This page is a basic introduction to lichens, so you can distinguish them from small plants and start learning the names of their structures — important for learning how to ID the lichens themselves.
A lichen is a composite organism formed by certain fungus species that join with certain algae species. The relationship between the fungus and the algae is quite intimate and integrated, and the lichen that is formed does not much resemble either of the components.
Lichens may have a variety of colors, depending on the types of algae associated with the fungus. The colors may change when the lichen becomes wet. For example, many species that usually appear gray, tan, or yellow turn green after a rain.
Depending on the species involved, lichens can have several different growth forms. They grow on rocks, trees, soil, or other surfaces. Some lichens look like old crusty, cracked, or peeling paint; others grow in multiply-branching, leafless tufts; others grow in flattened rosettes or shield shapes, with petal-like lobes; and others can be powdery, jelly-like, scaly, or hairy.
The growth form of lichens is a clue to the identity of a lichen species, but growth form (crusty, leafy, shrubby, etc.) is not a taxonomic distinction itself. It is similar to the situation with many familiar plants, where different species within the same family may grow as trees, shrubs, vines, or nonwoody plants. Also like plants — such as a shrub that could also be called a small tree — there are many lichen species whose form isn’t perfectly clear cut.
Lichens are plantlike, but they are not plants. They are a life-form made of at least two unrelated species: a fungus plus either a green algae or a cyanobacteria (blue-green algae). The fungus species and the algae and/or cyanobacteria species vary, so depending on which ones join together, there are many types of lichens. Each lichen takes the same scientific name as its fungal component. Not all fungus species form lichens. Those that do form lichens must partner with the correct type of algae, or they won’t survive. The algae species, however, may usually be able to exist independently.
The fungus and the algae that make up a lichen are usually thought to be in a symbiotic relationship. The fungus provides a protected substrate for the algae to live on and in, and the fungus also provides the algae with moisture and nutrients. The algae, which has chlorophyll, can make food (sugars) via photosynthesis, using air, water, and sunlight, and both organisms use this food. Often this relationship has been portrayed as a harmonious, equally beneficial relationship, but more recently biologists have pointed out that the fungus may actually be engaging in a sort of agriculture, growing, controlling, and skimming the nutrients from the algae for its needs.
New research has revealed that many lichens include a third species, a yeast, in their bodies (notably, the outer skin, or cortex). In many cases, the yeast seems a requirement for a lichen's survival. It's essentially a third symbiotic partner. The presence of different types of yeasts in otherwise identical pairings of fungus and algae helps explain why some kinds of lichens, which should look alike, end up looking quite different.
Vegetative and Reproductive Parts
The vegetative part of a lichen (as opposed to any reproductive, spore-producing structures) is called the thallus. It’s the main body of the lichen; it conducts photosynthesis and grows. There are distinct layers to the thallus. The outer skin, the upper cortex, is a thin protective fungal layer. Just under that is the thin algal layer, where the partner algae is concentrated, since it needs sunshine. Below the algal layer is the medulla, basically the rest of the fungal body, rather spongy. In many lichens, a lower cortex is the skinlike bottom surface. Rootlike or threadlike rhizines sometimes project from the lower cortex and help attach it to the substrate.
The other parts of a lichen are the fruiting bodies: reproductive structures that can take a variety of shapes, depending on what type of fungus is involved. Sexual (spore-bearing) structures may be clublike, cup-shaped, or stalked reproductive structures, or they may be disk-shaped, buttonlike, or warty structures, or they may take the form of squiggly lines on the surface of the lichen. See Life Cycle for more on specific types of reproductive structures.
Vegetative (asexual) reproductive structures — which break away to create genetic duplicates of the parent — may look like scales, patches or crescents of dust, or tiny raised dots, columns, or warts. See Life Cycle for more on specific types of reproductive structures.
Identification Tips
Precise identification of lichens can be tricky. You should note details of growth habit, colors, size, habitat (especially substrate), surface texture and patterns, and characteristics of the reproductive structures. Specialists use the above characters in precise ways, with technical terminology, and they frequently also test with certain chemicals that, when applied, cause color changes that help identify some species. They also frequently use microscopes to examine lichens and their spores.
Any description of a lichen begins with its general vegetative growth form (the thallus). Most of Missouri’s lichens fall into three main categories:
- Crustose (crusty) lichens are very tightly attached to the rock, bark, or other surface on which they grow. They often look like a thin or thick coat of paint, and the surface may be cracked. Sometimes they look like the result of a quick squirt from a can of flat spray paint. They have no lower surface. If you try to pry a crustose lichen from its substrate, you usually end up with a tiny pile of dust. To remove a crustose lichen from its substrate, you’ll have to damage the lichen, the substrate, or both. Roughly speaking, Missouri has about 200 species that could be called crustose lichens.
- Foliose (leafy) lichens are lobed, frilly, or leafy — somewhat like foliage. Like crustose lichens, they are basically two-dimensional, but unlike crustose lichens, they have a definite lower surface. The lower surface of foliose lichens is usually a different color than the top surface. They also have a branching, lobed growth pattern. Many form rosettes or shieldlike circular patterns on tree trunks or rocks; they branch outward with broad, leafy, or narrow, petal-like lobes. They range from no larger than a fingernail to circular patches several inches wide. In colonies, they can cover large areas. One subgroup of foliose lichens are the so-called navels, such as dog tooth lichen and rock tripe/stippleback lichens. These consist of floppy-looking leaflike structures attached to the substrate at a single point. Roughly speaking, Missouri has about 150 species that could be called foliose lichens.
- Fruticose (shrubby) lichens are three-dimensional, typically growing in multiply-branching tufts, resembling tiny leafless shrubs. The branches are round or sometimes flattened in cross-section. The reproductive structures can be club-shaped with colored tips, like fruits, or like tiny goblets or horns. Common Missouri examples include the easy-to-learn cladiform species, common on soil and/or rotting logs, such as reindeer moss, British soldiers, brown cap, and a variety of tiny-goblet-bearing species called pixie cups. Beard lichens (Usnea spp.) are also easy to recognize as a group, since they grow in branching, gray-green tufts on tree branches in forests. Roughly speaking, Missouri has about 55 species that could be called fruticose lichens.
Although the three most common growth forms of lichens are crustose, foliose (leafy), and fruticose (branching), there are other forms, too, including squamulose (crusty and attached to the substrate at the base, yet scalelike and free, like tiny leaves, at the tips); leprose (powdery-textured); and gelatinous (jelly-like).
Similar Species
- Some liverworts resemble certain lichens, especially the leafy ones. But liverworts, like mosses, are a type of nonvascular plant (not a fungus plus algae). They have a moister texture, different anatomical structures, and distinct growth patterns. They are usually found in moist, not dry habitats. Some of the most familiar liverworts have a flat, ribbonlike growth form with a surface that resembles reptile skin.
- Spanish moss resembles shrubby lichens in the genus Usnea, but it is neither a lichen nor a moss. It is a bromeliad — a member of the pineapple family of flowering plants. It doesn’t grow in Missouri, but you might have seen it hanging like gray-green beards from trees in Florida and other southern states.
Size varies depending on species. Many lichens develop colonies that can cover large areas of substrate.
Statewide. Different species may have different habitat preferences and therefore different regional distributions.
Habitat and Conservation
The different species of lichens have certain habitat preferences, especially when it comes to the substrate they grow upon. Some lichens are very generalized and can be found in a wide variety of places. Others are very specialized. Most lichens are found on either trees or rocks, but some grow on soil or other places. Some grow in places such as sidewalks, tombstones, fence posts, or cedar shingles. Ones that grow on rock may prefer certain rock types. Thus, the habitat and substrate can help to identify the species.
When you go hunting for lichens, carry a magnifying glass, or use a good closeup camera, which will help you get a better look at them. Keep in mind that many kinds of lichens look different when they’re wet. That’s because the fungus part of the lichen becomes transparent and allows the colors of the algae to show through. (Try splashing a dry lichen with some water to see it change color and texture.)
Winter and early spring is a great time to look for lichens because it’s wet and cool enough for them to make food and grow. In the summer, when it’s hot and dry, lichens often become dormant and less colorful.
The one type of habitat where you will not find many lichens is large cities: since they absorb nearly anything (both nutrients and toxins), lichens are sensitive to pollution, including air pollution, and only a few types can survive in city environments.
In Missouri, good places to see lichens on rocks include glades and the open, rocky tops (balds) of Ozark hills. For spectacular lichen viewing, visit places like Taum Sauk Mountain, Elephant Rocks State Park, Wildcat Glades, and Lichen Glade Conservation Area.
Older cemeteries can be wonderful places to enjoy lichens. In addition to the variety of stone surfaces for lichens to grow on, such as marble and granite, the trees in cemeteries grow in open situations, with the trunks and branches exposed to plenty of fresh air and sunlight.
You can also visit your nearest conservation nature center or other conservation area, where you are sure to see lichens on rocks, trees, and branches. In all seriousness, once you start looking for them, you will find lichens on nearly any surface that holds still long enough.
- Of the lichens that grow on rock or soil, some prefer calcareous substrates such as limestone or dolomite, or the alkaline soils they produce, while others prefer neutral rock and substrates like granite or sandstone and the mildly acidic soils they produce.
- Of those that grow on trees, some prefer bark substrates while others require plain wood. Some are more common on trunks, while others live on high branches or twigs (look for them on branches that have recently fallen to the ground), or at the very base of the tree. Those that live on exposed heartwood might also live on fence posts or wooden shingles. Some lichens prefer smooth, hard bark as on hickories or maples, while others prefer rougher, softer bark as on black walnut or ashes. Oaks may offer a slightly more acidic substrate and be preferable to certain lichens for that reason. Some lichens live in deep crevices at the bases of trees. Some lichens only occur on a single species of tree.
- Some lichens require more light and are found in sunny areas, while others are best adapted to shady places. Corresponding to light and shade is the amount of moisture: some lichens need a damp habitat, while others survive in the driest locations on Earth.
Food
The photosynthesizing portion of a lichen (the algae and/or cyanobacteria) makes its own food (sugars) using sunshine, water, carbon dioxide, and a photosynthetic pigment (such as green chlorophyll). Nitrogen is an important nutrient and can be a limiting factor. But cyanobacteria, like the bacteria in the root nodules of legumes, are able to “fix” (transform) atmospheric nitrogen into a form that can be used like fertilizer. But algae and cyanobacteria are usually limited to aquatic or very damp habitats. They can’t tolerate drying out.
Fungi are not capable of photosynthesis — like animals, they must digest organic materials (living or dead) in order to have energy (sugars) for life. With their network of threadlike strands (hyphae) and their tough outer cortex, however, fungi are very good at absorbing water and holding water.
Thus, the fungal component of a lichen provides the moisture for the algal component to survive, and the algae provides food for itself and the fungus. In this way, lichens are able to survive (even thrive) in some of the most inhospitable environments on earth: exposed boulders on hilltops, sheer cliff faces, alpine or polar tundra, and deserts. They only need sunlight, occasional moisture, and a surface to cling to; when life gets rough, they merely go dormant.
Status
Of Missouri’s approximately 436 species of lichens, 25 are listed as Missouri species of conservation concern, meaning that they are rare in our state and potentially vulnerable to extirpation (becoming extinct within our borders). Most of these 25 are ranked as critically imperiled, meaning they are extremely rare or are experiencing steep declines, making them especially vulnerable to extirpation. “Critically imperiled” is only one step away from “threatened.” Missouri’s lichen populations deserve broader and deeper study.
Taxonomy: Lichens present an interesting situation, since each one is a union of at least two separate species — a fungus plus a green alga and/or cyanobacterium (blue-green alga). Scientists use only the name of the fungal component as the official name of the lichen. The photosynthetic algal partner always has its own scientific name, however.
Giving a lichen the same name as its fungal component can be problematic. For example, several fungus species that form lichens have been known to link up with different species of algae or cyanobacteria, producing lichens that look quite different — yet because a lichen is named for the fungus no matter what its algal partner is, these all have the same scientific name.
Life Cycle
Lichens can reproduce sexually (via spores) or vegetatively (asexually).
Sexual reproduction, via spores, in lichens occurs in much the same way as other fungi. Special structures called apothecia give off spores, which are created as the result of a fusion of gametes (sperm and ova) that occurs in the apothecial tissues. Apothecia may look like circles, craters, cups, warts that erode to reveal cups, or simple holes or pores in the surface of the lichen. In several fruticose (shrubby) lichens, spores develop on special upright structures called podetia, which bear goblet-like or ball-like apothecia (British soldiers, pixie cups, and reindeer lichen are examples).
When lichens reproduce via spores, only the fungal component is reproduced. The germinating fungal spore must quickly attach to a suitable algal partner, or it perishes.
Vegetative (asexual) reproduction can happen in several different ways. In these cases, both fungal and algal components are included in what amounts to the offspring, so the new lichen is a genetic copy (clone) of both the fungus and the algae accompanying it.
First, there is simple fragmentation (you could think of it as “cuttings”): A piece of a lichen’s thallus, or the bark or rock on which it grows, can break away and begin growing elsewhere. Lichens, especially when dry, may easily be damaged, and the fragments may be transported to new locations by wind or water. Some lichens produce tiny green scale-like structures called squamules; these detach easily and form new lichens elsewhere.
Second, a lichen can also produce special tiny packages that contain a sample of both fungal and algal tissues. They readily break away to start new lichens elsewhere. These little structures can take two forms — soredia or isidia — and whichever of the two a lichen forms can help you identify the species: Most lichens that form one type of structure won’t form the other, although both types may form apothecia.
- Soredia are tiny ball-like packets of fungus and algae that form under the lichen’s surface (upper cortex), then erupt from breaks in the upper cortex that are like tiny pimples. The tiny pimple-like breaks in the cortex are called soralia. Places where clusters of soredia are forming make that part of the lichen look rough and dry: fluffy, grainy, or mealy.
- Isidia are another way lichens package portions of themselves to be broken off and distributed elsewhere. Isidia are little raised buds or tubes that form on the upper surface of a lichen, consisting of the upper, skinlike cortex and the middle (medulla) layers, so like soredia, isidia include samples of the algae. But instead of a pimple whose contents spew out to start a new lichen, think of a wart where the entire thing pops off to start a new lichen. Because isidia are covered with cortex tissue, they look rather shiny (use a hand lens), while soredia look dry and dusty.
Human Connections
Because lichens, with their soft, absorbent tissues, are extremely sensitive to air pollution, they are good indicators of air quality. Scientists use them to monitor air quality and forest health.
Dyes: Lichens can be used to make dyes for coloring cloth. The famous Harris tweed, made in Scotland, was traditionally colored using dyes made from lichens. Other lichens are used to make litmus, the mixture of dyes applied to paper test strips used to determine pH (acidity or alkalinity).
Model railroaders and others who create other miniature landscapes often use dried fruticose lichens, painted green or perhaps in autumnal colors, to represent trees and shrubs in their dioramas. The Dutch graphic artist M. C. Escher depicted fruticose lichens as landscape plants in some of his mind-blowing illustrations of impossible architecture.
Lichens can live for a long time. Because their growth rate is steady, lichens can be used to estimate dates in the past. It’s similar to tree-ring dating, but it works in rocky landscapes where trees are scarce. Many lichens begin as a dot on an overturned or newly exposed rock face and then grow outward, slowly, in a circular pattern. Measuring the diameter of the largest lichen on a rock face, and figuring in the growth rate, shows how long the rock has been exposed. This method of dating is called lichenometry.
Can you eat lichens? Reports vary. Certain types, in certain parts of the world, have been eaten historically, especially as a survival food. But many kinds of lichen contain toxic chemicals, so we do not recommend eating them.
Certain lichens have been used medicinally in the past. Researchers are investigating possible antibiotic value of some species. Some lichens have antimicrobial properties. Some manufacturers include them in deodorants, salves, and perfumes. Ancient Egyptians used certain lichens to stuff the empty body cavities of mummies, and they also used lichens to preserve and enhance the odor of spices used in the embalming process.
Lichens are common yet mysterious. Use a hand lens to examine these combo-organisms. The seemingly primitive nature of lichens, mushrooms, and mosses and other nonvascular plants makes them wondrous. They can thrive where other life-forms cannot, and they support other types of life, becoming a micro-habitat on their own.
Lichens contribute to a beautiful landscape. In desert, alpine, tundra, or other rocky habitats, they can coat rocks with a riot of bright colors — neon yellows, oranges, and greens — which intensify after a rain. In damp forests, the many fruticose species grow in interesting, mysterious masses on tree trunks and limbs.
In the mid-1800s, the celebrated essayist John Ruskin wrote of lichens and mosses, which are the first organisms to clothe newly exposed cliff faces with life: “Meek creatures! the first mercy of the earth, veiling with hushed softness its dintless rocks; creatures full of pity, covering with strange and tender honor the scarred disgrace of ruin — laying quiet finger on the trembling stones, to teach them rest. . . . And as the earth’s first mercy, so they are its last gift to us: When all other service is vain . . . the soft mosses and gray lichen take up their watch by the headstone. . . . These do service for ever.”
Do lichens damage tombstones or historic stone structures? People generally believe that the presence of lichens can accelerate weathering, but apparently it's not a simple issue. The type of stone, the fragility of any carving, irregularities in the stone, the extremes of weather, and the types of lichens are all factors. Also, many people cause greater damage to old gravestones in efforts to remove lichens, by scraping or by applying harsh chemicals. And the lichens will ultimately return. In many cases, it might be better to let the lichens grow. There is evidence that in some cases, the lichens might actually protect the stone from deterioration from the weather.
Certain types of lichens are vitricolous: they grow on glass surfaces, such as old bottles or broken glass left undisturbed in nature. When vitricolous lichens grow on the stained-glass windows of historic churches, they may hasten deterioration of the glass.
It wasn’t until 1867 that humans figured out that lichens are a combo-organism of fungal and algal partners. The Swiss botanist Simon Schwendener proposed the idea. It took a while for the idea to gain acceptance. And for a long time, people considered lichens to be plants: they were green, and they didn’t move. The components of lichens — fungi, green algae, and cyanobacteria — were for many years all lumped into the plant kingdom. Closer study of their cell structure and biochemistry cleared up the situation, and the three are now in different kingdoms.
Now is a great time to learn about lichens, because there is so much information available online. Before the Internet, you needed access to heavy reference manuals and scholarly journals, whose pictures were limited by publishers’ budgets. Today, great closeup photographs abound, and information is available at all skill levels.
Ecosystem Connections
Lichens are eaten by several types of animals. Lichen moth caterpillars feed on lichens, and so do lichen grasshoppers, which live in Missouri’s glades. Both the caterpillars and lichen grasshoppers are camouflaged to blend in perfectly with lichen-covered rocks. Then there’s the little white lichen moth (Clemensia albata) where both the caterpillar and adult are lichen-camouflaged. The caterpillars of this easy-to-overlook species (the only species in its genus) eat lichens. Many other animals eat lichens, too, including deer and caribou.
Because lichens provide cover for several types of insects, insect-eating animals often focus their hunting on lichen-covered places. Birds that forage for insects among tree lichens include nuthatches, the brown creeper, chickadees, titmice, woodpeckers, and numerous vireos and warblers. Other animals that look for insect prey among lichens include several types of spiders and the larvae of green lacewings.
For camouflage as they go around hunting aphids and other insects, the larvae of green lacewings decorate their backs with bits and pieces of detritus, and often they cover themselves with lichen. They look like little quarter-inch-long fragments of lichen that have decided to get up and walk around.
Many vertebrate animals have markings and coloration that help them blend in with the mottled background of lichen-covered bark or rock. Examples include the backs of birds such as yellow-bellied sapsuckers and brown creepers, gray treefrogs, and the prairie lizard (formerly called the northern fence lizard). Eastern collared lizards, with their mottled gray-tan backs and minty green legs, blend in with the lichen-coated rocks of their glade habitat.
Several types of birds, including ruby-throated hummingbirds, eastern wood-pewees, and many types of warblers and vireos, harvest lichens to build their nests. Often, they decorate the outside of the nest with lichens, which provides excellent camouflage. The nest, perched on a tree limb and covered with lichen, looks just like an old lumpy knot on the limb. Often, certain bird species prefer certain types of lichens.
In some places, the northern parula warbler’s numbers increased dramatically in the decades after the Clean Air Act was established in the 1970s: Many lichens are sensitive to air pollution and disappear when air quality is diminished. Apparently, the cleaner air allowed lichen populations to rebound. The parula relies on abundant lichens with which to camouflage its nests, so the rebound of lichens apparently led to the rebound of the sweet-singing little yellow-breasted warbler.
Tree trunks and branches are habitats unto themselves, and many insects that rest on tree trunks and limbs are lichen-camouflaged. Eyed click-beetles and eastern Hercules beetles are just a few of the many insects that blend in. Also, countless species of noctuid, underwing, sphinx, geometrid, and other night-flying moths rest unbothered during the day on tree trunks. They are hidden by their drab but exquisitely ornate wing patterns; a green cast in many species helps them mimic lichens and mosses. The minty-green and black-patterned green marvel moth (Agriopodes fallax) is a beautiful example. Often, caterpillars have similar camouflage.
A prominent lichenologist has said that “lichens are fungi that have discovered agriculture,” since lichen-forming fungi reap a harvest of carbohydrates that the algae or cyanobacteria produce, just as farmers harvest the nutritious products of vascular plants. But fungi are not alone in partnering with algae:
- Many species of sea anemones and corals (which are invertebrate animals) have developed a similar relationship with photosynthetic algae that live within their tissues. Nutrients from the algae supplement the diet of the anemones and corals. That is why people with saltwater invertebrate aquariums must use intense, full-spectrum lights.
- Several kinds of vascular plants, notably beans and other legumes, have root nodules containing bacteria or cyanobacteria that fix nitrogen—basically transforming nitrogen gas from the air into a form (ammonia or nitrate) that the plant can use as fertilizer.
- Hornworts, an oddball group that resembles some kinds of liverworts, often have cyanobacteria within chambers in their tissues. As with legume root nodules, they provide nitrogen fertilizer for the plant.
- Sloths have a symbiotic relationship with the algae that grow on their fur, creating a mini-ecosystem in their fur. The sloths provide a nice habitat for the algae; they can consume the algae as a nutritional supplement; and the green algae helps camouflage them.
Lichens are pioneer organisms. Over time, lichens that live on rocks gradually chemically degrade the minerals that compose the rock, slowly breaking down (weathering) the rock. This helps create soils from bare rock. In places where the landscape is almost totally bare rock, lichens begin the process of ecological succession. Once soils are created, mosses, grasses, and other small plants can become established. As soils are built up, these are followed by a succession of communities of larger plants such as shrubs and trees.
Soil-living lichens are important stabilizers, preventing soil from being washed away by rain or blown by the wind. In the desert southwest and in dune habitats, lichens are a big component of biological soil crusts — delicate, dirty-looking, crunchy-textured layers on the surface of the dirt. Few people notice them, but they are very important to the health of the land. Trampling these crusts opens the soil to erosion. It’s one more reason to stay on established trails.
In desert, tundra, and alpine habitats and regions, lichens may be the dominant form of life. It is estimated that 7 percent of Earth’s surface is covered by lichen.
The typical circular growth pattern of a foliose or crustose lichen is also the typical growth pattern of fungi. In the soil of a lawn, for example, the netlike fibers (mycorrhizae) of a fungus begin at a single point and grow outward in all directions, digesting organic material as the fungus grows, dying off in the center, and growing most vigorously at the perimeter. This is why you often see circular “fairy rings” of mushrooms, which arise at the outer edge of the fungus’s underground reach. Ringworm (dermatophytosis) is a fungal infection of the skin. It, too, has the familiar fungal growth pattern, infecting a circular area that often has a raised, reddish rim on the outer perimeter.
Lichens often grow very slowly. They can have extremely long lives. One lichen in the Arctic is more than 8,000 years old, making it the word’s oldest living organism.
Mosses, liverworts, hornworts, and lichens seem rather similar, but these organisms are in very different groups. Mosses, liverworts, and hornworts are small, low plants usually found in damp habitats. Unlike more familiar plants, they lack veinlike structures and do not produce flowers or seeds — instead, they produce spores. Meanwhile, lichens are not plants at all: they are a collection of different fungi that have photosynthetic algae living within their tissues.