Lichens are the group of plants made up of algal and
fungal components, living together in an intimate symbiotic relation ship.
Algal component is known as Phycobiont and
fungal component is known as Mycobiont.
The Term Lichen was first used by Theophrastus to denote
a superficial growth on the bark of olive trees
Based on the type of habitat colonised the
lichens are grouped as
Corticolous- grows
on the bark of the trees Eg: Usnea, Graphis, Parmelia
Saxicolous: Grows on
rocks Eg:
Dermatocarpa, verrucaria
Lingicolous: Grows
on wood Eg: Calicium, Chaenotheca
Terricolous: Grow on
the ground Eg: Lecidea, Cladonia
The two components of
lichens Phycobiont and Mycobiont
The algal components
belongs to Chlorophyceae or Myxophyceae where as the fungal
component belongs to Basidiomycotina or Ascomycotina
Nature of Association
The relation is
considered to be symbiotic in which both algae and fungi get benefitted. It is
believed that algae synthesized the organic food materials sufficient for both
algal and fungal components. Water and minerals are made available to algae by
the fungal component.
Although the association of both the components of
lichens is symbiotic, the fungal component shows predominance over the algal
component, and the latter is a subordinate patner. Such a association is known as Helotisim.
Structure
The plant body of lichens is a thallus, which is
irregular inshape. It is usually grey or greyish green in colour, but some
species are yellow, red, orange or brown in colour.
External Morphology
Lichens are broadly grouped into
three types, on the basis of their morphology
Crustose Lichens
The thallus is thin,
flat and crust like.The thalli are appressed to the substratum forming thin flat crusts.The thalli are partly or
wholly embeded in the substrtum and cannot be
removed from the substratum without injuring the thallus. Some times are fruit bodies
are visible above the surface of the substratum.
Eg; Graphis, Verrucaria, Haematomma and Leconora, Rhizocarpon, Caloplaca
Foliose Lichens
These lichens are flat with leaf like and lobed thallus
They are attached to the substratum with the help of
rhizoid like rhizines
Eg: Parmelia, Physcia, Peltigera, Gyrophora, Cetraria, Collema,Leptogium
Fruticose Lichens
These are brush like having cylindrical or star shaped
branched thallus.
The branches may grow erect or hand from the substratum
The plant body is attached to the substratum with the
help of basal mucilagenous disc.
Eg: Alectonia, Cladonia, Usnea, Ramalina
Internal Structure
Crustose lichens:Anatomically crustose lichens do
not show much differentiation.
In T.S the lichen
thallu
shows cortex , an algal layer and medulla.
The cortex is made up of fungal hyphae, beneath which is
the algal layer composed of algae and fungi in close association.
Below the algal layer is the medulla, made up of a loose
tissue of branching hyphae .
The lower cortex
is not distinguishable
Foliose lichen thallus
On the basis of
distribution of algal cells among the fungal tissue, two types of foliose thalli are recognised.
They are
1) Homoisomerous : The algae are more or less uniformly distributed
through out the thallus. Such forms are only few.
The algae is usually gelatinous and belongs to
cyanobacteria.
The outer protective layer of the thallus is formed by
the fungi.
Eg: Collema,Leptogium
Heteromerous type
The algal cells form a distinct layer within the thallus. Bulk of the
thallus
is made up of fungal hyphae.The thallus is differentiated in to four distinct regions
1. Upper cortex
2. Alagal Layer
3. Medulla
4. Lower Cortex
Upper cortex:
It is the outer most thich and protective zone of the thallus
It is made up of compactly interwoen fungal hypahe
The hyphae are arranged at right angle to the surface of
the thallus
There are usually no intercellular spaces between the
hyphae and if present are filled with gelatinous material.
In some lichens the upper cortex is interrupted at
intervals by pores which are meant for aeriation and are known as breathing ppores.
Eg: Parmelia
Algal Layer:
Beneath the upper cortex , is the algal layer also known
as gonidial layer.
It consists of looseely interwoven fungal hyphae, intermingled with algal cells
of a green algae (usually Chlorella, Pleurococcus, cystococcus) or Myxophyceae (Usually Gloecocapsa, Nostoc, Rivularia)
Medulla
It is the central part of the thallus, comprised
of loosely interwoen fungal hyphae with larger spaces btween them.
The hyphae are thick walled, oriented in different
directions.
Lower Cortex:
Below the medulla is the lower cortex consisting of
densely compacted hypahe.
Some of these hyphae become specialized and extended downward from the lower surface
of the thallus and help in attachment of thallus to the
substratum .
These specialized hyphae are known as rhizines. Rhizines may be
simple or branched.
Fruticose lichens:
The lower cortex does not occur in fruticose lichens
due to their cylindrical structure and medulla forms the central part of the
axis.
The external layer of a lichen thallus, or surface
is termed the cortex.
Beneath it is a layer of fungus-enmeshed algal cells
called the algal layer.
Below the algal layer is a region of cottony, loosely
woven fungal hyphae free from algal cells, the medulla
Reproduction
Lichens reproduce
both by vegetative , asexual and sexual
methods.
Vegetative methods: Common mode of reproduction
1.Fragmentation: Small fragments of thallus are formed
by accidental breaking or due to death and decay of older parts.Each fragment
develops into a new thallus, provided the fragment contains both algal and fungal
components.
2. Soredium:These are small bud like outgrowths which develop on the
upper surface of the thallus. A soredium contains one or few algal cells closely enveloped by a
weft of fungal hyphae.Both algal and fungal components are the same as in the
parent thallus.
Soredia forma
granular layer of greyish white colour on the surface of the thallus.
They detached from the thallus by the impact of wind or rain.some times the soredia develo in an organized manner in special pustule-like areas.
Then they are know as Soralia. Eg: Parmelia, Physcia. The soredia germinate on suitable substratum develop into new thallus
3. Isidium:
These are small coral like outgrowths which develop on
the upper surface of the thallus. Each isidium has an outer cortical layer of fungal hyphae, enclosing
the algal cells.
Isidia vary in their
shape
Rod shaped-Parmelia sexatilis
Coralloid shaped-Peltigera
Cigar shaped-Usnea compasia
Scale like- Collema crispum
Isida are mainly
meant for increasing the phtosynthetic area of the lichen thallus. However when get detached from the thallus , they
behave as reproductive structures.
The isidium germinates under favourable conditions and form new thallus.
Cephalodium:Some lichens are diphycophilous and
show three membered symbiosis(Two algal components+one fungus)
In such lichens one of the algal partner is segregated
into special external or internal swellings called cephalodia.
The cephalodia are small , hard, dark coloured, gall like swellings and
contain alwayas a different algal component than the lichen thallus.
For Ex: in Peltigera aphthosa, the cephalodium contains a blue green alga, but the algal component in
the thallus is a green algae.
Thus one phycobiont confined to cephalodium while the other one is present in the thallus. There is no
organic connection between them.
Eg: Lobaria, Solorina, Peltigera
Cephalodium
Asexual Reproduction
Some lichens develop flask shaped structures called Pycnidia immersed in
the thallus
The opening of the pycnidium is called Ostiole. The interior of the pycnidium is lined by a number of hyphae, which act as
conidiophores or pycnidiophores.
The conidiophores cuts of a chain of small cells called
conidia.
The conidia on being dispersed , germinate under
favorable conditions by sending hyphae in all directions.
These hyphae when come in contact with a suitable algae,
develop into a new lichen body.
Sexual Reproduction
In lichens sexual reproduction is exhibited by the mycobiont .
Phycobiont has no
contribution to the sexual process .
The mode of sexual reproduction in Ascolichens is
similar to that of Ascomycotina while that in Basidiolichens is similar to that of Basidiomycotina.
Sexual reproduction in Ascolichen
The female sex organs are known as carpogonia. A
carpogonium is differentiated into a
basal coiled ascogonium and an elongated multicellular hypha called trichogyne. The
ascogonium remain embeded with in the algal layer of the thallus, where as
the trichogyne projects over the surface of the thallus.
The male sexs organs are flask shaped spermognia. They form spermatia at the
tips of spermatiophores.
Spermatia act as
male gametes. They come out of the spermogonium along with gummy fluid.
They are deposited on the trichogyne of the ascogonium.
On dissolution of the walls between the spermatium and trichogyne, the
nucleus of spermatium migrates into the carpagonium through trichogyne. The male nucleus fuses with the female nucleus.
After fertilization many ascogenous hyphae develop from the
basal portion of ascogonium. This is followed by crozier formation and the
formation of asci and ascospores.
The asci bearing fruit body is called ascomata (Ascocarp). The ascocarp may be
either an apothecium (Parmelia, Anaptyahia) or Perithecium (Dermatocarpon, Verrucaria, Peltigera)
Structure of Apothecium
The apothecia are small, elevated cup shaped or disc shaped fruiting bodies found in the
many lichens.
They vary in colour from reddish, reddish brown , yellow to black depending
upon the species.
The bottom of the cup is lined by hymenium that consists
of number of upright ascii ntersprersed with slender paraphyses containing a reddish oily food-substance.
The wall of apothecium is composed of the vegetative
part of the thallus. If the vegetative part consists of both the algal and
fungal components it is called Lecanorine type Eg: Lecanora, Parmelia, Physcia.
If the apothecium contains only the fungal component, it
is called Lecideine type. Eg: Lecidea, Cladonia, Gyrophora
