6. Luxembourg Place 🌻

Lichens and medicine 

Below you will find the audio and the text of the content of the walk ! 
Have a good adventure !

A little bit of history….

The evolution of the lichen symbiosis is interesting because the co-evolution of the alga and the fungus has taken place several times at different periods and for different species. Lichens represent a success story of several million years. Fossils of lichens have been found and have been dated to approximately 418 million years ago (between 420 and 350 million years ago), in the Devonian period. To give you an idea, humans appeared 200 million years ago, and the big bang happened 13.8 billion years ago.  

If you could go back in time to that Devonian era, you wouldn’t see trees or plants – which would exist but would be very small – but Prototaxites. These organisms, sometimes the size of a house, dominated the landscapes in Africa, Australia, Europe, North America and Asia for 40 million years, 20 times the lifetime of humans. These huge organisms were large lichen-like structures created by the symbiosis between algae and fungi. Despite the Prototaxites being very similar to lichens, there is still much debate on what this organism was. The Prototaxites disappeared, probably due to the increased size of shrubs and trees which would have shaded the organisms and prevented them from using sunlight for photosynthesis. Photo and more info on the uncertainty surrounding these organisms here

Lichens have thus been around for a while and have changed their shapes and forms. 

What makes up a lichen? 

Lichens are full of chemicals. 

To differentiate one species from another, lichenologists – the people who study lichens – apply bleach or caustic soda (among other things) on the thallus which creates a chemical reaction and causes the colour of the lichen thallus to change. Based on this colour, we can distinguish between two lichens that are morphologically similar but structurally different. 

This type of test must be carefully done because it kills the lichens. Often the chemicals are applied to a very small part of the thallus and only to a few lichens found on the substrate – the surface on which the lichen is found.

In the past – and still today – lichens were used for dyeing. Lichens are full of chemicals and mixing them with other products, such as ammonia, creates colours which are interesting for colouring materials such as wool. In Scotland, a place where there was and still is a great deal of wool production, certain species of lichen were used extensively for dyeing, first on a small scale and then with commercial purposes. Three-legged cauldrons were often seen in the Scottish islands where lichens were boiled. The last producers of wool and traditional clothing ceased production in 1997.

The photo below shows the different colours that can be obtained by dyeing wool with lichens. The darker colours are obtained by dipping the yarns in the mixture several times. 

Orcein (pigment obtained from lichen) dyeing of wool from lichen. Credit: W. Carter on Wikimedia. CC0 1.0 Public Domain 

Despite this, lichens, as we have already said, grow very slowly and their harvest must be done properly, considering the time they need to grow.  

Traditional uses of lichens

Lichens have been used and are still used all over the world (Crawford, 2015), especially in traditional medicine. This is due to their second metabolites which are known to be physiologically active and can be used as antibiotics. Other uses of lichens depend on their carbohydrates.

In Europe, the origin of the use of lichens in traditional medicine can be traced back to the 4ᵉ and 3ᵉ century BC, when lichens were recorded by ancient Greeks. The most common uses of lichens in medicine are to treat wounds, skin and digestive problems as well as in obstetrics and gynaecology. The uses depended as much on the country and customs as on the species found in the area. For example, in the 18ᵉ century, the “highlanders” – the inhabitants of the Highlands, in Scotland – mixed the lichen Parmelia saxatilis (pictured below), with tobacco. In Sweden, the same species was used to remove warts and in Bhutan it was used to treat leprosy, uterine bleeding and ulcers in children. Other species such as Evernia prunastri are used to create perfumes. Lichens are also used in pharmacology and cosmetology. Evernia prunastri and Parmelia saxatilis are not rare species and can easily be found – sometimes in cities. Parmelia saxatilis can also produce a red-brown dye. 

Can we eat lichens?

Lichens are not digestible by animals, but some species have developed enzymes that allow them to digest the chemical components of lichens. 

Reindeer that feed on lichens (the Evernia type) have special enzymes called lichenases to digest lichens and their chemicals. Nevertheless, in some customs, lichens are a source of food. The Salish people (North western USA and Canada) consumed lichens. Mixing them with other food sources captured carbohydrates. For example, lichens of the genus Bryoria were rinsed and soaked in water for several hours before being worked by hand to remove the vulpine acids (lethal to some gastropods – the family of snails). The lichens were then cooked in a covered fire pit (with moss and soil) heated over a fire for several days. When the lichens were dug up, they were gelatinous and black.

In our society, it is very rare to eat lichens, but sometimes there are few other options….

During the war in Bosnia-Herzegovina (1992-95), some lichens of the genus Usnea and Evernia were used in porridges and to create flour (Redzic et al., 2010). Finally, in Japan lichens of the genus Umbelicaria are edible if prepared properly. They have been used as a food source during famines.

Lichens have therefore been part of human life for centuries, whether it is to feed us, to keep the ecosystem functioning or to stimulate thought.


Most of the lichens we have seen so far are foliose (leaf-shaped) lichens. Here, for the first time, you can see crustose lichens. They are very small and difficult to see, but if you keep an eye out, you will see them!

As a reminder, crustose lichens are lichens that are incrusted in the bark. It is impossible to remove the lichen without the substrate – the surface on which the lichen is found. The crustose lichen you see on the tree (see pictures below) are from the genus Lecanora. The species is complicated to determine as we would need a microscope to be sure to name it correctly. These specimens are difficult to find on the trunk because they are so small. Can you see them ? The powdery green specimens next to this lichen of the genus Lecanora are unicellular algae – they are not part of a lichen. 

Can you see the foliose lichens we already know such as Xanthoria parietina ? 

Another lichen you can see on the trunk is Phaeophyscia orbicularis. On its thallus there are structures looking like powder or small granules. These are the reproductive organs. We will talk about lichen reproduction at the next stop, here!

Turn around and take a closer look at the bench. Can you see the lichens embedded in the bench? Lichens are really everywhere…. The crustose species on the bench is Candelariella aurella. There are lots of lichens similar to this one (crustose, yellow and found on different surfaces). The main difference is the chemistry or the shape of the spores which can only be seen with a microscope. 

At the next stop, I’ll take you on a dive into the reproduction of lichens and like mushrooms, it’s exciting but not always obvious!

Check out the identification guide I have created for urban lichens. It contains the description of 28 urban lichens found on the tree bark. For each species, there is a description of its sensitivity to pollutants in the city !


  • Information on Prototaxites was first found in: 
    Sheldrake, M. (2020). Entangled life: how fungi make our worlds, change our minds & shape our futures. Random House.
  • Crawford, S. D. (2019). Lichens used in traditional medicine. In Lichen secondary metabolites (pp. 31-97). Springer, Cham.
  • Redzic, S., Barudanovic, S., & Pilipovic, S. (2010). Wild mushrooms and lichens used as human food for survival in war conditions; Podrinje-Zepa Region (Bosnia and Herzegovina, W. Balkan). Human Ecology Review, 175-187.

Let’s meet in Meeûs Square! As always more details on the location on the map 👇🏽

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