Rhizocarpon

Together with three small genera (Catolechia, Poeltinula and Epilichen), Rhizocarpon constitutes the family Rhizocarpaceae.[1][2][3] Historically, ca 389 names have been used.[4] However, many species concepts are ill defined, many names have been synonymized[5] and new species are regularly being described,[6][7][8][9] so true number of species is not entirely clear as of now, but is estimated to be around 200.[8][9][10][11] In molecular work, the genus has also been shown to be paraphyletic, with closely related genera being nested within Rhizocarpon.[2][12]

With so many species in a morphologically diverse genus it is difficult to say something general about morphology and anatomy and there will inevitably be some exceptions. However, they do share some key traits. They are all crustose and mostly saxicolous (rock-living), with some being lichenicolous (lichen parasites) on other saxicolous lichens. They are all lecideoid, meaning they have apothecia without a thallus margin containing algae.

The genus has a distinct type of ascus, the Rhizocarpon-type,[13] which is bitunicate with the inner ascus-wall being slightly apically thickened. Ascospores are considered important characters for determining species within the genus. They are either 2-celled (1-septate) or multi-celled (muriform) and are either hyaline or pigmented (green or brown), often with a characteristic halo (or “perispore”) visible when viewed in a microscope. Asci contain eight, two or rarely one spore.[5][7][10][12]

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  Rhizocarpon sp. on a mountain Zlatibor, Serbia
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  Rhizocarpon pycnocarpoides