Parmotrema robustum (Degel.) Hale forme corticole.
Parmelia robusta Degel.
Thallus foliose, similar to the thallus of Parmotrema perlatum but larger and more sturdy, to 25 cm diam., lobes well developed, 5-25 mm wide, indented, margins folded with soralia thin and not very thick, not pearly, sparse, and sparse cilia at the margin, upper surface sea-gray in dry weather, pale gray-greenish a little bluish in wet weather, typically somewhat discolored in places, lower surface blackish towards the center, brownish towards the margins with scattered and simple rhizines, black. Fertile forms not found and not described under our climate. Maritime species, uncommon, on bark of trees in oceanic woodlands and also on mossy rocks in highly exposed aerohaline heathlands: see Parmotrema robustum forme saxicole. which is slightly different in aspect and habitat, see also Parmotrema robustum forme sur substrat artificiel. Usually thought as rare but probably mistaken in the field with large specimen of Parmotrema perlatum with larger soralia and "in pearls" and with less pale colors ; not to be confused with large specimen of Parmotrema reticulatum of which the lobe upper surface is characterized by a network and with thallus K + yellow then red, and with large specimen of Parmotrema arnoldii of which the lobes are edged with black cilia and Pd-, K-.