Lichens: habitat, types and structure

  • Lichen is an intimate association of algae and fungi where two organisms live in close association with each other.
  • The association is so close that they appear to be a single organism.
  • The algal partner is called phycobiont, whereas the fungal partner is called mycobiont.
  • The fungal component forms the bulk of the lichen thallus.
  • The fungal hyphae form a close network with the algal cells often embedded in it.
  • The fungal partner is for protection against dessication, fog, excessive light intensity and fluctuations of temperature.

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  • It is also related with fixation and absorption of water and minerals to the plant body.
  • The algal partner prepares food material for both due to the presence of chlorophyll.
  • This type of association is called symbiotic relationship.
  • In lichen, both of them are benefitted from the association and neither can survive alone in the environment.
  • The reproductive organs however are entirely fungal in character.
  • The algal components of lichens are usually the members of Cyanobacteria (blue green algae) and sometimes Chlorophyceae (green algae).
  • The most common blue green algae are Nostoc, Gleocapsa, Rivularia, etc. and the most common green algae are Protococccus, Trentepohlia, Cladophora, etc.
  • The fungal components are usually the member of Ascomyctetes , sometimes of Basidiomycetes and rarely a Deuteromycetes.

Habit and habitat

  • They are world-wide in distribution growing in diverse habitats.
  • They are mostly confined to the tropics and subtropics where there is plenty of moisture.
  • They usually grow on tree trunks, walls, rocks, roofs of houses, branches of trees, etc.
  • They are usually xerophytic in nature and can withstand extremes of heat, cold and drought.
  • Some species are able to survive in such conditions where no other plants could survive.
  • They grow on high mountain elevation, barren hard rocks and in extremely cold alpine and arctic regions.

Types of Lichens

  • On the basis of their general habitat of growth, form and manner of attachment to the substratum, usually three basic forms of lichens are recognized. They are:

1.Crustose lichen

  • It is a crust like, thin, flat hard layer of thallus.
  • The thallus remains closely attached to the substratum by its whole lower surface.
  • The upper surface of the thallus bears a number of small cup-shaped fruiting bodies, the apothecia.
  • For examples: Graphis, Lecanora, Lecidia, Rhizocarpon, Strigula, etc.

Morphology of Lichens

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2.Foliose lichen

  • The thallus is broad, many lobed, leaf-like which remains loosely attached to the substratum by rhizinae.
  • The rhizinae act as an anchorage and absorptive organs.
  • The upper surface possesses several fruiting bodies the apothecia.
  • Apothecium is formed only by fungal components.
  • For examples: Physcia, Parmelia, Peltigera, Collema, etc.

3.Fruticose lichen

  • It is a most complex type of lichen where the thallus shows much branching.
  • The tips of branches bear fruiting bodies the apothecia.
  • The thallus remains attached to the substratum only at the base by a flattened disc.
  • The disc is composed of strands of densely packed hyphae.
  • The thallus may be erect or pendant (hanging from trees).
  • For examples: Usnea, Cladonia, Ramalina, etc.

Structure (External)

  • The plant body of lichens is called thallus which is irregular in shape.
  • It is usually bluish-green in colour, but some species are yellow, red, orange or brown in colour.
  • In form and habit the lichen thalli are highly diverse.

Internal structure

  • The internal structure of crustose lichens consists of uniformly distributed algal and fungal components.
  • Thalli are not distributed into layers of tissue due to which it is known as homomerous.
  • However, the thalli of most foliose and fruticose lichens are differentiated into several layers of tissue so called heteromerous.

Related image      Related image

  • A vertical section of foliose lichen shows four distinct layers which are as follows:

a)Upper cortex

  • It is the outer most thick and protective zone of the thallus.
  • It is composed of compactly interwoven fungal hyphae which form a tissue like layer.
  • The upper cortex may or may not be externally covered by an epidermis like-layer of hyphae.

b)Algal layer

  • The upper cortex is followed by algal zone.
  • It consists of a tangled network of loosely interwoven fungal hyphae intermingled with algal cells.
  • It is also called gonidial layer.
  • The algal region is the photosynthetic region of the thallus.

Lichens: Hybrid organisms - Encyclopedia of the Environment

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  • It forms the central core of the thallus which is composed of very loosely interwoven hyphae with very large interspaces.

d)Lower cortex

  • It is composed of closely packed, dark-coloured hyphae.
  • It forms the lower surface of the thallus.
  • The rhizines grow out from the underside of the lower cortex and attach thallus to the substratum.




Lichens: habitat, types and structure