Description ²

 Thallus: foliose, adnate to loosely adnate, 3-20 cm in diam., lobate; lobes: subirregular, elongate, slightly imbricate, plane, separate, 10-20 mm wide; apices: rotund, often ascending and wavy, ciliate; cilia: up to 5 mm long; upper surface: gray with some blackened areas, smooth, dull to shiny, faintly white maculate; soredia: granular, common, in linear, marginal to submarginal soralia; isidia and pustulae: absent; medulla: white with continuous algal layer; lower surface: black with brown to mottled white naked zone peripherally, centrally rhizinate; rhizines: scattered, simple; Apothecia: rare, substipitate, up to 10 mm in diam.; margin: crenulate; disc: brown, often perforate; ascospores: ellipsoid to broadly ellipsoid, 15-22 x 9.5-12 µm; Pycnidia: laminal, immersed; conidia: 10.5-11.5 µm long; Spot tests: upper cortex K+ yellow, C-, KC-, P-; medulla K-, C-, KC+ red or orange, P-; Secondary metabolites: upper cortex with atranorin and chloroatranorin; medulla with alectoronic acid (major) and α-collatolic acid (minor).; Substrate and ecology: on hardwood trees (often among mosses) in relatively exposed positions; World distribution: neotropics and extending well into temperate regions to the north and south, Macaronesia, throughout Europe and New Zealand; Sonoran distribution: common on live oaks in woodlands of southern California, especially the Channel Islands. 

Field Notes ³

The soredia on this lichen shine blue-white under UV light. A similar species, P. perlatum, is basically indistinguishable from P. arnoldii without the use of the UV test or a chemical test called the K test. The medulla of P. arnoldii has a negative K reaction.

Sources and Credits

1. (c) Richard Droker, some rights reserved (CC BY-NC-ND), http://www.flickr.com/photos/29750062@N06/4225423605
2. (c) Lichen Unlimited: Arizona State University, Tempe., some rights reserved (CC BY-NC-SA), http://eol.org/data_objects/10547678
3. (c) CALS, some rights reserved (CC BY-SA)

More Info

• iNat taxon page
• Biodiversity Heritage Library
• BOLD Systems BIN search
• CalPhotos
• Global Biodiversity Information Facility (GBIF)
• Mushroom Observer
• NatureServe Explorer 2.0
• SEINet Symbiota portals
• USDA PLANTS database
• Ways of Enlichenment

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