Nash, T.H., Ryan, B.D., Gries, C., Bungartz, F., (eds.) 2002. Lichen Flora of the Greater Sonoran Desert Region. Vol 1.
Thallus: foliose, loosely adnate to adnate, 3-10 cm in diam., lobate lobes: subirregular, elongate, slightly imbricate, plane, separate, 5-12 mm wide; margin: becoming dentate or lacinate with age; lacinae: 5-10 x 1-2 mm; apices: rotund, ciliate; cilia: sparse, simple, up to 2.0 mm long upper surface: gray to green-gray, smooth, dull, strongly reticulate maculate, becoming reticulately cracked with age along the maculae, isidiate isidia: laminal and marginal, cylindrical, simple to branched, tips often bearing cilia, more common marginally; soredia and pustulae absent medulla: white with continuous algal layer lower surface: black with brown zone peripherally, rhizinate to margin; rhizines: scattered, mostly simple, sometimes bifurcate to squarrose Apothecia: rare, up to 5 mm wide; margin: thalline, isidiate; disc: brown, imperforate or perforate ascospores: simple, hyaline, ellipsoid, 12-14 x 6-10 µm Pycnidia: common, immersed conidia: filiform, 10-12 x 1 µm Spot tests: upper cortex K+ yellow, C-, KC-, P+ yellow; medulla K+ yellow turning deep red, C-, KC-, P+ orange Secondary metabolites: upper cortex with atranorin and chloroatranorin; medulla with salazinic acid (major) and consalazinic acids (minor). Substrate and ecology: usually on trees World distribution: Arizona to Central America, east Africa Sonoran distribution: Sierra Madre Occidental of Chihuahua and Sonoran and southern Baja California Sur.
Type: Tanzania. Lushoto District: West Usambara Mountains, Bumba Mavumbi [original label data: Africa, Bumba: Usambara], Holst 8772 p.p., (G – lectotype!, designated by Hale & Fletcher 1990).
Description.Thallus corticolous or saxicolous; uppersurface white to whitish gray, dull to shiny, epruinose or pruinose, densely reticulate-maculate, sometimes becoming cracked; abundantly isidiate; isidia marginal, cylindrical, soon coralloid, i.e., densely to loosely and elaborately branched, typically with a brown tip, ciliate or eciliate, but along the lobe margin closely intermingled with the abundant cilia (see comments below); lobes moderate-sized, 3–9 mm wide, ± rotund, axils incised, margins abundantly ciliate; cilia short and slender, 0.3–2.5 mm long, black, mostly simple, very rarely branched; lowersurface black, gradually becoming dark brown towards the margin, the rhizines there becoming increasingly sparse and shortened, but rarely entirely missing and typically not developing into a distinctly erhizinate zone; rhizines long, slender, black, mostly simple, rarely sparsely branched; medulla white. Apothecia and pycnidia not observed among the Galapagos specimens.
Chemistry. Cortex with atranorin [P+ yellow, K+ yellow, KC–, C–, UV–]; medulla with salazinic acid [P+ deep yellow, K+ yellow, soon turning blood-red, KC–, C–, UV–].
Ecology and distribution. Africa, North, Central and South America (Dodge 1959; Krog & Swinscow 1981; Hale & Fletcher 1990; Brodo et al. 2001; Spielmann 2009; Egan et al. 2016). From Galapagos previously only reported online (Bungartz et al. 2016). A common species, mostly growing in the transition zone and humid zone, rarely in the lower or high altitude dry zone; mostly on bark (mostly native and endemic trees, shrubs and cacti; occasionally on introduced Cedrela), rarely on rock, one specimen on soil; typically at sunny, wind- and rain-exposed sites.
Notes. The Galapagos material of P. subisidiosum is somewhat heterogeneous in that some specimens form distinctly ciliate isidia, whereas others have cilia and isidia closely intermingled, the cilia forming along the lobe margin, being closely interspersed but not necessarily emerging directly from the isidia. These differences appear subtle but may be sufficiently distinct to recognize two separate morphotypes. It remains doubtful, however, whether this variation justifies recognizing these morphotypes as separate taxa.