Thallus: foliose, up to 6 (-7) cm in diam., irregular to more often orbicular lobes: linear to somewhat irregular-rounded and partly imbricate, 1-2 mm broad, ± flat to weakly and irregularly concave, prostrate upper surface: gray to gray-brown or brown, with a partial (lobe ends) or occasionally almost completely pruinose, sorediate soredia: in primarily marginal or terminal soralia, on short lateral branches, discrete and becoming weakly to strongly labriform as the upper cortex reflexes (at times the soralia become occasionally compressed (particularly at nodes along the segments); ± continuous, but the individual labriform portions still apparent), laminal soralia developing rarely; individual soredia granular to pseudocorticate and isidioid, pale or darkening upper cortex: scleroplectenchymatous medulla: white lower cortex: irregularly prosoplectenchymatous lower surface: of peripheral lobes usually distinctly white to off-white for a short distance, inward darkening to dark brown or black; rhizines: black and squarrosely branched Apothecia: infrequent, up to 4 mm in diam., the margin thick and entire, not usually sorediate but becoming lobulate, the lobules often forming labriform soralia at the end ascospores: 28-31 x 16-18 µm Spot tests: cortex all negative; medulla usually K-, C-, and KC- (very rarely C+ rose, KC+ reddish), P-; soralia K+ pale to distinctly yellow, C-, KC+ yellow to yellow-orange, P- Secondary metabolites: secalonic acid A (concentrated in and apparently restricted to the soralia). Substrate and ecology: bark and rock World distribution: North America and Asia Sonoran distribution: rather rare in Arizona, southern California and Baja California. Notes: This species is characterized by the marginal, weakly to strongly labriform soralia which contain secalonic acid A and usually react with both K and KC, by the lack of positive spot tests in the medulla, and by the scleroplectenchymatous upper cortex. Specimens with strongly labriform soralia have been commonly misdetermined as P. perisidiosa, an easy mistake if the distinctly corticate lower surface and reactive soralia are overlooked. Like essentially all sorediate species in the genus, the soredia of P. leucoleiptes sometimes become strongly isidioid and dark pigmented, which can obscure the K and KC spot test reactions. It is therefore essential to select younger and paler soralia for testing. Two other species with reactive soralia, Physconia enteroxantha and P. fallax, both have differently shaped soralia, as well as a paraplectenchymatous upper cortex.
I have concluded that P. kurokawae, which is morphologically identical and differs from P. leucoleiptes only by the production of gyrophoric acid in the medulla, does not merit taxonomic recognition, or at least not at the level of species. In eastern North America, where both are very common, there are no discernible differences in their ecology or distributions, and the two very commonly occur in mixed populations (and therefore in mixed collections). Furthermore, according to the intensity of the C and KC spot test reactions as well as the relative size and intensity of chromatogram spots, the concentration of gyrophoric acid in the medulla is extremely variable. The reactions vary from moderately strong to very faint, and in many cases can only be detected in the lower part of the medulla adjacent to the lower cortex. At the present time only one scrappy collection of the C+, KC+ variant has been found from the Sonoran region, that being a collection from the Santa Monica Mountains in California made apparently by Hasse (#161, NY).