Chrysothrix Mont.
Thallus generally leprose (rarely endogenous), either byssaceous or crustose-leprose, thin to thick, non-corticate, bright yellow to vivid yellowish-green, unstratified and coloured throughout, being composed entirely of a mass of pulverulent, convex to spherical granules or anastomosing filaments, thallus margin absent, without lobes, prothallus not apparent. Photobiont green, Protococcaceae, cells spherical to 22 µm diam., both singly and in groups. Apothecia rare, to 2.0 mm diam., scattered or clustered, innate or superficial, orbicular, concave to spherical with a thin, non-corticate margin or immarginate, disc smooth, green to yellow-green to light orange or brownish, often yellow-pruinose. Paraphyses 1-1.5 mm thick, colourless, septate, richly anastomosing above asci. Ascospores 8 per ascus, narrowly obovoid to narrowly ellipsoid, straight or curved, colourless, 3-septate (rarely only 1-2-septate) the second cell often the largest of the four, 9-14 × 2.5-5 µm. Pycnidia absent.
Chrysothrix is a cosmopolitan genus of 4 species found on trees, wood, and shaded acid rocks in all parts of the world except the larger deserts, and characterised by the bright yellow to yellowish-green colour (pulvinic acid derivatives), the unstratified thallus, apothecia which are either immarginate or with poorly developed non-corticate margins, and uniform 3-septate, narrowly obovoid to narrowly ellipsoid ascospores. Species of Leproplaca Laundon [ Lichenologist 6: 102-105 (1974)] differ from sterile plants of Chrysothrix in the presence of an anthraquinone, probably parietin, which reacts K+ violet-red, and by the possession of a white medulla. Specimens of Chrysothrix are commonly sterile, but are readily separated from Lepraria and other leprose lichens by their bright colour and chemistry. The genus is monographed by Laundon [ Lichenologist 13: 101-121 (1981)] who comments that it has few close relatives but may have affinities with Arthonia. One species is known from New Zealand.
