The lichens are slow growing, long living organisms,
occurring in wide variety of places ranging from sea shore to Arctic and
Antarctic regions of the earth. Theophrastus (300 B.C.) was the first to use
the word lichen.
Lichens are formed by close association of two different
partners (organisms) one of which is an algal component and the other is fungal
component. The algal component is called phycobiont or
photobiont and it mostly belongs to Chlorophyceae
(green algae) or cyanobacteria (blue green algae). The fungal component is
called mycobiont and it mostly belongs to
Ascomycetes and rarely Basidiomycetes or Deuteromycetes.
The Lichens are distributed in wide variety if habitats.
They grow on tree trunks, decaying logs, and on soil too. They are found in
extreme climatic conditions such as on dry exposed rocks and also at snow
covered Arctic and Antarctic poles. They are sensitive to air pollution and
generally do not grow near the cities. They are thalloid and the thallus is
greenish or bluish green in colour. Some lichens have additional pigments such
as yellow, orange, brown and black.
On the basis of fungal components the lichens are divided
into three catagories (Alexopoulos and Mims 1979) as-
A) Ascolichens : In this
category the fungal partner belngs to Ascomycetes.
B) Basidiolichens : Here the
fungal partner belongs to Basidiomycetes.
C) Deuterolichens : In this
category the fungal partner belongs to Deuteromycetes. These are sterile
lichens producing no spores.
On the basis of external form
(thallus organization), the lichens are of three types as –
A) Cruatose
Lichens : These are thin and flat lichens occurring as crust on the bark
or rock. e.g. Graphis, Lechanora, Haematomma etc.
B) Foliose
Lichens : These are lobed and dorsiventrally flattened, leafy lichens.
They are attached to substratum by hairy rhizoid like structures called
rhizines e.g. Parmelia, Collema and Peltigera.
C ruticose
Lichens : These lichens are commonly called shruby lichen. They have
cylindrical, branched and erect or pendulous thallus which gives them a shrub
like appearance. They remain mucilaginous disc e.g. Cladonia, Usnea and
Alectoria.
The lichens generally reproduce
by vegetative, asexual and sexual methods.
Economic
Importance of Lichens :
1) Lichens
as Food : Several lichens are used as food. The lichens contain a
substance lichenin which is similar to carbohydrate e.g. Lecanora esculenta in
Israel and Umbilicaria esculenta in Japan. Parmelia is used in curry powder as
well as in making chocolates and pestries.
2) Lichens
as Fodder : Lichens form a favourite food for reindeers and cattles.
Species of Cladonia, Citraia, Evernia, Parmelia are used as fodder.
3) Medicinal
Uses : Usnic Acid obtained from the Usnea and Cladonia species is used
as an antibiotic against Gram positive bacteria.
Lobaria,
Citraria species are useful in respiratory disease like T.B., Peltigera is
useful in hydrophobia, Parmelia is useful in epilepsy and Usnea species in
urinary disease.
Some lichens possess
anticarcinogenic property.
4) Industrial
Uses of Lichens :
In Sweden and Russia, lichens are
used for production of alcohol.
Lichens are used in the process
of tanning and dying.
In the litmus paper preparation
which is used as acid-base indicator extract
of species of Rocella and Lasallia are used.
Orcein,
a biological stain is obtained from Orchrolechia androgyna and O. tortaria.
In perfumery, species of Evernia
and Ramalina are the sources of essential
oils which are used in preparation of soaps and other cosmetics.
Ecological
Importance of Lichens : The lichens are the pioneers of vegetation on
rocks. Lichens are the first plants to settle on barren rocks. Lichens being
about weathering of rocks by releasing carbonic and oxalic acids. Weathering of
rocks leads to formation of soil and is called Pedogenesis. After lichens, some bryophytes and then
other higher plants can grow on such oil.