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X is for Xanthoria

We tend to think of lichens as being very sensitive to air pollution, but there are some species that are quite tolerant. The yellowy-orange coloured leafy lichen Xanthoria parietina is a species that prefers nutrient enrichment, particularly the nitrous oxides and ammonia produced by car exhausts. It is now one of the most common species around and is very noticeable in winter when the bare branches of trees alongside major roads show up as a yellow stripe where the lichens have colonised the polluted bark. It also grows on rocks, walls (the specific name parietina means ‘on walls’) and roofs where the yellow blobs often indicate places where nutrients are increased by bird droppings.

The yellow chemical xanthorin is thought to be produced as a defence against UV radiation, acting as a sunscreen to protect the algal cells inside the lichen. The name Xanthoria comes from ‘xanthos’, a Greek word meaning yellow. Extracts have been used as a dye to colour many things from fabric to Easter eggs. In the past it was used as a remedy for jaundice because of its yellow colour. For more information about Xanthoria and other lichens visit the SBIS pages