What is Lichen? - Definition, Types & Characteristics

Instructor: 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.

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.

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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