Lichens consist of a photobiont (usually an alga, but can sometimes be cyanobacteria) and fungi (usually an ascomycete, named so for their cupped apothecium) living as a mutualistic, composite organism. The fungi benefit from the carbohydrates produced by the algae via photosynthesis and the algae benefit by being protected from the environment by the filaments of the fungi, which also gather moisture and nutrients from the environment, and (usually) provide an anchor to it. The fungal partner protects the alga by retaining water, serving as a larger capture area for mineral nutrients and, in some cases, provides minerals obtained from the substrate. Lichens need a substrate to grow on, but since lichens generally don’t acquire nutrients from substrate, they can grow pretty much anywhere conditions are favorable, like rocks, tree bark, buildings, etc. 



The top layer of the lichen is referred to as the upper cortex, and both the upper and lower cortexes consist of tightly packed fungal cells. Underneath the upper cortex is the algal layer, named so because it houses the algae. Green algae give the lichen a bright green color, whereas cyanobacteria give lichen a dark green, brown, or black color. Beneath the algae is the medulla, which consists of loosely arranged filamentous fungal cells, and the medulla makes up the majority of the thallus, or body, of the lichen. Below that is the lower cortex and the rhizines, which are a continuation of the filamentous cells in the medulla and help anchor the lichen to its substrate. 



Crustose Form: lichens are thin and very tightly attached to their substrate. They usually grow in a circular pattern and do not grow thicker than a coat of paint. Most crustose lichens have stratified layers similar to the foliose lichen(like the anatomy lichen), except that there is no lower cortex or rhizines. The medulla sits directly on the substrate and in many instances actually penetrates the rock or bark it is growing on, making separation of the lichen from the substrate almost impossible. 

Foliose Form: These lichens are flat and leaf-like. Some foliose lichens lie very flat on the substrate while others may have edges that curl above the substrate. They can also grow together and coalesce into large patches. They consist of upper/lower cortex, algal layer, medulla, and the rhizine layer, the latter of which is absent from crustose lichens. 

Fruticose form: These lichen are upright like tiny trees or bushes, or they can also hang down like vines. Fruticose lichens are long and skinny with a round or flattened cross section. Just like the other two forms, these lichens have an outer cortex, algal layer and medulla, but not a definite top and bottom. The center is sometimes hollow or can have a more dense central strand.



In temperate regions, lichens are used by many birds as a nesting material for decoration, camouflage, insulation or to repel insects (because of their disinfectant properties). Lichens are extremely sensitive to air quality, although the degree of sensitivity depends on the species. This property makes lichens a good indicator species of air quality. Lichens absorb everything through their cortex – nutrients as well as harmful toxins and pollutants such as heavy metals. This means that they can be used to determine the levels of pollutants in the atmosphere, and the presence (or absence) of lichens has been used to map concentrations of certain pollutants. Lichens have also been consumed by humans historically, in the past people in Northern Europe used to cook Iceland moss into soups, porridges, etc. Lichens were also usually consumed during times of famine, since two obstacles prevent the regular consumption of most lichens: lichen polysaccharides are generally indigestible to humans, and lichens usually contain mildly toxic secondary compounds that should be removed before eating (,other%20tribes%20sought%20it%20out.).

Lichen Species

Reindeer Lichen (Cladonia rangiferina):These clumps of reindeer lichen were photographed on the forest canopy. Initially, the lichen grows flat along the ground and produces small, scale-like protrusions (foliose). Eventually, reindeer lichen develop upright, hollow, cylindrical, branching structures called podetia (fruticose). As the lichen gets bigger and forms a mat over the ground, resources like light and nutrients become less available to the center and base of the lichen. These areas die back and decompose while the outer edges continue growing. These lichen are named so because reindeer consume them as their main food source during the winter, due to the absence of ferns, mosses, and grasses.

Hooded Sunburst Lichen (Xanthomendoza fallax): This foliose lichen has a golden yellow upper surface with whitish yellow undersides.  The lobes are very narrow and crowded with soredia bursting from crescent shaped slits at the tips. This lichen can normally be found on non-acidic bark and alkaline rock located in full sunlight. 

Common Button Lichen (Buellia stillingiana): This crustose lichen lives on partly shaded rocks and trees. The thallus of this lichen is pale gray, and the numerous apothecia present appear as small, black dots. This specimen can even grown on other lichens present on its substrate, and in this photo you can see the areas where the hooded sunburst lichen and the common button lichen overlap. 


Bottlebrush Frost Lichen (Physconia detersa): This foliose lichen is one of 4 frost lichen species found in Ohio, it is by far the most common. These medium sized lichens have a brownish gray thallus, with long frosted lobes. This eastern species grows on bark in full sun to light shade. It is also commonly found on cemetery headstones. 



Snakeskin liverwort (Conocephalum salebrosum):  Snakeskin liverwort is a large, widespread, very common, bryophyte, and was quite abundant near the outcrops and caves of Deep Woods. It grows on calcareous soil in moist or wet areas that have full shade or dappled sun and are protected from wind. It often forms colonies of overlapping plants, sometimes creating extensive mats. This species gets its name for the scaly hexagonal scales on its surface, each of which have a small white-ringed pore in the center. Snakeskin liverwort creates a strong but pleasant odor when its leaves are crushed.  (


Haircap moss (Polytrichum commune): This moss grows in large patches and is distinguishable by its wiry spiked shoots. When moist, the spikes curve away from the stem, when dry, the spikes come inward and curl around the stem. The moss produces a 4 angled, box-like capsule in the summer that is covered in fine hairs, hence the Polytrichum classification (many-haired). This moss is found in a variety of acidic damp environments, such as moist forests.



Chinquapin Oak (Quercus muehlenbergii): This oak is common in limestone soils. The leaves are alternately arranged and toothed, with a pointy tip. This species can be distinguished from the chestnut oak, which has rounded teeth instead of pointy. 


Here is the list of sources I consulted to compile this webpage, additional sources outside of class resources cited in text where applicable. 

  • Peterson’s Guide to Trees and Shrubs (1986) by George A. Petrides