Flora of New Zealand Series

Holotype: New Zealand. Nelson, Arthur Ra., track from Graham Valley car park to Mt Arthur, rock fall at start of track, 41º11's, 172º45'E, c. 1000 m, on rock, Nothofagus menziesii forest, 13.iv.1991, B. Polly s.n. – WELT L3256.

Description : Thallus closely attached, in neat, flat rosettes, (1–) 2–5(–15) cm diam., or spreading irregularly, often coalescing and forming large, composite colonies 15–25 cm or more, with or without a fibrous, brownish, varnish-like marginal prothallus, projecting 2–4 mm beyond lichenised thallus. Upper surface of small (0.1–0.5–1 mm diam.), contiguous to scattered, flattened squamules, discrete and scattered at margins to closely contiguous to areolate centrally, squamules or areolae with scattered spots or patches of glistening white pruina, or the whole surface thickly white-pruinose, central, epruinose areolae and squamules minutely maculate (×10 lens), olive-green to greyish or olive-brown when moist, or completely white through thick deposition of pruina, pinkish brown to grey-brown or dirty greyish when dry, to uniformly white-pruinose, without isidia, pseudocyphellae or soredia. Medulla white. Photobiont green, chlorococcoid, cells rounded, 7–10 μm diam. Cephalodia orbicular, flattened, level with or projecting only slightly above thallus surface, 0.5–2(–2.5) mm diam., scattered towards central parts, smooth to slightly wrinkled, not plicate-cracked or fissured, epruinose, bluish purple when moist, pale yellowish pink to brown-pink or brownish when dry; cyanobiont Scytonema, in chains, cells oblong to bean-shaped, somewhat compressed, 8.5–13.5 μm diam. Apothecia prominent, sessile, constricted at base, submarginal to central, widely scattered to crowded, rounded to somewhat irregular through mutual pressure, 0.5–1.5(–2) mm diam, disc subconcave to plane, roughened–granular to occasionally minutely fissured, dark brown-black to black, colour not noticeably changing when moist or dry, epruinose. Thalline margin prominent, thick, persistent, swollen and obscuring disc at first, entire, white-pruinose, contrasting strongly with disc. Proper margin always present, thin, entire to minutely warted, slightly glossy, raised above margins of disc and slightly above enclosing thalline margin, epruinose. Epithecium granular, brown, black, 16.5–20 μm thick. Hymenium colourless to pale yellowish brown above, 125–150 μm tall. Hypothecium densely interwoven, nubilated, pale yellowish brown, 100–150 μm thick. Asci cylindrical–clavate, noticeably tapering at foot, 100–112 × 17.5–20 μm. Ascospores uniseriate to partly biseriate in ascus, ellipsoidal, apices pointed, straight or slightly curved, contents colourless, minutely granular, (21.5–)23–28(–32) × (7–)8–10 μm, wall thin, smooth, 0.5–0.8 μm thick. Pycnidia widely scattered to crowded, 1–2 per areole, ostiole brown-black, minute, punctiform-depressed. Conidia filiform, arcuate, 15–20 × 0.5 μm.

Chemistry : Thallus K−, C+ red, KC+ red, Pd−; containing gyrophoric acid.

S: Nelson (Mt Arthur, Buller River), Westland (Haast River), Otago (Makarora, Matukituki Valley, Routeburn), Southland (Eglinton River, Piano Flat Waikaia River). It is a fast-growing species of high-rainfall areas in South I., both E and W of the Main Divide. It is found on rocks and pebbles in riverbeds and along stream sides, and also on rock slides and rock outcrops, close to or above treeline, and on roadside rock faces, where it often forms complex, interlocking mosaics with other species of Placopsis such as P. clavifera, P. elixii, P. hertelii, P. illita, P. perrugosa and P. stenophylla, and is also frequently associated with Trentepohlia and species of Stereocaulon in these disturbed habitats.

Endemic

Illustrations : Malcolm & Galloway (1997: 105 – as Placopsis sp.); Galloway (2004a: 159, fig. 5).

Placopsis pruinosa is characterised by: an areolate thallus, an orbicular to spreading, squamulose–areolate thallus delimited by a fibrous, brown, varnish-like marginal prothallus; the squamules or areolae with scattered spots of white, glistening pruina, or the entire thallus thickly white-pruinose; small, orbicular, flattened cephalodia that are not radially cracked; prominent, widely scattered apothecia with a brown-black, epruinose disc, with a persistent, prominent white-pruinose thalline exciple; ellipsoidal ascospores (21.5–)23–28(–32) × (7–)8–10 μm, and gyrophoric acid as the major secondary compound.

It is distinguished from P. hertelii (q.v.) by the much less prominent and narrower, marginal prothallus; the larger and flatter (never papillate) thalline squamules that are also much more densely white-pruinose; the prominent, white-pruinose thalline exciple and the darker apothecial discs; the slightly broader and shorter ascospores, and a simpler chemistry [P. hertelii has additional 5- O -methylhiascic acid (major) and 5-methoxylecanoric acid (minor)].

Placopsis pruinosa differs from P. illita in several characters, in the presence of pruina [P. illita is never pruinose]; in the smaller (0.5–2 mm diam.), flatter, smoother cephalodia [in P. illita cephalodia are fewer but much larger (2–6 mm diam.), central, plicate–ridged to deeply radiately cracked]; apothecia with a prominent, persistent, swollen white-pruinose thalline margin [in P. illita the thalline margin is generally absent or reduced to a thin collar below disc and is epruinose]; and larger spores [in P. illita spores are 12–18 × 5–7 (–8.5) μm].