Flora of New Zealand Series

Holotype: New Zealand, Campbell I., cliffs around Mt Lyall pyramid, 393 m, 2.i.1970, H.A. Imshaug 46441 - MSC 126755.

Description : Thallus rosette-forming to irregularly spreading, or in coalescing patches, closely attached to substratum, 50–150(–200) μm thick, tartareous, (1–)3–7(–9) cm diam., with or without a thin, black, irregular, marginal prothallus. Upper surface pale-pinkish or pinkish white when dry (pale lettuce-green when wet); uniform, smooth or shallowly undulate, minutely maculate through regular discontinuities in photobiont layer, small, isolated clumps of photobiont appearing as minute, green maculae (×10 lens) when wet; patchily white-pruinose (×10 lens); continuous or crazed with a reticulum of fine cracks or becoming areolate centrally. Margins entire to flabellate, abruptly and neatly delimited, swollen, here and there pale-brownish. Cephalodia very characteristic, immersed, to very slightly raised above surface of thallus, surrounded by and also intersected by minute cracks (×10 lens), narrow (0.2–0.5(–1.0) mm wide), elongate, vein-like, subdichotomously branched, to dendroid with periodic constrictions along length and appearing "jointed", occasionally somewhat dispersed as small "islands", radiating from centre of thallus to close to margins, pink to pale purplish-blue when wet, ivory to pale pinkish white when dry; containing Scytonema, cells compressed, 6.5–8 μm diam., in parallel chains. Apothecia scattered, rarely 2–3-together, rounded to slit-like, 0.2–0.8(–1.0) mm diam.; thalline margin prominent, pale-whitish, commonly obscuring disc, surrounded by a narrow crack; disc deeply urceolate, pale red-brown, epruinose. Epithecium pale yellow-brown. Hymenium colourless, 225–275 μm tall. Asci cylindrical with a tapering foot, 120–165 × 15–17 μm. Ascospores uniseriate in ascus, broadly ellipsoidal to ovoid, 17–24(–25.5) × (10–)12–14(–15) μm. Pycnidia minute, punctiform, 0.05 mm diam. or less, widely scattered to occasionally crowded in clumps or lines, immersed, ostiole minute, dark-brown to black, surrounded by a pale or whitish halo. Conidia filiform, slightly curved, (20–)25–30(–33) × 1 μm.

Chemistry : Thallus K+ yellow→red or −, C+ red, KC+ red or −, Pd + orange or −; containing gyrophoric, salazinic (±), norstictic (tr.), and protocetraric acids.

A: C: On hard, smooth, subalpine to alpine rocks, associating with Pertusaria spp., Placopsis lambii, P. macrophthalma, P. perrugosa, P. subcribellans, species of Porpidia, Stereocaulon caespitosum, and Verrucaria sp.

Endemic

Illustration : Galloway (2004c: 111, fig. 8).

Placopsis venosa is characterised by: the saxicolous habit; a thick, whitish to creamish, or pinkish (lettuce-green when moist) tartareous, smooth, regular, closely encrusting thallus, distinctly white-pruinose in patches (×10 lens); narrow, subdichotomously branching, vein-like, immersed cephalodia; immersed and somewhat constricted–contorted apothecia; and broadly ellipsoidal to ovoid ascospores, 17–24(–25.5) × (10–)12–14(–15) μm. The Fiordland endemic, P. murrayi (Galloway 2004a), another species with long, vein-like cephalodia, differs from P. venosa in that it has deeper thalline cracks, larger apothecia, slightly larger ascospores, (20–)23–27(–28.5) × 15–17.5 μm; wider cephalodia (1–3 mm wide), and a different chemistry [gyrophoric acid (major), 5- O -methylhiascic acid (minor), lecanoric acid (minor), and a characteristic bright-yellow pigment (comprising two unidentified anthraquinones (minor) reacting K+ red-purple) developed in the medulla and visible at base of thalline cracks in central parts of the thallus].