Diagnosis. Differs from P. conformatum by the more angular, more flattened, often abundantly lobulate and sparsely to abundantly ciliate lobes, with rounded lobe axils.
Type: Ecuador. Galápagos: Isla Pinta, along the trail up to the summit from the S-coast, 0°34′15″N, 90°45′2″W, 289 m alt., transition zone, open woodland of Zanthoxylumfagara, Pisoniafloribunda, and few Tremamicrantha, grasses, Justiciagalapagana, and Alternantherafilifolia in the understory, old cactus pad lying on the ground, sunny, wind- and rain-exposed, 27-Feb-2007, Bungartz, F. 5888 (CDS 33565 – holotype!).
Description.Thallus corticolous (on cactus pads); uppersurface pale greenish yellow, dull, emaculate, not cracked; sparsely to abundantly isidiate; isidia marginal to sublaminal, cylindrical, mostly simple to sparsely branched, 0.05–0.1 mm in diam., concolorous with the thallus to deep brown at their tip, eciliate; lobes small to moderate-sized, 2–5 mm wide, angular, ± flattened, with rounded lobe axils, margins sparsely to abundantly ciliate; cilia short and stout, 0.2–0.5(–0.6) mm long, black, mostly simple, rarely branched; lower surface often blackened throughout or becoming dark brown towards the margin, typically with an erhizinate 0.5–1(–1) mm marginal zone; rhizines short and stout, black, mostly simple, rarely sparsely branched; medulla white. Apothecia and pycnidia not observed among the Galapagos specimens.
Ecology and distribution. Currently only known from the type, possibly endemic.
Notes.Parmotrema cactacearum is in many respects very similar to P. conformatum. The two species share almost the same chemistry. In Galapagos all specimens of P. conformatum contain fumarprotocetraric acid, whereas this secondary metabolite is absent from P. cactacearum. However, this chemical difference alone may not constitute the most reliable character to distinguish the two species. Hale (1965), Ribeiro (1998) and Canêz (2005) all report fumarprotocetraric acid from P. conformatum, but Winnem (1975) suggests that she found usnic acid, protocetraric acid and an unknown substance in the type (at BM). This suggests that the type material should be re-examined. More important than the chemical differences, however, is the distinctly different growth morphology of the two species. Parmotrema conformatum forms broadly rotund, loosely undulating lobes with incised axils; these lobes are typically not lobulate and are mostly eciliate (only a few specimens occasionally bear cilia). Parmotrema cactacearum has flattened, appressed, angular lobes with rounded axils. These lobes are often abundantly lobulate with sparse cilia; where the lobes lack lobules they are more abundantly ciliate. The two species also differ in their isidia ontogeny. In both species, isidia are most abundant and best developed at the lobe edges, but in P. conformatum the isidia generally emerge from a much broader gnarled surface area along the lobe margins. Towards the margin this irregular surface becomes increasingly granular and the lobe margins are then abundantly cylindrically isidiate. In P. cactacearum the isidia appear more confined to the lobe edge; the thallus surface generally remains smooth and only occasionally bears a few immature, scattered isidia. In Galapagos both species are not common.