|
Lichen
Photo: Courtesy of
David Catcheside
-
These vary in appearance, resembling small volcanoes, cups
or jam tarts.
-
Lichens grow on rocks, walls, trees, fences, roofs and on
the ground, as well as on leaves.
-
They are very hardy in natural conditions, living in the
tropics, on mountain tops, in salty seashore splash zones and even polar
regions, where they survive temperatures well below freezing.
Fungus and Algae
- Lichens were previously regarded as individual plants, just like any other member
of the plant kingdom.
- However the nature of lichens is now better understood, and they
are classed separately as an association of a fungus and an alga that form a single
thallus because the two organisms are so intertwined.
- The fungus component of the lichen is known as the mycobiont, and the algal
component is known as physobiont.
- The lichen thallus represents a mutualistic symbiosis from which both organisms
benefit.
- The fungus draws up water and minerals (with
hyphae) that the algae can
use.
- The alga uses its chlorophyll
for photosynthesis to manufacture food.
- It seems
the algal cells are protected from high light intensity.
- Generally the fungal threads create the body of the lichen
with the algal cells living within them.
- This gives the lichen strength and prevents the alga from
drying out.
- The fungi also manufacture a type of acid which serves to
eat away at underlying rock and provide the lichen
with a foothold.
- Lichens therefore erode
rock, very
slowly.
- It is a very important balance that exists between the fungus and the
alga in the lichen thallus. If disturbed in favour of either, disassociation may result
and perhaps so might the destruction of the less-favoured organism.
- This symbiotic relationship can also enable the combination to survive in environments where
neither partner is likely to survive alone.
- They are limited by light, needed by the algae for
photosynthesis, and they also need clean air.
Habitat and Distribution:
- Lichens occur in a variety of habitats from the Arctic to the Antarctic and all
regions between.
- They can be seen on exposed rocks in the desert, solidified larval flows,
the bark of trees, the leaves of plants, and particularly in the tropics.
- Lichens that grow on rocks are called Saxicolous, and are important in initiating
soil formation. This is either by chemical means (secreting lichen compounds that weather
the rocks on which they live) or mechanical means (directly destroying the rock by the
physical action of the lichen thallus).
- Phycobionts have been induced to undergo rapid cell division in laboratories. The
green algal symbionts multiply by vegetative cell division and by the formation of
aplanospores.
Types of Lichen:
- There are three main types of lichens:
- Crustose lichens, Foliose lichens, Fruticose lichens.
- Crustose lichens form flat crusty
plates.
- They grow very slowly outwards around the margin, increasing their radius
by as little as 0.5mm a year.
- Foliose lichens are leafy in appearance, although lobed
or branched structures are not true leaves.
- They often grow near mosses.
- Fruticose lichens are even more finely branched and may
hang down like beards from branches or grow up from the ground like tiny
shrubs.
- These last two are particularly fussy about clean air but grow faster than
the crustose lichens.
Additional Information:
- It has been estimated that they produce a centimetre of
topsoil in two thousand years.
- Many lichens produce small powdery granules called soredia
on their surface.
- When washed or blown away, or carried by insects, they grow
into new lichens.
- Sometimes the fungal partners make their own fruiting
bodies.
- When these burst, the fungal spores are dispersed but
if they do not pair up with the correct algal cells they cannot develop.
- The algal cells, on the other hand, are able to live
independently.
- The presence of sulphur dioxide. produced when fuels such
as coal and oil are burned, and which is responsible for acid rain damage,
kills lichens.
- These plants are therefore good indicators of a clean
environment.
- Since some are more tolerant of pollution than others, the
type of lichen growing can tell us how bad the pollution is.
- They can stop growing if conditions are dry, resuming when
moisture returns and are usually the first organisms, to invade barren
ground, such as new lava flows..
Human Uses:
- Lichens were used traditionally in many countries to make
dyes for cloth.
- The dye litmus is derived from lichens.
- It changes to red when exposed to an acid, and blue when
exposed to an alkali.
- An infusion of the lichen lungwort was used for treating
coughs and asthma.
- Reindeer `moss', the staple diet of reindeer and caribou in
winter (when nothing else grows) is actually a lichen.
- Unfortunately, because lichens tend to accumulate certain
environmental contaminants, after the Chernobyl nuclear power plant accident
in 1986 reindeer `moss' accumulated so much radioactivity that the Saami
people who live in the Arctic were unable to eat their reindeer.
(Courtesy of:
Environmental Protection Agency, Cairns.)
Additional Information:
Damon Ramsey
Lichen is actually a life-form
that is the result of an inter-dependent relationship between two very different
organisms; usually a fungus and an algae, (or a cyanobacteria, a common
occurrence in the lichens lower down in the rainforest strata). While biologists
are still not exactly sure what the true dynamics of the relationship are,
generally speaking the algae usually provides the food through photosynthesis,
while the fungus provides protection from the elements as well as mineral
nutrients. The result is a very successful and diverse group of organisms that
can inhabit habitats all over the world, including some of the harshest
environments on the planet.
-
In the tropical rainforests,
different types inhabit different zones in the rainforest, as can easily be
observed by examining a typical trunk with it's different colours at different
levels (left).
-
In one survey, biologists found over 170 species of lichen on
one single individual Quandong tree in the New Guinea tropical rainforest.
-
They are similarly common in the
Australian rainforests, from the micro lichens that look like paint on the tree
trunks, to the splashes on every leaf (right), to one of the most recognizable
groups of macro-lichens, the 'Old Man's Beards', Usnea spp., which are
bunches of stringy material hanging from the branches.
-
One of the most recognizable
groups of macro-lichens is the Old Man's Beards which are bunches of stringy
material hanging down from branches. Can be seen hanging down from branches
over the water on the Lake Barrine cruise.
Script:
Courtesy of Damon
Ramsey BSc.(Zool)
Biologist Guide
|
