Lichens 101

Introduction to Lichens


Local lichen (Photo: Karen McDonald)

Lichen Basics

During the Winter and early Spring,  it's often hard to find anything that is green in this part of the Eastern US, outside of a few evergreen ferns, scattered cedar trees, and holly trees. However, there is still color in the woods, on the trunks of trees and on rocks, walls, and boulders. It comes from lichens.   Lichens are fascinating, because they look so simple and boring when you first see them, but if you dig just a bit deeper you'll find they're really quite amazing, so let's dig into lichenology.


Reindeer lichens (Photo: Wiki Commons).

Lichens aren't plants, and they aren't just one organism, they are symbiants (things living together) and can be two or three species all cohabitating at once. The main body of a lichen consists of a fungus (mycobiont) and a photosynthetic buddy (photobiont). These photosynthetic partners can be green algae or cyanobacteria (which are really bacteria but used to be called blue-green algae). I always remember the relationship between algae and fungus using the phrase, "Alison algae and Freddie Fungus took a lichen to each other" (you can insert a corny groan here).

The relationship between the fungus and the lichen can be parasitic, where one benefits more than the other partner, or mutualistic everyone wins.   In all lichens the fungus makes a house-like structure for the photosynthetic partner(s), This is called a thallus. The outer part of the thallus is called the cortex while the inside has a photosynthetic layer (where the algae or bacteria live) which  is called the medulla. Lichens aren't plants and they don't have roots like plants do for absorbing nutrients and water. They get what they need by absorbing it from the air (more on this later).

Shapes and Forms

Lichenologists (yes, this is a real title) group lichens into three main forms:

Crustose, which are crusty just like their name, and form a hard crust on the wood or rocks they are on. You can't get them up without damaging the object they're on. Crustose lichens can grow on rocks, bricks, or plants (called enndophlodic). Sometimes they may even look powdery.

j brew flicker of map lichen

Crustose lichen (Photo: J. Brew Flicker Sharing).

Foliose (Foe-li-ose) which sounds like "foliage"  are green and leafy, with two definite leafy sides and lobes. They usually grow parallel to the thing they're growing on (making them low-riders). Many are are umbilicate (yes, it's just like it sounds) and are attached at a central point.

foliose lichen wiki

An example of foliose lichen (Photo: Wiki Commons).

Fruticose (fruity-cose) even though it sounds like they should be shaped like fruit or Fruit Loops,  they are really shrubby and have branches and stalks, or even strands. These guys are the 3D ones you think of hanging from the trees in swamps or in clumps on the ground. If you look at them in cross section they're also usually round.

Fruticose lichen

Fruticose lichen (Photo: Wiki Commons).

So What's The Deal With the Symbiosis?

So why exactly do you have a a living version of "Friends" in lichens? The fungus could survive on its own, decomposing objects and growing, and the algae/bacteria could also survive on their own, but by living together (or in some cases parasitizing), they mutually gain benefits. The fungus makes a protective home for the algae and the algae get nutrients and water provided to them by the fungus that absorbs what it needs from the atmosphere. The algae photosynthesize and create food for the fungus. About 90% of lichens have green algae as their primary photosynthetic partner.


British soldier lichen (Photo: Wiki Commons).

What Good Are Lichens?

First ones in ("put me in coach!"):

Lichens are one of the most severely under-appreciated of wee creatures. For example, crustal lichens are an important part of the rock and mineral cycle. They grow in areas where other things can't. They collect moisture from the air in their bodies (which are attached to the micro-surfaces of the rock), and as the water inside their bodies expands and contracts with freezing and thawing, rock and mineral particles are released from the rock's surface (yes this is really slow). When it rains, these particles are washed out and then enrich the mineral and nutrient content of soil, and over time can allow for colonization of plants. Lichens are what scientists call primary succession organisms. Basically they're the first things "in" in area that can grow, a lot of times in inhospitable conditions.

Air pollution monitors:

Lichens also get most of their nutrients and moisture from the air (remember, no roots). This makes them super sensitive to air pollution. They are sort of like oysters, in that their tissues hold heavy metals and pollutants and don't get rid of them easily. They are especially sensitive to sulfur dioxide, which can lower the lichens internal pH, killing its internal algae (chlorophyll) and ultimately the symbiant. Lichens are often used as the "canary in the coal mine" by scientists studying air pollution. Here's one example of a book used by lichenologists.


Pollution monitoring using lichens from Pelagic Publishing.

Living Clocks:

Some of the oldest creatures on earth are lichens and many are very long-lived. Their slow and consistent growth rate can be used to date events in time, this is called lichenometry (geochronologic dating). For example, if you find a rock-slide of exposed rock. You can measure the diameter of the largest known lichen on a rock, and determine its growth rate to figure out how long the rock has been exposed. This can be useful for dating between 500-10,000 years!


Caribou surrounded by lichens (Photo: Wiki Commons).

Food and Dyes:

Lichens are a staple food for many organisms, especially those that live in rocky arctic areas like the tundra. Caribou need lichens to survive, and lichens can be up to 90% of their winter diet (they paw through snow for them). Reindeer also eat lichens. Flying squirrels, some monkeys, deer, grouse, turkeys, and mountain goats also eat them. In the western US mule deer and elk eat lichens in winter too.

Native Americans have used lichens in medicine and dyes for centuries. Navajo weavers use lichen dyes to make their own yarn colors. This can be done through boiling the lichen or using ammonia to ferment them.  You can read a post all about mycopigments and lichen dies here:   Below is a wonderful picture of lichens and the different dye colors that they produce (from Fibershed).


Lichens are very simple organisms, without much to speak for them on the surface, but they are important early succession organisms, clocks, and air pollution indicators. If you would like to learn more check out the page on "Lichens of North America" or purchase the book, "Lichens of North America" by Brodo, Sharnoff and Sharnoff" (it's a bit pricey but there are cheaper books too).

lichen book

You can also use the online guide to lichens found here:, that can help you identify them by shape and color.


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About Infinite Spider

My name is Karen and I am currently the Education Program Coordinator at the Smithsonian Environmental Research Center, working with students K-gray and doing outdoor science education based on Smithsonian research. I have also been a curriculum developer for the Smithsonian Science Education Center and a contract curriculum writer for the Discovery Channel. In my spare time I love to explore nature topics that I want to know more about, which has lead me to blogging here on "The Infinite Spider" ( I've designed it to be a science and nature blog for every-day people, naturalists, and outdoor educators. Currently I live in Annapolis, MD. If you have questions you can reach me at Let me know if you enjoy the blog or if you would like to see a particular topic covered. Thanks for reading!