A close-up view of a foliose lichen, Physciaceae family.
Close-up View of a
by Brian Johnston (Canada)
Although the crusty structures that grow
on tree trunks appear to be a single organism, lichens are
associations of a fungus, and either a blue green alga, or a
cyanobacterium. The fungus (called the mycobiont),
structural support, while the many blue green algae (called the
photobionts) use photosynthesis to produce food for the
In turn, the fungus provides the algae with nutrients absorbed
tree’s surface, water, and surrounding air. Such a
beneficial dependency is referred to as ‘mutualism’.
Depending on their appearance,
lichens are divided into three categories, Crustose, Foliose,
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,
lobes which rise above the surface. Fruticose lichens
shrubby, and often possess stalks, or thread-like structures
from, or hang from the tree.
Since lichens have no roots,
not need a nearby reservoir of water, they are able to grow in
environments where normal plants would perish: bare rock,
tombstones, etc. In environments where extreme desiccation
occurs, lichens can enter a metabolic suspension called
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
not true. No part of the tree is consumed, and the tree is
poisoned by its presence.
The lichen photographed for
article was growing on the lower trunk of an approximately
old maple tree. Near its base, large pieces of bark were
loosely attached, and could easily be pulled from the trunk
damaging the tree. At the time, the weather had been
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
bark, reveals both colourations.
A closer view of the lichen
on the bark’s surface shows its leaf-like, lobed nature.
these characteristics that allow the species to be designated as
Two images of another piece of
show the two colourations clearly. Note that there is only
species of lichen growing here. Although the colours are
different, the structural characteristics are identical.
For comparison, here are
views of the back surface of the piece of bark shown in the
two images. Since sunlight is necessary for the
the blue-green algal component of the lichen, one doesn’t expect
any evidence of lichen here.
In an image like the one shown
below, most of what you are looking at is the fungal component
lichen, since it typically comprises the majority of the
volume. If you are extremely sharp-eyed, you may be able,
this low magnification, to discern small greenish-yellow areas
bottom right corner. These contain the reproductive
the lichen. We will see these in more detail later.
The fungal component of a
usually has a different appearance than that of a normal fungus
separately. In the lichen the fungus surrounds the algal
often enclosing them with tissues unique to lichen associations.
At the centre of the image
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
monochromatic – yellow-orange in this case. The lichen’s
surface, called the lower cortex, is usually covered in short
which act as a form of root, attaching the structure to the
Unlike real roots however, these hairs do not absorb nutrients
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
of a piece of bark into water. After several hours, the
absorbed enough water to turn both the blue-gray and
appearing lichen bodies, (thalli), a greenish colour.
At the centre of the three
that follow, several round, orange, structures called apothecia,
visible. These are the reproductive organs of the fungus,
is in these disks that its spores are produced. The disks
raised above the surface as they are in this species.
The higher magnification
that follow show these apothecia more clearly. To begin,
apothecium edge is complete, and it forms a wall that surrounds
bright orange, relatively flat surface. As time passes,
begins to disintegrate, as does the material it surrounds.
process reveals the granular looking spore producing bodies,
which is called an ascus. The ascus contains many
small spores that are released to be carried out into the
by the prevailing winds.
Additional images showing
processes can be seen below. A second type of reproduction
also be seen in the macro-photographs. Here, portions of
thalli (leaflet-like structures), and apothecia break away from
lichen and fall to the ground, or get caught in the crevices of
section of bark. This process is called vegetative
The following sequence of
taken with increasing magnification, shows what happens to the
appearance of the lichen when its environment becomes
piece of bark shown was placed in a glass containing about 2.5
water. Over a period of several hours, the lichen changed
dramatically as the moisture diffused up through the porous
A similar sequence shows the
process happening with a different piece of bark.
Still higher magnifications
just how dramatic is the disintegration of both thalli and
There are believed to be more
20 000 species of lichens growing on Earth, from the coldest
environments, to the hottest desert ones. How strange it
learn then, that these fascinating organisms are some of the
known forms of life on the planet!
The low magnification, (to
macro-photographs were taken using a 13 megapixel Canon 5D full
DSLR, using a Canon EF 180 mm 1:3.5 L Macro lens.
An 10 megapixel Canon 40D
equipped with a specialized high magnification (1x to 5x) Canon
lens, the MP-E 65 mm 1:2.8, was used to take the remainder of
A Flower Garden of
A complete graphical index of
of my flower articles can be found here.
The Colourful World
A complete graphical index of
of my crystal articles can be found here.
Microscopy UK or their contributors.
Published in the
September 2012 edition of Micscape.
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