This is one of the foliose jelly lichens containing cyanobacteria. There are two genera with foliose species ie Collema and Leptogium. The former tend to be dark kaki coloured and form rather thick pulpy lobes when wet whilst the latter when wet or dry are normally relatively thin. L. brebissonii when dry is unmistakably a Leptogium with its thin minutely wrinkled blue-grey thallus. If in any doubt it may be necessary to cut a section through its thallus.
Photo: Alan Orange
Leptogiums have a characteristic well-differentiated cellular cortex to the thallus. Once established as a Leptogium this species has lobes that are thin and blue-grey when dry and become more swollen and dark green-brown when wet. In both conditions they are strongly but minutely ridged. Tiny granular concolorous isidia are scattered over the thallus but are concentrated on the lobe margins. Apothecia are unknown in Britain. L. hibernicum can be separated by its tomentose lower surface but has yet to be recorded from Wales. (RGW)