2. What are lichens ?
A lichen is a composite organism that
arises from algae or cyanobacteria living
among filaments of multiple fungi in a
symbiotic relationship. The
combined lichen has properties different
from those of its component organisms.
4. Importance of lichen
lichens enable algae to live all over
the world in many different climates,
they also provide a means to convert
carbon dioxide in the atmosphere
through photosynthesis into oxygen,
which we all need to survive.
Lichens can provide us with valuable
information about the environment
around us.
5. Physcia
Nitrogen-
loving Physcia thrives in
polluted air environments,
so expect to see
these lichens in big cities
and near major roads.
7. Structure and life history
It is a foliose lichen.
Fungal partner belongs to
ascomycetes that’s why it is also
called as ascolichen.
It is found growing on rocks, old
wood, trees mostly.
Rarely found on plain grounds.
8. Ascolichen (physcia)
ascolichens (lichens in which
the dominant mycobiont is an
ascomycete) form fruits called
ascocarps that are similar to
those of free-living
ascomycetes, except that the
mycobiont's fruits are capable
of producing spores for a
longer period of time.
9. Plant body
It is leaf like ( foliose)
Rizhines are present for
attachment with substratum
Fungal partner is ascomycete and
green alga ( Trebouxia) is an algal
partner
Trebouxia --------- (green
alga)
11. Upper cortex and algal layer
Upper cortex contains more or less
vertical hyphae, that round off in some
species to form pseudo parenchyma. It
may or may not contain intercellular
spaces. These spaces when present are
filled with gelatinous material.
Algal layer : beneath the upper cortex,
there is intermingles layer of algal partner
with hyphae. This is called algal layer.
12. Medulla and lower cortex
Medulla : it contains layer of very
loose interwoven hyphae. This
layer is present below the algal
layer.
Lower cortex: in this region, the
hyphae is present parallel to the
lower surface of thallus. Below
this layer, rhizines are present
that are involved in fixing of
thallus with substratum
13. Life cycle : reproduction
It involves following methods
Vegetative reproduction
Asexual reproduction
sexual reproduction
14. a) :Vegetative reproduction
Fragmentation : thallus is broken into smaller
pieces , due to death of older cells or
accidental injury. Each fragment develops into
new lichen body.
Soredia : these are produced as white pustules
on the upper surface of thallus. Each soridium
contains algal cell enveloped in fungal hyphae.
It develops into new thallus upon detachment.
16. b) Asexual reproduction
Pynidia and pynidiospore:
Flask shaped asexual reproductive
bodies , the pycnidia are present in
embedded form in Physcia thallus .
Small rounded , unicellular spores
called as pycnidiospores are
produced in pycnidia.
17. c) Sexual reproduction
Apothecium Sexual reproduction take place
by the formation of cup shaped fruiting body
called apothecium .inside these , septate
ascopores are produced.
18. c) Sexual reproduction
Ascogonoum :
The female reproductive organ is an
ascogonium (carpogonium)
It develops from hypha deep in the algal layer.
It is a long multicellular hypha, the coiled base
of it is the oogonium and the straight portion
above it the trichogyne.
The trichogyne in some species projects
beyond thallus.
More than one ascogonia may develop at a
point where an apothecium is later formed but
only one becomes fertile.
19. c) Sexual reproduction
Spermogonium :
The male reproductive body is
spermagonium (pycnium).
It is flask-shaped cavity immersed in
the thallus and opens to the exterior by
small ostiole.
The fertile hyphae lining the inner
surface of the spermagonium produce
large number of small non-motile
gametes spermatia.
The spermatia are functional male
gametes.
20. fertilization
The spermatia are carried to the
receptive hyphae of ascogonium the
wall of contact between two dissloves
The male nucleus migrates into the
oogonium .
The male and female nucleus arrange
them in pairs at the periphery of the
wall.
21. Ascus formation
From the wall of oogonium, the development of
ascogenous hyphae starts.
The diploid nucleus migrates and starts division and
moves towards these ascogenous hyphae that becomes
septate.
The asci develop from the terminal cell of the ascogenous
hyphae.
The karyogamy takes place in ascus mother cell.
8 ascospores are formed.
These spores arrange themselves in elongated asci in layer
that is called hymenium.
The sterile hyphae develops and are called paraphyses.
In the mean time the hyphae present below the
spermagonium develops and form a wall of the fruiting
body.
The fruiting body is called apothecium.
22. Reproduction in algal partner
These are produce by aplanospores or the division of
ordinary cell.
Synthesis of thallus
The ascospores upon release from asci, germinate to form
hyphae.
They grow in all directions and when come in contact with
suitable alga, these produce additional branches and enfold
the alga, an in this way the new lichen thallus develops
