A general account on lichens

1. LICHENS
By
Jayalakshmi P.S.
Dept of Botany,
Sree Sankara College, Kalady

2. LICHENS
Lichens are the most important symbiotic living organisms /
plants on the earth.
Theophrastus (371-284 BC), the Father of Botany, to
denote the superficial growth on the tree bark, coined the
term 'LICHEN'.
Lichenology is a branch of Botany that deals with the study of
Lichens. Acharius (1798), a Swedish lichenologist, is the
Father of Lichenology

3. Lichens are one of the largest group of plants consisting
about 500 genera and 18000 species included in 9 orders.
A lichen can be defined as "the dual organism with symbiotic
(mutualistic) association of a fungus with an alga".

4. Mycobiont: is the fungal partner. It is dominant,
colourless, protective and reproductive partner.
Phycobiont: Is the algal partner. It is the inhabitant, recessive,
coloured autotrophic partner.

5. In a typical lichen, the fungal partner (the Mycoblont) is the
most abundant component, which accounts for 90-95% of the
total biomass, whereas the algal partner( Phycobiont) accounts
for 5-10%.
The fungal hyphae form a close network resembling a tissue
like mass with the algal cells often embedded in it.

6. Although, lichens are the symbiotic associations, the
relationship between a fungus and an alga is like a Master
and Slave. It is known as "HELOTISM". Fungus is the
Master and alga is the Slave.

7. Habit and Habitat:
Occurrence: Lichens are one of the most widely distributed
plants ranging from the polar to the deserts, and aquatic
bodies.
Lichens are the dominant vegetation both in the Arctic and
Antarctic regions.
They will grow as lithophytes, xerophytes, epiphytes,
mesophytes, and hydrophytes.

8. Some of these are Cladonia rangiferina (Reindeer moss),
Peltigera (Aquatic lichen), Gyrophora (White lichen), Graphis,
Parmalia, Haematomma etc.
They show great variation In their size, shape, structure,
morphology, growth rate and ecology.

9. Classification of Lichens
Lichens are classified basedon their habitat (distribution),
external morphology, Internal anatomy, and the partners
Involved (fungal and algal) are classified into several types.

10. Based on Habitat
a) Saxicoles (which grow on the rocks)
Eg. Porina, Xanthoria
b) Corticoles (which grow on bark)
Eg. Parmelia, Usnea
c) Terricoles (which grows on soil)
Eg. Cladonia
Porina
Parmelia
Cladonia

11. Based on Habit/Morphology
There are three types of lichens viz.,
a)Crustose lichens: These are the most
common type with thin, flat and crust-like
thalloid plants, firmly attached to the
substratum. Ex: Graphis, Haematomma.
It is very difficult to dislodge it from the
substratum.
The pioneers of the xerosere type of plant
succession.
Haematomma
Graphis

12. b)Foliose lichens: These are flatten, broad, much lobed and leaf like or
scale like thallus loosely spread over the substratum.
They are attached to the substratum through rhizines. Ex: Parmelia,
Xanthoria, Peltigera.
Rhizines may be branched or unbranched and serve as the organs for
anchorage and absorption.
Peltigera. Parmelia

13. c)Fruticose lichens: They are the most conspicuous type.
Thallus is slender, much branched and shrubby. Branches may
be cylindrical to ribbon like.
They attach to substratum using single basal disc and grows
erector hangs down from the substratum.
Ex: Cladonia, Usnea
Cladonia Usnea

14. d) Leprose lichens: They have no organised structure and comprise a
mass granules of fungal hyphae and algal cells, which gives a
powdery appearance.
no fruiting bodies has been observed.
Eg. Lepraria, Chrysothrix
Chrysothrix

15. e) Squamulose lichens: small and leafy
with loose attachments to the substrate
and are usually considered to be a special
type of crustose lichen.
Here their thallus is lifted off from the
substrate to form ‘squamulae’ (small
scales).
Eg: Vahliella, Pannaria
f) Gelatinous lichens: the fungus grows
inside the gelatinous membranes of algal
cells (cyanobacteria) and ramifies there.
Eg: Collema
Pannaria
Collema

16. Based on Algal partner
Generally, either green alga or the blue-green alga will be
the algal partner of lichens. Hence, lichens are
a) Chlorophycophyllous lichens
b) Cyanophycophyllous lichens.

17. In Chlorophycophyllous lichens, the
algal partner Is green alga. 80% of
lichens contain green alga as
phycobiont. Trebouxia (unicellular)
is the most abundant partner
followed by Chlorella.

18. In Cyanophycophyllous lichens, the
algal partner belongs to BGA or
cyanobacteria which accounts for
~8%. Some of these are Nostoc,
Stigonema, Rivularia, and Gleocapsa.
However, Nostoc is the most common
partner.

19. Based on Fungal partner
Lichens can be classified into three types viz.,
a) Ascolichens
b) Basidiolichens
c) Deuterolichens

20. a)Ascolichens: The fungal partner
belongs to the class Ascomycotina and
algal partner is a blue-green algae.
These are the abundant type. Over
98% lichens are Ascolichens.
Eg: Parmelia, Graphis.
Apothecium and perithecia are the
common fruiting bodies.
They may be Gymnocarpae
(Apothecium) or Pyrenocarpae
(Perithecium).

21. b) Basidiolichens: The fungal partner
belongs to the class Basidiomycotina
and algal partner is a blue-green
algae.
They are very less in number (<1%).
Eg. Multiclavula and Dictyonema.
They produce pycnidial cups.

22. c) Deuterolichens: The fungal partner
belongs to the class
Deuteromycotina.
These lichens are sexually sterile
and does not produce sexual sopres.
Eg. Cystocoleus,
Lepraria and Lichenothrix.

23. Based on internal structure
Lichens may be of
a) Homoiomerous/Homomerous
b) Heteromerous
In Homoiomerous lichens, the algae and
fungal components are loosely arranged
and more or less uniformly distributed.
Several crustose lichens belongs to this
category.
In Heteromerous lichens the algae and
fungal components are thick and
compact and arranged in distinct cellular
layers.

24. Structure of crustose lichen
Homomerous where algal and
fungal components are irregularly
distributed in a gelatinous matrix
without differentiation into distinct
layers.

25. Structure of foliose lichen
Heteromerous since algae and fungal components are
organised into distinct layers such
Upper cortex- vertical and closely packed fungal
hyphae.
Algal layer-algae intermixed with fungal hyphae.
Medulla- thick walled and loosely interwoven fungal
hyphae.
Lower cortex- dark-coloured compact and closely
packed fungal hyphae arranged parallel or
perpendicular to main axis of thallus

26. Structure of fruticose lichen
Thallus is radially symmetrical.
3 distinct zones present.
Pseudocorex- thick walled and
compactly arranged fungal hyphae.
Algal layer- photosynthetic. algae
and fungal hyphae.
Medulla- formed by compact fungal
hyphae lying parallel to the main axis
of the thallus.

27. Peculiar vegetative structures in Lichen
a) Breathing pores: localized depressions or
cone-shaped areas on upper surface of
foliose lichens for gaseous exchange btw
thallus and atmosphere.
Here he compact cortical hyphae are
replaced by loosely interwoven hyphae to
facilitate gas exchange.

28. b) Cyphella: small, circular pits or depression in the lower cortex of some
foliose lichen.
Have definite form with apical rims.
Function as aerating organs and appear like cup-like white spots on thallus.
Hyphae from medulla may extend into thee depressions and provides
surfaces for aeration or gas exchange.
Pseudocyphellae- Cyphella like cavities without definite form and apical rim.

29. c) Cephalodium: small, hard and dark, gall-
like or wart-like internal or external
swellings on the upper surface of the
thallus.
Formed of algal cells enclosed by fungal
hyphae.
Lichens that have both green algal and
cyanobacterial symbionts restrict the
cyanobacteria to cephalodia.
They retain moisture and helps in nitrogen
fixation.

30. d) Isidia: stalked, branched or unbranched
papillate outgrowths from the upper surface of
the thallus.
Consist of algal and fungal components covered
by definite cortex.
Primary function is to increase photosynthetic
efficiency by increasing surface area.
They may also detach from parent thallus and
grow into new thallus.

31. e) Soredia: minute, rounded and bud like outgrowths or bodies
on surface of thallus or in pustule lie areas called soralia.
They are seen in the form of a greyish white or greyish
green powder in extensive patches.
It consists of a small group of algal cells surrounded by
fungal hyphae.
Soredia detach from thallus and give rise to new thallus.

32. REPRODUCTION IN LICHENS:
► Both vegetative/asexual reproduction and sexual reproduction are present
in Lichens.
• The Vegetative Reproduction: takes place very frequently either by the
fragmentation process or by the formation of different types of
diaspores (having both alga and fungus) such as Soredia and lsidia.
• In the vegetative propagation, both the partners are involved and develop into
a new daughter Lichen.
• Fragmentation is the common mode of vegetation where the fragments are
arise either due to the progressive death and decay of older portion or due to
the Involvement physical factor.

33. Asexual reproduction by Oidia, conidia and pycniospores.
Oidia: small and thin walled bodies formed from fungal hyphae.
Conidia: in several lichens, fungal partner forms conidia on
conidiophores. They get released and when comes in contact
with suitable algae they form new lichens.
Pycniospores: conidia formed in flask shaped structures called
pycnidia remain embedded in the thallus.
Pycniospores get liberated and germinate into fungal hyphae.
when comes in contact with suitable algae they form new lichens.

34. Sexual Reproduction in Lichens
Sexual reproduction occurs in many Lichens and in this, only
the fungal partner (the mycobiont ) is involved. The role of algal
partner is passive or absent.
The mode of sexual reproduction in lichens maybe spermatogamy.
Ascolichens produce the sex organs namely Spermogonium
(male) and Carpogonium (female). After fertilization, they
produce the special fruiting bodies such as Apothecium,
Perilhecium and some times Puesdothecia. Eg:Parmelia

35. While, the Basidiolichens, produce basidiospores followed by the
formation of the Pycnidia, as the sexual apparatus.
After spermatogamy, the sexual fruiting body develops as
Basidiocarp like in Agaricus. Ex: Cordelia.

36. Sexual reproduction In Ascolichens
• Since majority of the lichens are with Ascomycetous
fungal partner, it will reproduce sexually by the formation of
two different types of sex organs.
• The male reproductive structure is Spermogonium and the
female reproductive sex organ is Carpogonium.

37. •The spermogonium is a flask shaped structure
that usually present upper surface of the
thallus. It is with a pore called ostiole.
•Spermogonial cavity is filled with fertile and
sterile hyphae.
•Fertile hyphae - spermatiophores or
pycniophores.
•They produce spermatia or pycniospores.
•Spermatia are unicellular, uninucleate non
motile colourles male gamates produced in alage
numbers.. They area released through ostiole
and disseminated by wind.

38. The carpogonium is a special cellular
filament.
It consists of two portions, the lower
coiled portion usually called
Ascogonium and it is multi-cellular and
monokaryotic.
It lies deep in the medullary region of the
thallus.
Helically coiled basal portion is called
oogonium and has the egg nucleus.
Erect and extended portion is called
trichogyne.

39. • Spermatia is wind dispersed.
• The trichogyne receives the spermatia (the male
gametes) which in turn penetrates and finally
reaches the carpogonium and subject to
fertilization.
• Plasmogamy follows resulting in a dikaryon. Male
nucleus gradually passes down to oogonium.
• Several septate ascogenous hyphae are developed
and the ultimate cells of each ascogenous hypha
develops into ascus.

40. •karyogamy occurs in this
ascus mother cell .
•the diploid nucleus undergo
meiosis and produce haploid
ascospores.
•Simultaneously, sterile hyphae
are formed around ascogonium,
ascogenous hyphae and
developing asci resulting in
fruiting body – Ascocarp.

41. Fruiting body or ascocarp
Two types of fruiting bodies: Apothecium and Perithecium
Ascolichens classified as
Gymnocarpae: apothecium cup shaped or bowl shaped. Eg: Physcia
Pyrenocarpae: perithecium pitcher shaped or flask shaped.
Eg.Acrocordia

42. Apothecium
Wall of ascospores formed from hyphae alone or both algae and
fungal hyphae.
Central part is formed od asci and sterile tissue called hamathecium
consists of 4 different elements.
Paraphyses: arise from base of ascocarp and grow vertically upward.
Periphysis: arise from side of the ostiole canal and protrude out of the
ostiole.
Paraphysoids: arise by stretching of the tissue of ascocarp.
Periphysoids arise from the roof of ascocarp and grow vertically
downward.

43. Internally ascocarp has three zones:
Thecium: central zone formed by asci and hamathecium.
Epithecium: terminal zone formed by the tips of paraphyses projecting
beyond asci.
Hypothecium: basal zone formed by loosely packed fungal hyphae.

44. •Each ascus usually contains eight ascospores (bi-celled).
The liberated ascospores liberated, if come in contact with suitable
alga, produce the new daughter thallus. Otherwise they perish.

45. Perithecium
This is another sexual fruiting body of the
Ascolichens.
It is much smaller than the apothecium and
flask shape structure and appears as a small
dot on the surface of the thallus.
It is embedded in the elevations of the thallus
on the upper side
A dark coloured wall bound it and the cavity
is lined with asci.
The paraphyses are few in number or totally
absent.
It opens through a small pore called an
Ostiole.