The foliose lichen, Peltigera membranacea (a so-called "Dog lichen"), growing on moss-covered rock in Western Scotland. Lichens arise from a symbiotic association between a fungus and a photosynthetic alga. Foliose lichens grow as an extended thallus ( resembling a leaf ), which is attached to the substrate by means of rhizines, seen here as small pointed structures where the underside of the thallus is visible due to the reflex curling of developing apothecia. The upper surface of the apothecia is orange-brown, here seen in places above centre.