Rhizocarpon purpurascens Fryday
Biblioth. Lichenol. 88: 140. 2004.

Thallus areolate; areoles creamy-white to grey, flat, 0.4-0.6 mm across, dispersed or contiguous; medulla I. Photobiont chlorococcoid, cells globose 10-14 m diam.
Apothecia black, lecideine, orbicular, (0.5-)0.7-0.9 mm diam., sessile, flat to convex; margin thin and barely raised, 0.03-0.04 mm wide. Excipulum thin, composed of radiating hyphae 5-7 m wide, hyaline within, with dark brown cortical cells. Hypothecium thin, 35-50 m, mid brown to dark brown (K, N), mainly composed of vertically orientated hyphae. Hymenium vertically streaked red-brown (K+ purple, N), 130-150 m thick, I+ blue; epihymenial zone dark blue-black (K, N+ red), 15-20 m thick. Paraphyses slender, 1.5-2.0 m thick, branched and anastomosing, conglutinate, swollen at the apices to 5-6 m with a dark blue-black (K, N+ red) cap. Asci cylindrical to clavate, c. 85 x 35 m, Rhizocarpon-type. Ascospores initially hyaline, soon becoming blue-black (K, N+ red), sub-muriform with 3-5 transverse septa and 1 longitudinal septum, (21-)24-30(-34) x 12-14 m, halonate, halo 7-8 m thick in young ascospores, thinner in mature ones.
Conidiomata not observed.

Chemistry: K+ yellow or red, C, Pd+ yellow or Pd+ orange. Norstictic or stictic acids by TLC.

Distribution: Siliceous rocks on Kerguelen, Campbell Island and South Island (NZ).

Notes: The New Zealand specimens have a thick, creamy-white thallus composed of contiguous areoles that contains norstictic acid, and flat apothecia, whereas the holotype from Iles Kerguelen has thinner, grey, dispersed areoles, containing stictic acid, and convex apothecia. However, the two populations have an identical apothecial anatomy and so I have no hesitation in including them in the same taxon.
The degree of red-brown pigmentation in the hymenium varies from dense, colouring the whole hymenium, to scattered granules that are only readily visible on the application of K, when they turn bright purple. Even in the absence of this pigment the species would be readily identified by the combination of relatively small, pigmented, sub-muriform ascospores, blue-black (N+ red) epihymenium, and I ve medulla.
Lichens associated with the type collection include Candelariella sp., Fuscidea asbolodes, and Pertusaria sp.

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Thallus & apothecia