LICHENS
Human beings have an overwhelming compulsion to sort and
name things. This helps us decide where we fit in the universe and where
everything else fits as well. This process is called classification and
currently the trend is to use this as a thread to follow evolutionary change
and development. In order to determine where things fit in the grand scheme of
things they must be very accurately described, then named, and this is called
taxonomy.
The Flora of New Zealand course is where you will learn these
skills and techniques to classify and identify plants.
This is a botany course and will show you how to look at
plants with a view to describing them in a scientific manner. There is a lot of
new terminology to learn and a list of terms and their definitions is added at
the end.
When you collect specimens you will need to use a hand
lens to discover other features to help identify them. A hand lens is an
essential tool in plant identification and will become a constant companion.
The hand lens is held very close to your eye, about 1 - 3
cm away. In order to focus on the specimen you need to hold this really close
to the lens as well, about 1 - 3 cm away.
You will look weird, but it won't show as everyone else will look weird
as well.
Video clip
of handlens
One of the first groups you will look at are what are considered to be 'primitive' plants. This
includes lichens, liverworts and mosses. Here is the first problem! Often
things that are happily accepted in normal usage are, in fact, incorrect in
science.
Lichens are not actually plants. They are the result of a fungus and an alga
(sometimes more than one alga) forming a symbiosis.
What is remarkable is that the 'lichen' that is formed is
entirely different from either the fungus or the alga when these are
free-living, and yet, it is an identifiable and classifiable organism in its
own right. Lichens are described using terms for fungal taxonomy so there are
many terms that are only used for them and not other plants.
Another remarkable thing about lichens, leafy liverworts
and mosses is that they can dry out completely, then
come back to life when re-wet. This is termed poikilohydric.
When you think about higher plants this is impossible for them – pot plants
wilt in a most alarming manner when not watered – so this is a significant
attribute. The physiology that enables them to re-hydrate and continue to metabolise is the subject of a lot of scientific study.
The first step in identifying a lichen
is to look at their basic shape and growth pattern – termed the morphology. It
is likely that you have looked at lichens and never realised
what they were. They are often those small greyish
things that grow on tree trunks.
Plate 1. Lichens are
everywhere. Examples here include a headstone and trees showing lichen on
trunks and branches.
One of the most common species is found on fruit trees as
well as other trees and looks like shall shrubs, about 20mm in height. This
upright growth habit with a holdfast is described as fruticose.
Sometimes lichens hang down (pendulous) but if they have a holdfast then they
too are described as fruticose.
Figure 1. Simple diagram
showing the thallus, apothecia and holdfast in an
upright lichen and the holdfast of a pendulous, fruticose
lichen. Copyright
Plate 2. Fruticose lichen (Usnea sp.) showing holdfast
Other lichens may be a lot larger and look more like
leaves. They have a distinct upper surface and a different lower surface
(termed dorsi-ventral). These are termed foliose.
Photo of Pseudocyphellaria
Plate 3. Different
lichen morphology; foliose (leafy) on the left and crustose
on the right.
Others that you will have noticed are flat and stuck firmly
(closely appressed) to the substrate. Often these can't be removed without
taking the substrate with them, and these are termed crustose.
Plate 4. Crustose
lichen species ‘embedded’ in stone and tree bark.
Collect several specimens to look at and amongst these
you need to have the one in the photo below, Ramalina celastri.
Plate 5. Commonly
found lichen (Ramalina
celastri).
Using the hand lens inspect the lichen. You will see that
it is made up of small flattened pieces. This is termed the thallus and forms the main 'body' of the
lichen.
There are also small bumps all over the thallus. These are reproductive structures, called apothecia (singular = apothecium) and reproductive spores are
produced inside.
Figure 2. A
lichen apothecium in cross-section view to show
spores. Copyright S McCurdy, University of Waikato,
2003.
Cut the thallus across and have
a look at the exposed end with the hand lens. You should see that the thallus has layers. There is a thin green layer on the
outside and a white inner layer. These are the outer layers of green algae and
the inner layer of fungal hyphae.
Where the thallus is divided
into layers it is called heteromerous.
Figure 3. Diagrammatic
cross-section of a heteromerous
lichen showing distinct upper and lower tissue layers. Copyright
Usually the partner that photosynthesises
is outermost, nearer the light, and is called the photobiont. The fungal
partner (mycobiont)
does not photosynthesise and derives its nutrients
from the photobiont. Thus it does not need to be near the light so is
usually towards the middle of the thallus.
Some lichens do not have a thallus
that is divided into layers - termed homoiomerous. They are generally dark, often black, in colour and are gelatinous.
Figure 4. Diagrammatic view
of a cross-section of a homoiomerous
lichen. Note that there are not separate upper and lower layers of tissue as in
heteromerous lichens. Copyright
Plate 6. Lichen
with a cyanobacterium as the photosynthetic partner (Leptogium sp.).
The colour is derived from the photobiont, which is a cyanobacterium rather than a green alga. The cyanobacteria are held in a matrix formed by the mycobiont that dries out to a thin and insignificant
looking film but swells up when wet to 500 times its dry weight.
Identification
We are lucky to have some superb books on identification
of plants. These often use what is called a dichotomous key. This is like a
series of ‘clues’. The term dichotomous means divided into two and in this
context it means that each ‘clue’ is a question with only two possible answers.
An example is ‘has hairy stems’ or ‘does not have hairy stems’. Your answer
determines which step is next. Gradually as you determine the attributes of
your specimen against the key, you reach a final identification.
The Flora of New Zealand Volume 4 is the lichen key. This
is now a little out of date but an updated publication is available and this is
called Lichens of New Zealand, by Malcolm and Galloway (1997). Most of what you will look at in these laboratory
exercises will be covered by the Flora.
There is a comprehensive glossary at the back of each
volume and you will need to refer to this all the time, so put a bookmark in
there now to save yourself some time later. The newer book also has a wonderful
glossary together with drawings of the actual feature so it is often easier to
figure out what they mean. There are also some lovely pictures that can be an
invaluable help.
The first lichen we will look at will be the most
commonly found and widespread species, Ramalina. This is the one pictured in Plate 4. You may never have noticed it before but look
at your local trees, particularly fruit trees and old fence posts.
One of the first steps in identifying lichen is to
determine their gross morphology, i.e. what shape are they? They can be fruticose, foliose,
crustose or squamulose. This is
where the key begins.
Using New Zealand Flora Volume 4 (lichens of
Your next step is to
go to page 496 and read the genus description. Take special note of where it is
most commonly found – this is always a valuable clue.
The second very
common lichen we will look at is called Usnea. This is often called ‘old man’s beard’ as generally
they festoon trees giving them a bearded appearance. There is one species in
this genera can grow to extraordinary lengths - up to several metres
long! These species are also used extensively in natural dying providing many
beautiful colours.
You have seen from
the first foray into the key that the first step is to determine the overall
shape (morphology) of the lichen. In this case you can see that it has a
holdfast and looks like an upside-down tree. Therefore it is fruticose. Go directly to 234. (Do not pass GO and do not
collect $200).
Now go to page 596
in the Flora and check that the description of the genus fits your specimen.
The next specimen
we’ll try and identify is commonly found in fairly humid areas of bush,
particularly manuka/kanuka forests. The first step is
always to determine the morphology – does it have a holdfast? – does it have a
distinct top and bottom surface? – what colour is it dry and wet? – what
other distinguishing features are there? – ripples;
apothecia; grainy areas, holes?
Often, particularly in large foliose species, there are
small ‘hollows’ on the dorsal side. You need to look at these carefully,
firstly with your hand lens and then under a dissecting microscope. There are
two types of hollow. The first one has a distinct rim and the base of the
hollow is smooth. This is caused by the fungal hyphae
lying in a transverse fashion. These are called cyphellae
and the genus that has cyphellae is called Sticta.
Photo cyphellae and
pseudo
The second type of ‘hole’ looks a little different. There
is no rim and the base is slightly roughened. This comes about because the
fungal hyphae are ‘end on’ rather than lying flat.
These are called pseudocyphellae (false cyphellae). The genus most renowned for the presence of pseudocyphellae is called Pseudocyphellaria.
As you have
discovered already the Flora is divided into sections based on the morphology
of the lichen. Your specimen is dorsi-ventral, has no
holdfast, is somewhat leaf-like, green when wet and,
if you look carefully on the back with your handlens
you can see if it has pseudocyphellae or cyphellae. Begin at 162, page lxii..
References
Galloway, D.H. 1985: Flora of
Malcolm, W.M., and