Parmotrema reticulatum (Taylor) M. Choisy

    Foliaceous thallus, loosely attached to the substrate, reaching 15 cm in its greatest length, formed of lobes 5-15 mm wide, with rounded and crenate or incised ends; upper surface glaucous grey, pale grey, finely reticulated (magnifying glass sometimes necessary), these reticulations being able to simulate fine cracks on old and dry lobes, presence of swollen soralia, more or less elongated and rarely capitate at the margin of the lobes which very rarely bear black cilia (and then very short); lower surface dark brown to blackish (especially in the center) with simple rhizines reaching to the margin. Apothecia rare, strongly sessile and almost stipitate, 5-8 mm in diameter, brownish disc, smooth or sorediate margin. Simple spores, elliptic, 13-19 x 8-10 µm. Cortex: C-, KC yellow then red, K+ yellow (red after a few minutes due to lysis of the cortex and damage to the medulla), P+/- yellow; Medulla: C-, KC+ yellow then red, K+ yellow then red, P red-orange, UV-. Species with oceanic and thermophilic tendencies, fairly common in Brittany, especially near the coast, growing on tree trunks and also on mossy rocks, rarer on artificial substrates. See:

Parmotrema reticulatum corticolous form, growing on tree bark.

Parmotrema reticulatum saxicolous form, growing on coastal rocks.

Parmotrema reticulatum grows on artificial substrates, particularly on shipwrecks and also on street furniture in ports.

N.B. Beware of Parmotrema perlatum, which is very easy to confuse, but whose lobe tips are not incised and bear pearl-like soredia formed from mealy soralia, without rhizines towards the margins of the lobes on the underside, without a network on the upper surface of the lobes, and yellow K+, and Parmotrema arnoldii, P-, with long black cilia on the margins of the lobes.

N.B. Parmotrema pseudoreticulatum, very similar to Parmotrema reticulatum, was described in 1974 and can only be distinguished with certainty by DNA analysis. This species differs macroscopically by the absence of rhizines towards the margins of the lobes, which are thicker and less upturned, with a network on the upper surface that is less distinct and incomplete or replaced by spots or dots. However, these characteristics are unreliable, and there appear to be intermediate species that cannot be determined macroscopically.

N.B. It is preferable to speak of the "Parmotrema reticulatum complex" because this "species" is polyphyletic and there are at least 7 crypto-species in the world.