Physcia tribacia (Ach.) Nyl. forme saxicole.

    Thallus foliose, usually irrgular in outline, forming lying irregular or rounded tufts, to 4-6 cm across, potentially able to merge with other conspecific colonies, lobes 1-2 mm wide, apices twice incised giving a wrinkled aspect, upper surface minutely pruinose, grey-whitish, pale greyish, grey-yellowish isidia -and later soredia- over the entire surface and at the margin and at the apice of the lobes and then may appear to arise from the underside. Apothecia very rare and most frequently absent, underside whitish to pale brownish. Apothecia very rare and most often absent, 1-1.5 mm diam.. Ascospores broadly ellipsoid, 15-24 x 9-11 µm. Pycnidia more or less abundant. Cortex : C-, K+ yellow, KC-, Pd- ; Medulla : C-, K-, KC-, Pd-. Fairly rare species, most often coming, in our prospection area, on slightly acid rocks by the sea, more rarely on the bark of trees by the sea in somewhat nutrient places.

N.B. Sometimes difficult, or even impossible to differentiate from Physcia dubia except by performing a microscopic examination (lower paraplectenchymal cortex).

Cf. Physcia caesia which has distinct pseudocyphellae, a yellow K+ medulla and grey-blue soralia.

Cf. Physcia tribacioides, the globular soralia of which are whitish, laminate and scattered throughout the thallus.