Lichen ID

Thanks for your question, Bob — lovely photo. It would be helpful to have more of an overview, but your lichen gives the strong impression of Physcia tenella. It’s a very common leafy or ‘foliose’ lichen on bark and other substrates, especially in lowland UK. Indeed, it’s probably fair to say that it’s been on the increase, as it thrives on the excess nitrogen that our agricultural activities have injected into the environment.

You can read more about P. tenella on the British Lichen Society website at https://britishlichensociety.org.uk/sites/default/files/Physcia%20tenella.pdf” class=”bbcode_url”>https://britishlichensociety.org.uk/sites/default/files/Physcia%20tenella.pdf. You can also see an updated distribution map (April 2023) at https://britishlichensociety.org.uk/resources/species-accounts/physcia-tenella” class=”bbcode_url”>https://britishlichensociety.org.uk/resources/species-accounts/physcia-tenella.

P. tenella tends normally to reproduce asexually or ‘vegetatively’, through powdery grains called ‘soredia’, which contain cells from both the fungus and the alga that make up the symbiotic organism. These soredia develop in masses called ‘soralia’, which on P. tenella tend to form on the tips of the lobes shown in your photo. They disperse in wind and rain to colonise other surfaces and become full lichen bodies or ‘thalli’.

One must be careful to distinguish between P. tenella and the look-alike P. adscendens. They’re best told apart by the shape of their lobe tips and their soralia. The lobe tips of P. adscendens tend to be closed and ‘hooded’ rather than open and ‘fingery’, and the soralia develop in the ‘hoods’ rather than on the tips of the ‘fingers’.

Your dark ‘claws’ are called ‘cilia’, which are hair-like structures or outgrowths of the thallus. P. adscendens has them as well, as do some other lichens. Their purpose isn’t entirely clear — they don’t help with attachment to the substrate, as similar outgrowths on the underside called ‘rhizines’ do. Perhaps they help with the condensation of moisture, as lichens take their water from the air or from rain. Perhaps they have some other purpose as well.

By the way — the whitish spots on the lobes of your lichen are called ‘pseudocyphellae’. They represent a thinning of the lichen’s surface layer or ‘cortex’, which helps to facilitate gas exchange. The alga inside the fungus body draws in carbon dioxide to photosynthesise sugars for itself and for the fungus, and oxygen must be released.

If you could post a photo showing a bit more of the lichen, it might be possible to confirm that you have P. tenella. That said, immature thalli of P. tenella and P. adscendens, which haven’t yet developed soralia, are generally indistinguishable.