Flora of New Zealand Series

Thallus foliose, lobate to squamulose, dorsiventral, heteromerous, orbicular to spreading, loosely to closely attached, with or without a conspicuous, black prothallus visible beyond margins of lobes or squamules. Lobes adjacent, flabellate to cuneate, often imbricate or ± crowded or areolate-squamulose, margins entire to variously incised, notched, phyllidiate or lobulate, sometimes ± ascending, sometimes ± hirsute, tomentose or scabrid. Upper surface convex to plane, smooth to wrinkled-plicate or ± bullate, with or without soredia, isidia or phyllidia, shining or matt, scabrid or tomentose. Medulla white. Photobiont green, ? Myrmecia. Cephalodia (Nostoc) on upper or lower surface, or on hyphae of prothallus, superficial or immersed, sometimes sorediate, simple, ± globose to spreading, placodioid-plicate. Lower surface corticate, ± pale, rhizinate. Rhizines simple or branched, pale to black, often dense, entangled, byssoid, extending beyond lobe margins as a black prothallus. Apothecia laminal or marginal, lecanorine, with a prominent, persistent, crenulate thalline margin, concolorous with thallus, sometimes hirsute, disc red-brown to pale yellow- brown or orange-brown, concave at first then mostly plane to subconvex, smooth, continuous or gyrose-contorted or etched or with plugs or concentric rings of sterile, thalline tissue, sometimes white-pruinose. Asci clavate to sub-cylindrical, 8-spored, often with a well-developed manubrium. Ascospores simple, colourless, oval-ellipsoid, often with a thickened or decorated epispore.

Psoroma is a predominantly Southern Hemisphere genus of c. 50 species included at present in the family Pannariaceae. One species P. hypnorum also ranges widely in the Northern Hemisphere [Jørgensen Opera Bot. 45: 26-32 (1978)]. Two main centres of speciation are known, southern South America, and New Zealand where at least 30 species are found. In this account 25 taxa are discussed, though at least five additional species remain to be described. Psoroma as understood at present is a heterogeneous assemblage of taxa and may well prove to be divisible into two or more independent genera. Psoromidium is one such segregate. No modern account of the genus is available although an account of the New Zealand taxa is in preparation, and Prof. Henssen is monographing the species in South America. Typification of Psoroma is discussed by Jørgensen [ Opera Bot. 45: 13 (1978)], and the earlier accounts of Nylander [ Syn. Meth. Lich. 2: 21-26 (1863); Lich. N.Z.: 51-56 (1888)], and of Hue [ Nouv. Archs Mus. Hist. nat. Paris. sér. 4, 10: 169-224 (1909)] are still useful. Chemistry in the genus has yet to be investigated in depth but preliminary reports [Sargent et al., Aust. J. Chem. 29: 2263-2269 (1976); Elix et al., Aust. J. Chem. 31: 2057-2068 (1978); Renner et al., Mycotaxon 13: 433-449 (1981); Elix et. al., Aust. J. Chem. 35: 2325-2333 (1982)] indicate that chemotaxonomy may be important in Psoroma.

Taxa in Psoroma in New Zealand, as in Pseudocyphellaria and Menegazzia, seem sensitive to particular changes in microclimate and microhabitat, and some species have a very restricted distribution. In the field most species are a bright lettuce-green, this colour being lost on storage with the result that most herbarium specimens are ± uniformly cinnamon-brown. Terricolous species, P. hypnorum, P. buchananii and P. hirsutulum are exceptions, as are the yellowish species P. durietzii, P. pholidotoides, P. soccatum and P. xanthomelanum. Laminal cephalodia are always pale or dark blue-grey in freshly gathered material, and in consequence are easily seen in the field, whereas in herbarium specimens they may be difficult to detect as their colour fades rapidly to the uniform colour of the dried specimen.