Peltigera horizontalis
A distinctive old woodland indicator Dogtooth, characterised by the rounded apothecia held horizontally. These combined with the dark brown anastomosing veins bearing fasciculate dark rhizines on the underside are also diagnostic. Found frequently in humid woods in the west, but also on veteran trees in drier situations.
Thallus 10–20 (–40) cm or more in diam., forming wide-spreading patches; lobes to 3 cm broad and 5 cm long; upper surface blue-grey, often tinged brown, ± glossy, margins ± entire or indented, rarely sublobulate and crisped; lower surface white towards the margins, becoming darker towards the centre, with conspicuous anastomosing dark brown-black veins with numerous starkly white elongate interstices; rhizines fasciculate, often brown-black or black, concentrically arranged as if in annual rings. Apothecia rounded, occasionally broader than long,
horizontal, discs flattened, red-brown. Ascospores 30–46 × 3–7 µm, 3-septate. Conidiomata unknown. Thallus with tenuiorin, methyl gyrophorate, ± gyrophoric acid (C+ red), T3, T4 and unidentified terpenoids.
A distinctive old woodland indicator species, characterised by the rounded apothecia held horizontally. The dark brown anastomosing veins bearing fasciculate dark rhizines are also diagnostic; the montane Peltigera elisabethae differs in having marginal schizidia and a more evenly felted dark but non-veined underside.
On mossy trunks of old trees, rotting logs, stumps and mossy rocks, often in sheltered valleys, especially in woodlands with a long history of ecological continuity. Quite mobile in humid ravines, but more veteran tree depenant in less humid woods.
Locally abundantin western Britain but extending locally eastwards. The eastern most extent of its range ha been lost to past acififying pollution. Less frequent in Ireland and rare in the Central Plain.
Cannon, P., Magain, N., Sérusiaux, E., Yahr, R., Coppins, B., Sanderson, N. & Simkin, J. (2021). Peltigerales: Peltigeraceae, including the genera Crocodia, Lobaria, Lobarina, Nephroma, Peltigera, Pseudocyphellaria, Ricasolia, Solorina and Sticta. Revisions of British and Irish Lichens 20: 1-34.
Text by Neil A Sanderson based on Cannon et al (2021)