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DescriptionThallus: foliose to weakly subfruticose-ascendant, up to 67 cm in diam., +orbicular to occasionally irregular; lobes: elongate, 1-3 mm broad (rarely up to 5 or 6 mm, but narrowing basally), usually somewhat concave or canaliculate, rather prostrate or more often loose and ascending; lobe edges: irregularly crenate; upper surface: tan-green to olive-brown or brown, rarely darkening, epruinose, usually somewhat glossy at least toward the lobe tips, older parts sometimes becoming papillate (usually with pseudocyphellae on top), becoming warty-papillate or (rarely) foliolate, without soredia or isidia; upper cortex: paraplectenchymatous; medulla: white; lower cortex: paraplectenchymatous; lower surface: almost white to pale tan, often +smooth in younger parts, becoming rugose or trabeculate at least in older parts, dull or weakly shiny toward the lobe ends; rhizines: sparse, often nearly absent in peripheral parts, mostly concolorous with the lower surface; Apothecia: marginal or terminal, frequent, up to 6 (-8) mm in diam., +concave or irregularly convex with age; margin: crenate-papillate (sometimes grossly papillate-foliolate in large specimens); ascospores: globose to sub-globose, 4-5.5 x 4-5 µm; Pycnidia: black, mostly strongly marginal or submarginal (usually only a few laminal ones present, at least in peripheral parts), distinctly emergent (usually as tall as they are wide); conidia: bifusiform, 4.55 x 1 µm; Spot tests: all negative in cortex and medulla; Secondary metabolites: lichesterinic type fatty acids.; Substrate and ecology: bark or wood, rarely on rock; World distribution: North America; Sonoran distribution: seen only from Santa Barbara Co. in southern California.; Notes: Typical normal-sized specimens of Tuckermannopsis orbata aren't likely to be confused with any other cetrarioid lichens, although small or abnormally prostrate specimens might be confused with fertile members of Tuckermanella. Careful attention to the placement and form of pycnida and to the size and shape of the ascospores should help distinguish members of that genus from Tuckermannopsis orbata.