#lichens #crustose #lirellae apothecia This is hard to describe - from macro to micro. A lirellate crustose lichen (probably Opegrapha calcarea group - O. saxatilis(?)) grows abundantly on this exposed sandstone coastal outcrop. The sandstone is porous and friable. First image shows the nature of the substrate, regularly dosed with seawater spray. Second image shows details of the black lirellate apothecia on the RHS with the white thallus exposed in the centre - from a distance these lichens appear whitish on the red sandstone. Third image shows how apothecia appear to grow around individual grains, i.e. along grain boundaries. As the lichen matures the apothecia appear to fill remaining areas. Fourth image is a cut through the rock (ie a cross section through the substrate below the apothecia) showing the endolithic nature of the thallus as the hyphae fill the cracks and pores of the rock. The dark part at the top of the image is the non-illuminated surface. Scale bar 1000microns.