What is Lichen? - Definition, Types & Characteristics

Julie Zundel

Julie has taught high school Zoology, Biology, Physical Science and Chem Tech. She has a Bachelor of Science in Biology and a Master of Education.

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Christianlly Cena

Christianlly has taught college physics and facilitated laboratory courses. He has a master's degree in Physics and is pursuing his doctorate study.

While you have probably seen lichen, how much do you know about it? Is it a plant? A fungi? This lesson will define lichen, describe some common shapes and end with some characteristics lichen share. Updated: 12/27/2020

What is Lichen?

There once was a little patch of fungi that took a lichen (pronounced liken) to a beautiful batch of algae. This terrible joke has helped me remember what two organisms make up lichen. While you're probably not rolling on the floor laughing, hopefully it'll help you remember, too. Lichen, if you haven't gathered already, is formed by the mutualistic relationship between a fungus and an alga. Mutualism is a type of relationship where both organisms benefit from being together. So, the algae help the fungi and the fungi help the algae. Before we go on, something to note (in case you're asking yourself what in the world an alga is):

  • Algae is plural and alga is singular.
  • Fungi is plural and fungus is singular.

You're probably wondering how the algae benefits the fungus and vise versa. The fungi provide the home, and the algae provide the food. For example, the fungi hold in moisture, and provide a structure where the algae can grow. In turn, the algae use sunlight (and photosynthesis) to produce food for the fungi.


How does it look when the fungi takes a lichen to the algae? Don't worry, I'll stop with the bad joke soon. There are thousands of types of lichen, but just a handful of lichen shapes. We will focus on three of these: crustose, fruticose and foliose. Let's check out each shape.


The first shape is crustose, which is lichen that sticks closely to whatever it's growing on. Like its name suggests, it is crusty and even forms a crust on its growing surface (can include rocks, trees or even dirt). This crust can be difficult to remove.

Crustose lichen example...makes a crust over the surface in which it grows.
crustose lichen


The next type, fruticose is like a little branching shrub, kind of like a miniature leafless tree. It looks somewhat like a bushy coral and grows on rocks, trees and soils. While fruticose does not sound like shrub, the Latin origin of the word actually translates into bush or shrub, so the name makes sense.

Example of a type of fruticose lichen
fruticose lichen


The next group is called foliose, which has a leafy shape and can be easily removed from its growing surface. They get their name because they look like foliage (plant leaves).

Example of foliose lichen
foliose lichen


We know that lichen is the result of a mutualistic relationship between fungi and algae. And we know a few of the shapes of lichen, but what other characteristics does this diverse group share?

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Additional Activities

Lichen: Crossword Puzzle

In this activity, you'll check your knowledge regarding the types and characteristics of a lichen.


Complete the crossword by filling in a word that fits each of the given clues. For this activity, you'll need a printer to reproduce the following page. With a pencil and an eraser, neatly write your answers in the boxes provided.


2. __________ are a group of living organisms that have mutual associations with algae, bacteria, and plants.

3. A __________ is a complex organism with a plant-like appearance that can conduct photosynthesis.

6. Lichens are extremely sensitive to sulfur dioxide, which is a by-product of atmospheric __________.

8. __________ are a diverse group of aquatic organisms with the ability to produce their food, a process known as photosynthesis.

10. Mutualism is an interaction between two different species in which each organism __________.


1. Due to pollution, the species of lichens in __________ have dwindled in numbers.

4. __________ strongly adheres to any surface, such as on rocks and tree barks, and is well adapted to dry climates.

5. A fruticose lichen is identified by its __________-like shrubby or bushy growth structure.

7. __________ serves as a food source for many animals and are characterized by their flattened, leafy appearance.

9. __________ are mammals that live in the northern regions of Europe, feeding on plants and lichen to survive.

Answer Key

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