This crustose Placynthium with black lecideine apothecia, three-, or exceptionally more-septate spores, grey-pruinose thalli without, or only little developed, marginal lobes, had not been distinguished until recently. The lack of marginal lobes distinguishes it from a similar P. garovaglii with a conspicuous placodiod thallus. In a study of Placynthium in the Alps and Carpathians (Czeika & Czeika 2007), it was introduced as newly described P. garovaglii var. subtile (Nimis et al. 2018). Jørgensen (2005) separated it from P. garovaglii due to differences in spores and ascus apical apparatus. Based on the latter, a forgotten genus name Collolechia was resurrected. However, a recent molecular study (Košuthová et al. 2016) showed that both lichens are distinct but phylogenetically very close, forming a monophyletic group. Pronounced differences in the asci and ascospores were not confirmed because both characters are very variable.
Placynthium caesium is associated with sun-exposed calcareous rocks with periodic water seepage. It was found only recently in the Czech Republic, in the Pálava region and limestones near Horažďovice (Malíček et al. 2018).
Literatura: Jørgensen P. M. (2005): (2005): Placynthium garovaglioi not present in Scandinavia. – Graphis Scripta 17: 3–5 . Czeika H. & Czeika G. (2007): Placynthium in den Alpen und Karpaten sowie in benachbarten Gebieten. – Herzogia 20: 29–51. Nimis P. L., Hafellner J., Roux C., Clerc P., Mayrhofer H., Martellos S. & Bilovitz P. O. (2018): The lichens of the Alps – an annotated checklist. – Mycokeys 31: 1–634. Košuthová A., Fernández-Brime S., Westberg M. & Wedin M. (2016): Collolechia revisited and a re-assessment of ascus characteristics in Placynthiaceae (Peltigerales, Ascomycota). – Lichenologist 48: 3–12. Malíček J., Palice Z. & Vondrák J. (2018): Additions and corrections to the lichen biota of the Czech Republic. – Herzogia 31: 453–475.taxonomic classification: most frequented synonyms:Collolechia caesia
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