Rhizocarpon geographicum (L.) DC.
Thallus up to 15 cm across, areolate and mosaic-shaped, areoles angular, 0.2-2 mm diam., flat to slightly convex, smooth, yellowish, yellow-greenish, seldom with orange reflection, prothallus black, distinctly visible at the margin or between the areoles. Apothecia innate, iregular, to 1-1.5 mm, immersed in the thallus, circular of slightly angular, disc flat to slightly convex, not pruinose, black, exciple variable, rather thick at first but not very distinct at maturity. Ascospores brown-black, broadly ellipsoid, muriform with 5-22 cells in section, 20-50 x 10-20 µm. Photobiont: Chlorococcoid. Medulla : C+/- red, Pd+/- yellow-orange, K -. This species is not especially maritime, and presents a high level of polymorphism. Common on nutrient-rich horizontal or low sloping acid rocks, dry and well-lit where it may develop large colonies sometimes in mosaics. Most often this species grows mixed with other crustose lichens displaying less bright colours: blackish, brownish, etc. In that case, its bright yellow colour makes it easy to identify. N.B. The high level of polymorphism resulted in the description of several taxa among which some may have maritime trends as for example Rhizocarpon geographicum subsp. geographicum of which the apothecia are angular with bright yellow colour (see piectures). Up to now, the parameters allowing identification are not very effective and even different according to some authors, and consequently we did not go further. Rhizocarpon lecanorinum is a similar species with peculiar areole shape, partially or totally enclosing apothecia or typically horseshoe-shaped, areoles more convex, and sometimes difficult to distinguish on the field. The latter has montane trends and is rather rare in our prospection area.*