Micarea xanthonica

Thallus light yellowish green, leprose, of ± globose goniocysts 16–28 μm diam. which sometimes coalesce to form granules to 40 μm diam.; hyphae colourless, K–; photobiont cells 4–7 μm diam. Apothecia very rare, (0.2–) 0.3–0.6 mm diam., or forming tuberculate clusters to 1.2 mm across, at first flattish and weakly marginate, but soon convex, immarginate or becoming tuberculate, whitish to pale straw; true exciple soon reflexed; epithecium colourless; hymenium 40–50 μm tall, colourless; hypothecium colourless; paraphyses 0.7–1 μm diam., to 1.5 μm diam. towards the apices, numerous, richly branched. Asci 36–38 × 8–12 μm. Ascospores 9.3–14 (–14.3) × (3.5–) 3.8–4.2 (–4.7) μm, cylindric-ovoid to cylindric-ellipsoidal, (0-)1(-3)-septate. Pycnidia not seen. Thallus C+ and KC+ persistent orange, K–, Pd–, UV+ dark orange, K/UV (wet)+ bright lime green (thiophanic acid, plus some additional xanthones in trace amounts); apothecium sections C± red (gyrophoric acid).

Distinguished from other members of the M. prasina group by its KC+ persistent orange thallus, which usually has a slightly yellowish tinge; otherwise similar to M. viridileprosa and light green forms of M. micrococca. The C+ pink reaction of M. viridileprosa often looks orange because of its bright green thallus, so filter paper should be used, and promptly because of the fleeting reaction. The K/UV (wet)+ bright lime green test for xanthones definitively separates M. xanthonica from all other M. prasina group species.