Flora of New Zealand Series

Holotype: New Zealand. Canterbury, Boyle River near Lewis Pass. On bark of Nothofagus menziesii. 13 September, 1981. DJ. Galloway, CHR 381118!

Thallus ± orbicular or in irregular rosettes, 5-10(-18) cm wide, ± loosely attached. Lobes numerous, 0.5-1.5 cm wide, 1-2 cm long, ± convex, thin and rather delicate in texture, overlapping or at least marginally contiguous except for apices, margins crenate-incised, isidiate, uneven, often sinuous or subascendent. Upper surface bright lettuce-green when wet, pale grey-green, buff to yellowish-olive or pinkish when dry, often becoming dark red-brown on storage, apices slightly darkened, brownish ± uniformly to irregularly tomentose, tomentum, white, silky rather delicate and often thin and abraded in older parts, cortex below tomentum smooth to scabrid and ± reticulate-cracked, undulate, wrinkled, never faveolate, isidiate, without soredia, maculae or pseudocyphellae. Isidia small, delicate, granular, flattened, coralloid, marginal, occasionally and rarely laminal in older parts. Medulla white. Photobiont green. Lower surface pale, whitish to pale pinkish-tan centrally, uniformly white- tomentose to margins, tomentum, soft, silky rather short. Pseudocyphellae white, scattered to numerous, tubercular, minute, inapparent, 0.4-0.8 mm diam., central decorticate area very small. Cephalodia internal, often visible as distinct, ± hemispherical swellings on both upper and lower surfaces. Apothecia very rare, sessile to subpedicellate 0.5-2.0 mm diam., disc concave to plane, dark red-brown, shining, epruinose, margins entire, pale flesh-coloured, tomentose. Ascospores biseriate, 1-3-septate, brown, ellipsoid with pointed ends, 25.5-29 × 10.2-13.6 µm. Chemistry: TLC nil.

N: Gisborne (Urewera National Park) to Wellington (Manawatu Gorge). S: Nelson (Lake Rotorua, Inangahua Junction) to Fiordland (Manapouri and Lake Hauroko) and eastwards to Catlins. St: (Port Pegasus).

Endemic

Named in honour of Greta Du Rietz who collected lichens widely throughout New Zealand in 1926-27 with her husband G. Einar Du Rietz. Greta Du Rietz prepared all of the plant collections, took the photographs on their expedition, and made the botanical drawings to illustrate Einar Du Rietz's accounts of New Zealand plants. She maintained an active interest in lichens, especially those of New Zealand and in the preparation of this flora until her death in 1981.

P. gretae is a very distinctive species characterised by the dense tomentum on both upper and lower surfaces, the delicate, coralloid, marginal isidia which are also densely tomentose, white, rather indistinct pseudocyphellae and a negative chemistry. This last character is noteworthy since it appears to be the only species in the genus with no metabolites detectable on acetone extraction. It is distinguished from P. pubescens by the marginal isidia, the epruinose apothecial discs which are red-brown and not black, by the white pseudocyphellae and by the chemistry (P. pubescens has a complex chemistry, see below).