Description & Identification
This is usually a relatively conspicuous and easily recognised lichen, recorded only four times from Wales, and only once since the nineteenth century. Two morphotypes occur, a more frequently seen one, containing green algae, and a rarer one containing blue-green algae.
The green morphotype comprises single, or several overlapping, fan-shaped lobes 5-20 mm wide, which are bright green when wet and grey-green to light brown when dry, and smooth above. The lower surface has conspicuous, pale to dark brown radiating veins. There are no rhizines. Dark brown or grey cephalodia containing Nostoc occur on the underside, and are rounded or irregular and wart-like. Blackbrown apothecia are usually present on the lobe margin.
The blue-green morphotype comprises small grey, brown or brown-black squamules, c. 0.4-1.2 mm wide, flattened, with somewhat indented and slightly crisped margins. Microscopically the central part of the squamule has filamentous hyphae (i.e. the structure is paraplectenchymatic) mixed with the cyanobacterium Nostoc, and the squamule is bounded by a cortical layer one cell thick. This morphotype resembles very compact and reduced morphs of small Leptogium species, and it is uncertain how it can be confidently identified unless it is directly attached to the green morphotype.
The morphotypes can be found together, sometimes with one developing from the other. Both can arise from an undifferentiated prothallus containing fungal hyphae, green algae, and cyanobacteria, forming a coherent slimy layer on the substratum. The cyanobacteria-containing squamules can also arise from the upper surface of the green squamules, or from the cephalodia on the underside of the green squamules.
P. venosa upper surface. © E. Timdal.
Underside with veins and cephalodia. © E. Timdal.
The green morphotype should be easily recognised by the small, neat lobes, bright green when fresh and wet. Peltigera britannica and P. leucophlebia are also green when fresh and wet, but they are much larger, with lobes 20-40 mm wide, and with the cephalodia occurring on the upper surface, where they are easily visible. Peltigera didactyla often has small thalli, but the upper surface is grey to grey-brown, the photobiont is Nostoc, the underside has pale veins and rhizines are present.
The blue-green morphotype is similar to very reduced forms of Leptogium species, including L. imbricatum and L. gelatinosum.
Records from Wales
VC 49, 23/56: no locality or recorder, pre-1960.
VC 49, 23/65: Ogwen Valley, in base-rich flushes, 1840, John Ralfs (BM); det. O. Vitikainen.
VC 49, 23/65, Twll Du, July 1883, J.E. Griffith (NMW 27.76.131).
VC 49, 23/63.58: Glyder fawr, Twll Du, 1895, recorder unknown.
VC 49, 23/65: Llyn Llydaw, 1981, Tony Fletcher; blue-green morphotype, det. P.W. James.