2. Michelangelo Avenue

The complexity of lichens and their forms

You will find below the audio and the text of the walk!
Have a good adventure !

So far, we learned that a lichen is a symbiotic form between a fungus and an alga… 

A nice discovery ! But it is not that simple.

The world of fungi is complex, and its forms are variable and sometimes incongruous. Look at the lichens on the tree with a magnifying glass or get closer to the lichen to see them with the naked eye… Another world, isn’t it ? 

 Credit: Xanthoria parietina, photo taken by the author (2020). CC BY-SA 4.0.

The multispecies assemblages 

Researchers tried to recreate a lichen in a lab by assembling the different organisms – algae and fungi. But…. a lichen did not emerge from this combination. Further research has shown that the lichen is probably composed of a third organism, a single-celled yeast of the Basidiomycetes phylum (see Box 1).

Two fungi ? Yes… For years, lichens were thought to be an association between an Ascomycete fungus (see box 1) and an alga. This idea had already created controversy at the time in the well-founded taxonomic science. The cooperation between several organisms caused us to question ideas in evolutionary theories. Indeed, evolutionary theories are based on the natural selection of organisms that were most well-adapted to their environment. So, the idea that a species physically seen as one individual was, in fact, composed of many organisms was just unthinkable. We’ll talk more about these issues and discoveries later in the walk, see here.

The recent discovery of the presence of two fungi was initiated by Toby Spribille (2016) when he and his colleagues studied a yellow lichen (Bryoria tortuosa) that produces a poison called vulpinic acid (which helps to keep snails – including other gastropods- away) and a brown lichen (Bryoria fremontii) that does not produce these toxic substances. These two species had always been considered different… Yet, they are formed from the same species of algae and fungus. 

Then what makes them so different ? By analysing the genes of the lichens, Toby Spribille discovered the presence of genes from basidiomycetes fungi and when these were removed, everything related to the presence of vulpinic acid disappeared. Spribille continued to do more research analysing different types of lichens and found that this Basidiomycete fungus was present everywhere ! Since then, the definition of a lichen has changed.

For more information on Spribille’s research on lichen symbiosis, here is a link to an article.  

Lichens contain many mysteries that we are just beginning to discover thanks to the current genetic technologies. For example, we know that the bacteria that form the thallus certainly have an important function in the functioning of the lichen.


In front of this tree, what do you see ?

A trunk with some lichens. Different shapes. Various colours.

The trunk is the substrate on which the lichen grows. The lichen does not have a negative effect on the tree. Indeed, lichens do not have roots. They have rhizines which are small attachments made up of fungal cells that allow the lichen to attach to the substrate. Rhizines are one of the ways to identify different species of lichens, as they can have different shapes – they can be single or double.

Rhizines of a lichen in the genus Peltigera. Credit: Peltigera praetextata by Ed Uebel on Wikimedia. CC BY-SA 3.0

The forms of lichens

The thallus can have very different shapes. This characteristic is the first important step to differentiate and identify the species.

  • Crustose lichens: these are lichens that form a crust on the substrate and that can hardly be detached from this surface because the rhizines are so deeply embedded. This type is the most common, but also the most difficult to identify because of their various shapes. To take the lichen with you, you would have to take a piece of the substrate: the rhizines of the crustose lichens can go into the substrate, both trunk and stone. The photo below is of a lichen often observed in polluted environments.  The name of the lichen is Lecidella elaechroma.
Credit: Lecidella elaechroma, photo taken by the author (2020). CC BY-SA 4.0.
  • Leprose lichens: these are lichens that have their crust (similar to crustose lichens) entirely formed of small mealy granules. These leprose lichens form a kind of leprosy on their substrate and are characteristic of a habitat sheltered from rain.
Probably Phlyctis argena surrounded by Parmelia sulcata (green grey on the sides) and Evernia prunastri (blurred in front), photo taken by the author (2020). CC BY-SA 4.0.
  • Foliose lichens: these are leaf-shaped lichens. These lichens can be detached from their substrate relatively easily. You can run a fingernail under the lobe to check. The lobe is the outer part of the lichen thallus, shaped like a leaf. The upper cortex of foliose lichens has a different colour than the lower cortex. This difference in colour is due to a different distribution of algae as the lower cortex has no algal layer.
Physconia grisea, photo taken by the author (2020). CC BY-SA 4.0.
  • Fruticose lichens: these are shrubby (shrub-like) lichens. These lichens have radial symmetry, from all their sides the lichen looks the same. Because of the homogeneous distribution of the algal layers, the lower and upper cortexes have the same colour. The first species on the diaporama is a Ramalina farinacea. The second picture on the diaporama is Evernia prunastri. This lichen is often considered as a fruticose lichen, however, despite its apparent radial symmetry, its lower cortex is of a different colour compared to the upper cortex. It is thus a foliose species. Both pictures were taken by the author (2020) and are under a CC BY-SA 4.0 license.

There are still many different lichen forms, but for now you have enough information to differentiate a few types.

Lichens can have many different colours. The colour of the thallus depends on the type of algae that forms the symbiosis with the fungus. If it is a green alga in association with a fungus (for example, lichens of the genus Physconia – see photo above), the thallus will probably be green when wet and blue when dry. If the algae are red, then the lichen’s thallus will most likely be brown (like lichens of the genus Melanelixia or Melanohalea – we will see this species later). It is important to know that lichens change a lot depending on the humidity. If it rains the lichens can be saturated and change their shape, sometimes becoming unrecognizable. 


Can you differentiate between the different forms of lichens on this tree ? Are there different coloured lichens ?

To look at lichens, you need a magnifying glass. If you don’t have a magnifying glass, you can admire the lichens with the pictures below. If you have a magnifying glass but don’t know how to use it, I recommend this video from the British Lichen Society. 

We will introduce a common lichen species. The name of the lichen is Xanthoria parietina. The lichens of this genus (it is like the family, Xanthoria) are yellow as you can see on the specimen on the tree. Sometimes these lichens are blue and have yellow edges. Their characteristic is their yellow or blue colour. Their reproductive systems, which are like small yellow mushrooms (see photo) are also another important feature to identify lichens. We will learn more about these structures, called apothecia, later in the walk (here). Apothecia are structures that allow sexual reproduction and contain spores in sacs called asci – structures that define one of the types of fungi that form the lichen, the Ascomycetes. 

Also, check out the identification guide I have created for urban lichens. It contains the description of 28 urban lichens found on the tree bark.

On the way to our third stop, think about the question below :

Why do you care about lichens ? Why should we care about lichens ?


  • Honegger, R. (1991). Functional aspects of the lichen symbiosis. Annual review of plant biology42(1), 553-578.
  • Spribille, T., Tuovinen, V., Resl, P., Vanderpool, D., Wolinski, H., Aime, M. C., … & McCutcheon, J. P. (2016). Basidiomycete yeasts in the cortex of ascomycete macrolichens. Science353(6298), 488-492.

We’ll meet at the Cinquantenaire Park at the location below 👇🏾👇🏾

If you don’t continue the ride, can you give us feedback on your experience here ? It will help us improve !

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