Lichens: habitat, types and structure

March 21, 2019

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.

Image source: flatrock

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.

Image source: berkeley

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.

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.

Image source: encyclopedie

c)Medulla

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.

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