Petractis clauHamb1.jpg

Petractis clausa, growing on an old limestone memorial, VC 24, Buckinghamshire. The thallus has been scratched and the photobiont layer is seen to be a dull, bluish-green colour due to the presence of the cyanobacterium Scytonema. This colour in the scratch is different from the fresh, grass green of lichens containing Trebouxioid green algae, or the yellow- or orange-green of lichens containing Trentepohlia. The surface of the thallus in P. clausa often has a yellowish orange tinge originating from the coloured gelatinous sheaths of the Scytonema photobiont.