Although the crusty structures that grow on tree trunks appear to be a single organism, lichens are symbiotic associations of a fungus, and either a blue green alga, or a cyanobacterium.  The fungus (called the mycobiont), supplies the structural support, while the many blue green algae (called the photobionts) use photosynthesis to produce food for the fungus.  In turn, the fungus provides the algae with nutrients absorbed from the tree’s surface, water, and surrounding air.  Such a mutually beneficial dependency is referred to as ‘mutualism’.

Depending on their appearance, lichens are divided into three categories, Crustose, Foliose, and Fruticose.  Crustose lichens are attached over their entire surface area to the bark, and have few if any projections.  Foliose lichens, like the one studied here, appear leaf-like, and have lobes which rise above the surface.  Fruticose lichens appear shrubby, and often possess stalks, or thread-like structures that grow from, or hang from the tree.

Since lichens have no roots, and do not need a nearby reservoir of water, they are able to grow in environments where normal plants would perish:  bare rock, walls, tombstones, etc.  In environments where extreme desiccation occurs, lichens can enter a metabolic suspension called cryptobiosis where almost all biochemical activity stops.

It may appear that the lichen growing on the tree-trunk in your front yard is a parasite, but this is not true.  No part of the tree is consumed, and the tree is not poisoned by its presence.

The lichen photographed for this article was growing on the lower trunk of an approximately eighty year old maple tree.  Near its base, large pieces of bark were only loosely attached, and could easily be pulled from the trunk without damaging the tree.  At the time, the weather had been particularly dry for a couple of weeks.  The lichen was a bright yellowish-orange colour, but small areas had a bluish-gray colouration.  The first image in the article, showing a section of bark, reveals both colourations.

A closer view of the lichen growing on the bark’s surface shows its leaf-like, lobed nature.  It is these characteristics that allow the species to be designated as the foliose type.


Two images of another piece of bark show the two colourations clearly.  Note that there is only one species of lichen growing here.  Although the colours are different, the structural characteristics are identical.


For comparison, here are several views of the back surface of the piece of bark shown in the previous two images.  Since sunlight is necessary for the functioning of the blue-green algal component of the lichen, one doesn’t expect to see any evidence of lichen here.



In an image like the one shown below, most of what you are looking at is the fungal component of the lichen, since it typically comprises the majority of the lichen’s volume.  If you are extremely sharp-eyed, you may be able, even at this low magnification, to discern small greenish-yellow areas in the bottom right corner.  These contain the reproductive components of the lichen.  We will see these in more detail later.


The fungal component of a lichen usually has a different appearance than that of a normal fungus growing separately.  In the lichen the fungus surrounds the algal cells, often enclosing them with tissues unique to lichen associations.



At the centre of the image below, the granular structures visible contain both fungal and algal components.  They are referred to as ‘soredia’.


The top surface of a lichen is called its upper cortex.  It is, for the most part, smooth and monochromatic – yellow-orange in this case.  The lichen’s lower surface, called the lower cortex, is usually covered in short hairs which act as a form of root, attaching the structure to the bark.  Unlike real roots however, these hairs do not absorb nutrients from the surface that they are imbedded in.  (They may aid in the absorption of water from the bark by capillary action.)



As an experiment, I placed the base of a piece of bark into water.  After several hours, the bark had absorbed enough water to turn both the blue-gray and orange-yellow appearing lichen bodies, (thalli), a greenish colour.


At the centre of the three images that follow, several round, orange, structures called apothecia, are visible.  These are the reproductive organs of the fungus, and it is in these disks that its spores are produced.  The disks may be raised above the surface as they are in this species.



The higher magnification images that follow show these apothecia more clearly.  To begin, each apothecium edge is complete, and it forms a wall that surrounds a bright orange, relatively flat surface.  As time passes, this wall begins to disintegrate, as does the material it surrounds.  This process reveals the granular looking spore producing bodies, each of which is called an ascus.  The ascus contains many microscopically small spores that are released to be carried out into the environment by the prevailing winds.





Additional images showing these processes can be seen below.  A second type of reproduction can also be seen in the macro-photographs.  Here, portions of the thalli (leaflet-like structures), and apothecia break away from the lichen and fall to the ground, or get caught in the crevices of other section of bark.  This process is called vegetative reproduction.




The following sequence of images, taken with increasing magnification, shows what happens to the appearance of the lichen when its environment becomes moist.  The piece of bark shown was placed in a glass containing about 2.5 cm of water.  Over a period of several hours, the lichen changed colour dramatically as the moisture diffused up through the porous bark.





A similar sequence shows the process happening with a different piece of bark.






Still higher magnifications reveal just how dramatic is the disintegration of both thalli and apothecia.




There are believed to be more than 20 000 species of lichens growing on Earth, from the coldest alpine environments, to the hottest desert ones.  How strange it is to learn then, that these fascinating organisms are some of the least well known forms of life on the planet!



Photographic Equipment

The low magnification, (to 1:1), macro-photographs were taken using a 13 megapixel Canon 5D full frame DSLR, using a Canon EF 180 mm 1:3.5 L Macro lens.

An 10 megapixel Canon 40D DSLR, equipped with a specialized high magnification (1x to 5x) Canon macro lens, the MP-E 65 mm 1:2.8, was used to take the remainder of the images.


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 All comments to the author Brian Johnston are welcomed.