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Images of British Lichens

Rhizocarpon reductum Th.Fr.
(= R. obscuratum auct., in part)

Thallus a rather thin, matt, pale- to brown-grey or grey-brown, usually cracked-areolate crust, often rather dull and inconspicuous; apothecia inially set into the thallus surface, becoming only slightly raised, variable in shape, flat, black with rough surfaces and grey-pruinose rims (more obvious in rounded apothecia), usually rather randomly dispersed on the thallus, but sometimes arranged partly in concentric circles (if then with grey thallus, may be var. cinereum (comb.ined.)). Generally common throughout Britain, on base-poor rocks, stonework and pebbles

Refs: Smith et al. (2009), 805; Purvis et al. (1992), 539 (as R. obscuratum); Dobson (2005), 389 (photo); Dobson (2011), 393-4 (photo); Whelan (2011), 140 (photo); van Herk & Aptroot (2004, 2013), 338-9 (photo); Valcárcel et al. (2003), 338-9 (photo, as R. obscuratum); Wirth et al. (2004), 204 (photo, as R. obscuratum, centre, grey, with R. geographicum); Brodo et al. (2001), 637-8 (photo, as R. obscuratum). Detailed description in Fryday (2000), Lichenologist 32: 207-224.

A variable species that can be confused with the equally variable Buellia aethalea, but the latter forms generally smaller thalli, bounded by more conspicuous, black prothalli, and the apothecia lack pruinose rims (though may be surrounded by a "collar" of thallus). Microscopic examination of the mature spores, if they can be found, provides ready separation, 1-septate in B. aethalea, compartmentalised by both cross-wise and shortly longitudinal septa (muriform) in R. reductum.
The account in Smith et al. (op. cit.) mentions named varieties, including"cinerea", but these were published as variants of the former name, Lecidea petraea (in part), and have not been formally republished (with necessary change in spelling) under R. reductum and have no current nomenclatural validity. However, the variant once called "Lecidea petraea var. cinerea", probably shown below, may be worthy of recognition.

Rhizocarpon reductum
Rhizocarpon reductum
Rhizocarpon reductum
On urban, sandstone, river bridge, Paisley, Renfrewshire, March 2011 (var. cinereum?)
Rhizocarpon reductum, wet
Rhizocarpon reductum, damp
On siliceous, stone wall, Balmaha, Stirlingshire, October & November 2008, on both days wet and green after heavy rain

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© A.J. Silverside
Uploaded December 2013, updated January 2014