Flora of New Zealand Series

Thallus rosette-shaped, grey, olive or blackish, lobate, some species with a marginal prothallus, saxicolous or terricolous or muscicolous, rarely corticolous. Lobes squamulose or stellate-radiating, flattened or filiform, with or without isidia. Photobiont blue-green, Rivulariaceae or Scytonemaceae. Lower surface and rhizines bluish-green or pale. Apothecia hemiangiocarpic, lecideine with proper or thalline margin. Asci cylindrical with apically thickened wall, 8-spored. Ascospores colourless, 2-, 3- or 4-celled, ellipsoid or fusiform.

Species of Placynthium are saxicolous, growing in ± moist places such as drainage cracks on rock faces, and the edges of lakes or rivers. Some species occur on dry limestone. P. nigrum is occasionally found on soil, or epiphytic on bark or other plants. Twenty-five species are described world wide and are included in the family Placynthiaceae. The North American species are monographed by Henssen [ Can. J. Bot. 41: 1687-1724 (1963)] and this account contains much useful information on the genus and its taxonomy. Species are distributed mainly in the Northern Temperate zone, and in the Southern Hemisphere the genus seems to be rare. At least one species is known in New Zealand from limestone.