Description: Thallus foliose, heteromerous, dorsiventral, broad-lobed, potentially >40 cm across, loosely attached at one end, sorediate and/or isidiate. Lobes strap-shaped, 1-3(-5) cm wide, more or less dichotomously divided, at first adpressed, then mostly free from substrate in apical parts, the apices truncate, sinuate-indented; upper surface grey-green to grey-brown, glossy, bright green when wet, strongly reticulately ridged, with marked depressions. Cylindrical isidia or granular soredia often present along margins and on ridges. Lower surface white to pale brown at margins and on the swellings corresponding to the depressions on upper surface, otherwise pale to dark brown, tomentose, except on the swellings, mostly erhizinate. Upper cortex pseudoparenchymatous; medulla white, often with internal cephalodia. Apothecia rare, laminal, most frequent on ridges, 2-5(-7) mm across, with a brown-red disc and a thin thalline margin. Epithecium reddish brown; hymenium colourless or pale brown in lower part; paraphyses mostly simple, not capitate; hypothecium brownish. Asci 8-spored, clavate, fissitunicate, the thickened apex with a K/I+ blue ring, Peltigera-type. Ascospores (1-)3(-5)-septate, fusiform to narrowly ellipsoid, hyaline, 18-30 x 5-9 µm. Pycnidia immersed on the ridges. Conidia bacilliform, c. 5 x 1 µm. Photobiont chlorococcoid (cyanobacterial in the internal cephalodia). Spot tests: upper cortex K-, C-, KC-, P-; medulla K+ yellow-orange (often faintly), C-, KC+ yellow-orange, P+ faintly orange-red. Chemistry: stictic and constictic acids (major), norstictic, cryptostictic, and salazinic acids (minor).
Note: a mainly temperate, holarctic species found on bark and on epiphytic and epilithic mosses in humid forests; extinct in the plains of northern Italy, it is still abundant in humid montane forests of central and southern Italy, reaching the coast in undisturbed areas of Tyrrhenian Italy (e.g. in the Castelporziano Estate near Rome). A distribution map in Italy was published by Nascimbene & al. (2016). It is included in the Italian red list of epiphytic lichens under the “Least Concern” category (Nascimbene & al. 2013c).
Growth form: Foliose, broad lobed
Photobiont: green algae other than Trentepohlia
Reproductive strategy: mainly asexual, by soredia, or soredia-like structures (e.g. blastidia)
Most common in areas with a humid-warm climate (e.g. most of Tyrrenian Italy)