Lichens are small unusual plants found on rocks, soils, tree trunks, on leaves, fences, and paths. They are of slow growth, are long lived, and are found all over the world, from polar regions to the equator, and from sea level to mountain tops. They are able to withstand extremes of temperature and long periods of drought. These small plants do not have roots, stems, or leaves but appear as spreading crusts, leaf-like mats, or hanging threads, mainly grey or yellow-orange in colour.

A lichen is a composite plant consisting of two organisms living together for mutual benefit. A particular species of fungus lives in symbiotic association with a particular species of alga to form a distinct plant body. When a cross section of a lichen is examined under a microscope, a layer of green or blue-green algae is seen beneath the tangled formation of colourless fungal hyphae. The fungus forms the protective layers which shelter the alga while it carries on photosynthesis and makes food for both partners.

Reproduction may be from fragments of the plant body containing both the fungus and the alga, as in Sticta and Parmelia. Spore capsules, either apothecia or perithecia, are formed by most fungi, and from these are ejected fungal spores which, however, must combine with the particular alga to form a new plant. In some species, such as Cladonia, algal cells become surrounded by fungal hyphae and form powdery masses (soredia) on the surface of the plant. These are distributed by wind and rain.

Many lichens are cosmopolitan and may not be restricted to any particular substratum or habitat. Many are endemic to New Zealand; there are over 1,250 species belonging to 150 genera already known, and it is probable there are many more to be identified.

Lichens may be classified, according to their form, into three main groups as foliose, fruticose, and crustose, with many intermediate forms.