LICHENS
Lichens are symbiotic associations between algae and fungi.
The algal component (autotrophic) is called phycobiont
and fungal component (heterotrophic) is mycobiont.
Lichens are found in habitat like walls, window panes,
barren rocks, cooled volcanic lava, tree bark, soil and
aquatic habitats.
Different forms of lichens are formed based on their habitat
such as saxicolous, lignicolous, corticolous, terricolous,
marine and freshwater.
Morph010kv
Lichens vary in colour like orange, brown, dark brown,
grey, yellowish green, etc.
Based on their morphology, lichens can be of various types
(i) Crustose These lichens are crust-like, closely
attached to the substratum, e.g., Graphis, Lecanora.
(ii) Leprose These are like minute scales attached
superficially to the substratum, e.g., Lepraria.
(iii) Foliose These are like twisted and crinkled leaf,
e.g., Parmelia, Physcia.
(iv) Fruticose These are branched, erect with bushy
appearance, e.g., Evernia, Ramalina, Usnea.
(v) Filamentous These lichens consist of chains of algal
cells wrapped around by fungal hyphae,
e.g., Racodium.
Internal Structure
The maior part of lichen body constitutes fungus and only 50/0
part is algal cotnponent. The body is divided as upper cortex,
algal zone (gonidial layer), middle medulla, lower cortex and
rhizines.
Only the algal zone contains photosynthetic partner. In about
98% lichens, the fungal partner belongs to Ascomycetes. In
the rest, the fungal partner belongs to Basidiomycetes and
Deuteromycetes.
Symbiotic Relationship
The algal partner or phycobiont plays following roles
(i) Nitrogen-fixation, if cyanobacterial type.
(ii) Photosynthesis
(iii) Provides vitamins and other growth substances.
The fungal partner or mycobiont is involved in
(i) Outer covering for protection.
(ii) Attachment to substratum.
(iii) Protection against harmful radiations.
Ou the basis of above mentioned roles of algal and fungal partner,
lichens show symbiotic relationship, i.e., symbiosis or mutualism.
Sometimes, mycobiont sends haustoria into algal part. It
prevents alga to secrete pectic substances or induces alga to
secrete nutrients. In this case, fungus is considered to be a
controlled parasite over the alga. This phenomenon of
controlled parasitism is called helotism.

Reproduction
Lichens may reproduce vegetatively or sexually. They
reproduce vegetatively by the following ways such as
fragmentation, death and decay, etc.
Lichens reproduce sexually by following ways
(i) Formation of ascomycetous fruiting body.
(ii) Formation of basidiocarp.
Sexual reproduction is not considered to be a common means
of reproduction to form a new lichen.
Importance of Lichens
Lichens have widespread importance for the mankind. Some
of the important uses of lichens are as follows
(i) Lichens contain a complex carbohydrate called
lichenin. It is used as food for many animals.
(ii) Many lichens are useful in producing medicines.
Usnea and Cladonia (antibiotics and usnic acid).
Cetraria islandica (laxative mucilage).
Parmelia (curing epilepsy).
Lobaria (treating lung diseases).
(iii) Lichens like Roccella tinctoria, Parmelia
omphalodes are used in making different types of
dyes. Litmus was also previously obtained from
Roccella montaingne.
(iv) Extracts of Lobaria pulmonaria and Cetraria
islandica are used in tanning leather.
(v) Some lichens like Usnea are used in brewing industry.
(vi) Lichens are best indicators of pollution. They do
not grow in polluted environments.
(vii) Lichens help in understanding the process cf
biological succession by the ecologists.