Any of a group of composite organisms made up of a fungus and an alga living in symbiotic association (called obligate mutualism). The fungus provides a structure that may protect the alga from drying and harsh conditions; the alga synthesizes and excretes a specific carbohydrate that is taken up and utilized as food by the fungus. Lichens do not feed off or damage host plants thus efforts to control or eradicate them should not be undertaken

Lichens are extremely slow-growing and very susceptible to air pollution thus it is not likely that you will find them in cities. Lichens have no roots, obtaining moisture instead from the evening air and the dew. Below are Foliose (leaf-like) lichens on lemon tree bark.