English: A lichen - Parmelia saxatilis. This is an extremely common lichen in Britain. It can be found on moderately acidic bark as well as on rocks and walls.

(Compare a similar species: 1184066. Another species, P. ernstiae ( https://www.geograph.org.uk/photo/1213636 ), is very similar to P. saxatilis, but is less common, and often has little lobules on its lobes.)

The example shown here was growing on one of the stones of a dry-stone wall.

On the upper surface, the outer layer (cortex) of the lobes is covered with a network of white lines; this network consists of pseudocyphellae (places where the cortex is thinner). Little stubby projections called isidia form on these lines; they are clearly visible through a lens, and, in this photograph, they give the centre of the lichen (which is densely covered with isidia) a rough appearance.

Isidia contain both fungal and algal cells (the two components of a lichen); they are designed to break off and propagate the lichen without the need for sexual reproduction. In some lichens, isidia are narrowed near the base so that they can break off more easily.

Parmelia saxatilis and P. sulcata are both assigned to zone 4 of the Hawksworth and Rose scale (see 914821), which means that they are fairly tolerant of sulphur dioxide pollution. However, they are much more vulnerable to other forms of air pollution: both of these species (as well as Hypogymnia physodes – 1049273), "contain salazinic acid and seem to be especially sensitive to fluoride exposure" [Chapter 5 ("Fluorides") in "Pollution monitoring with lichens" by D.H.S.Richardson, 1992].

Historically, this species was used to produce a purple-brown dye. The lichen is sometimes said to have the common name Crottle, a name that is derived from the Gaelic "crotal"; however, both the Gaelic word and its English derivative can be broader in meaning. Under the headword "crotal", Dwelly's "Illustrated Gaelic-English Dictionary" gives the meaning: "general name for the varieties of lichen used for producing dyes of various shades of red and brown"; it also gives the more restricted meaning: "the lichen, stone- or heath-parmelia – Parmelia saxatilis and Parmelia omphalodes".