Porpidia tuberculosa (images of British lichens)

A crustose, 'lecideine' lichen with a thin, continuous or cracked-areolate, white to pale blue-grey thallus, rarely tinged orange, surface pale- to dark-blue-grey-sorediate, often densely so (as in lower photograph), margin often delimited by a narrow, brown or black prothallus; apothecia convex, black, thinly pruinose when young, solitary or grouped 2-4 together, somewhat constricted at the base to give them a tubercle-like appearance. Widespread and common on exposed, base-poor rocks, commonly seen only in the barren, densely sorediate state.

Refs: Smith et al. (2009), 748; Purvis et al. (1992), 499; Dobson (2005), 363 (photo); Dobson (2011), 366 (photo); Dalby & Dalby (2005), 20 (photo); Whelan (2011), 134 (photo); van Herk & Aptroot (2004), 318-9 (photo); Wirth (1995), 2: 766, 772 (photo); Thomson (1997), 499.

Porpidia is part of the very large 'Lecidea group' of lichens – crustose species with dark or black apothecia that lack distinct 'thalline' margins, though they will usually have differentiated 'proper' margins the same colour as the disk. Microscopic examination of mature apothecia (and sadly they are often not mature) is usually necessary even to be sure of the genus, and field identification of all but a few distinctive species is risky. Dobson (2011), pp. 236-7, gives an invaluable analysis of the component genera.

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Images of British Lichens