+/- yellow. Very common taxon on rocks from marine shores to mountains.

_ Rhizocarpon geographicum subsp. prospectans (Räsänen) D. Hawksw & Sowter:

  Taxon with yellow, yellow-greenish colors, thallus very divided into angular, thin, flat areoles, separated by cracks revealing the black hypothallus. Apothecia few in number, rounded coming only into the fissures. Muriform spores with 8-12 visible cells, 17-28 x9-12 µm. Oceanic taxon found on rocks by the sea or under oceanic influence, not very rare in the Armorican Massif.

_ Rhizocarpon geographicum subsp. lindsayanum (Räsänen) Ahti.

 Taxon with poorly divided, flat, angular, greenish-yellow areoles. Muriform spores with 6-12 visible cells, 20-32 x 9-16 µm. Taxon of humid climates, from the plains to the hills. N.B. This taxon is considered by many authors as a simple humid climate morphotype of Rhizocarpon geographicum (L.) DC. subsp. geographicum

_ Rhizocarpon geographicum subsp. tinei (Torn.) Clauzade and Cl. Roux.

  Mediterranean xerophilous taxon with flat, thin pale yellow areoles. Muriform spores with 12-15 visible cells, 24-40 x 12-22 µm.

_ Rhizocarpon geographicum subsp. diabasicum (Räsänen) Poelt & Vezda.

 High mountain taxon with thick, convex, bright yellow areoles. Spores with 12-20 cells. 20-42 x 10-22 µm.

_ Rhizocarpon geographicum subsp. frigidum (Räsänen) Hertel

 Alpine taxon with areoles with a very smooth and shiny surface. Muriform spores with 5-10 visible cells, 20-28 x 9-15 µm.

_ Rhizocarpon subsp. kittilense Räsänen) Ahti

Very rare.

N.B. Possible confusion with all yellow Rhizocarpons and in particular:

Rhizocarpon lecanorinum whose apothecia are surrounded in a "horseshoe" or completely by rounded areoles.

Rhizocarpon viridiatrum which is greenish-yellow and parasitic on crustose lichens, particularly Aspicilia.