Lichens are symbiotic associations between fungi and algae or cyanobacteria. There are between 18,000 to 20,000 known lichen species that can be found growing on rocks, soil, trees and other surfaces. Lichens have three main growth forms - crustose which are thin and crust-like, foliose which are flat and leaf-like, and fruticose which are erect and branching. Lichens reproduce through both asexual and sexual means and play important ecological roles as pioneers in plant succession and as indicators of environmental pollution.

2. LICHENS
Presented by ADS (I) To Prof: Saba Arshad.

3. Lichens
• Lichens are association consisting of
phycobiontiant and mycobiant.
• There are 18000 to 20000 species of lichens
are known.

4. Occurrence
• On the basis of habitat lichens are of three types
• (I) Saxicolous:Those lichen found in bare rocks
and deserts.
• Terricolous: Those lichens that grow on soil.
• Corticolous: These are lichens that grow on
woods and barks of trees.

5. Components
• A lichen is consist of a two members one is
algae and other is fungi.
• Algal component is called a photobionts.
• Fungal component is called a mycobionts.

6. Photobionts
• About 24 species of algae are involved in
lichen formation
• These members belongs to chlorophyceae and
cyanophyceae e.g
• chlorophyceae= cocomyxa, cladophora
• Cyanophyceae= nostoc seytonema

7. Mycobionts
• In lichens three groups of fungi are found
• Ascomycetes,Basidomycetes and
Deutromycetes e.g.
• correla, Cora, peltizera

8. Morphology of Lichens
• Lichens are thallophytic which vary in color
from grey to greyish green
• On the basis of nature of attachment to
substratum and growth lichens are classified
in to three types
• (i) Crustose
• (ii) Foliose
• (iii) Fruticose

9. Crustose Lichen
• The thallus is thin flat, crust like and closely
appressed to substratum e.g. Graphis.
• They grow on bare rocks, walls and deserts.

10. Foliose Lichen
• The thallus is flat, leaf like lobed and branched
• They attach to substratum with help of
rhizines e.g in physcia

11. Fruticose Lichen
• The thallus is erect, cylindrical or branched
• The thallus remain attach to substratum with help
of densely packed hyphae, called a hold fast e.g in
Usnea

12. Internal structure of Thallus
• On the basis of distribution of algal cells lichens
are of two types
• Homiomerous
• Heteromerous

13. • In these thalli the algal and fungal
components are uniformly distributed
• The fungal hyphae form a thin outer
protective layer .
•

14. Hetromerous
• In these thalli the algal cells form a distinct
layer usually toward the upper side
• The main bulk of thallus is formed by fungal
hyphae
• The following tissues can observed in these
• The upper Cortex
• The Algal layer
• Medulla
• The lower Cortex

15. The upper Cortex
• It is a dense layer formed of fungal hyphae
• It act as a protective and water retaining layer
• The fungal hyphae woven to form a parenchyma
like a tissue called a pseudo parenchyma tissue.

16. The Algal Layer
• Below the upper cortex algal layer is present
• These algal cells are enveloped by hyphae
• This is a photosynthetic zone of lichen

17. Medulla
• It is the central portion consisting of fungal
hyphae
• The hyphae are interwoven to form a
prosenchyma.
• These are conducting cells

18. The Lower cortex
• The lower cortex is made of fungal hyphae
• The hyphae run either perpendicular or
parallel to the surface of thallus
• The rhizines arise from lower cortex

19. Reproduction
• Lichens reproduce by three ways
• (1) Vegetative reproduction
• (2) Asexual reproduction
• (3) Sexual reproduction

20. Vegetative Reproduction
• Lichen reproduce vegetatively by following
methods
• By Fragmentation
• By Isidia
• By Soredia

21. Fragmentation
• In this thalli of lichen break in to small units called a
fragments due to mechanical stress or death of
older portion
• This result in separation of growing areas
• The each fragment grow to form new lichen

22. Isidia
• The small papilla like outgrowth present on upper
surface of thallus called a isidia
• When they detached the thallus they grow to form
new lichen

23. Soredia
• The bud like outgrowths on thalluas are called a
soredia
• The soredia is consist of a fungal and algal cells
surrounded by cortex
• The cortex rupture and soredium exposed as
pustule like areas called sorellia
• These soredia disperssed by wind and grow to form
new lichen

24. Asexual Reproduction
• The component organisms reproduce
undependably by formation of asexual spores
• These include Conidia, Oidia formation etc.

25. Sexual Reproduction
• As lichen thalli have a dual nature, therefor
reproduction of both partners discussed here
• Reproduction of Mycobionts
• Reproduction of Ascolichens

26. Reproduction of Mycobionts
• Fungal partner of lichen form a there
characteristic spores
• These spores release from lichen thallus and
grow to form new mycelium

27. Reproduction of Ascolichens
• The ascolichen reproduce by ascospore produced
in asci which are arranged in ascocarp

28. Importance of Lichen
• Lichens are pioneers of succession they establish a
terrestrial ecosystem
• These are pollution indicators
• They cause a forest fire due to dry nature
• Many lichens are rich in carbohydrates and they
use as a food
• Many commercial products like perfumes,litmus
alcohol,beers and soaps are manufacture by
lichens
• These are used to prepapare medicines