Parmelia saxatilis

Overview

Pale grey to blue-grey, often browner at the lobe tips, Parmelia saxatilis has the same network of raised white lines (pseudocyphellae) as P. sulcata, but along that network it grows tiny, dark-tipped cylindrical isidia instead of powdery soredia. It is common on acid rocks, walls and trees across Britain and much of Europe.

Lobes are up to about 3 mm wide and widen slightly at the tips. The underside is black, paler at the margin, with simple rhizines. Dark red-brown apothecia up to 1 cm across are occasional.

What most field guides call P. saxatilis is really an aggregate: P. saxatilis in the strict sense, P. ernstiae and P. serrana look alike, and in practice need DNA to separate.

Identification

• Grey foliose rosettes with a raised net of white lines and dots (pseudocyphellae) on the upper surface.
• Cylindrical isidia, often grey-brown at the tips, along the network and spreading over the centre; no soredia.
• Underside black (paler at the margin) with simple rhizines.
• Cortex K+ yellow (atranorin); medulla K+ yellow-orange turning red, P+ orange, C− (salazinic acid).
• Tell it from P. sulcata by isidia (cortex-covered pegs) versus soredia (loose powder). In eastern North America, the bark species P. squarrosa has bottle-brush (squarrose) rhizines.

Ecology & Habitat

Typically on acid rock and walls, and on the bark of acid-barked trees, becoming more abundant in upland and western districts of Britain. The UK's lichen-based nitrogen air-quality index lists Parmelia species, including P. saxatilis, among the nitrogen-sensitive lichens, so it becomes scarcer where ammonia and other nitrogen pollution is high.

Sources

• British Lichen Society: Parmelia saxatilis
• Irish Lichens: Parmelia saxatilis
• APIS: Guide to the lichen-based nitrogen air quality index (PDF)
• Ohio Moss & Lichen Association: Parmelia squarrosa