Dog-tooth lichen (Peltigera canina) is often found growing on lawns and rocks. Like all lichens, it consists of an alga or cyanobacterium and a fungus living together in a symbiotic relationship. The fungus provides a structure for taking up moisture and nutrients; the alga or cyanobacterium is capable of photosynthesizing and producing food for both itself and the fungus. The brown structures in the photograph are the fruiting (spore-producing) bodies of this lichen, and their resemblance to dog teeth gives this lichen its common name. In the Middle Ages, dog-tooth lichen was used to treat rabies — it was felt at the time that this lichen’s resemblance to dog teeth indicated that it could cure dog-related ailments.