Structure of Lichens (With Simple Diagram) | Lichenology

In this article we will discuss about structure of lichens

Structure of Lichens

• In lichen, the fungal partner produces a thallus, which accommodates the algae.
• Lichens exhibit a wide range of colors, some are greyish green, grey, white, orange, yellowish green, brown, black and red.
• Based on their external morphology, lichens are grouped into three types: crustose, foliose and fruticose lichens.

1. Crustose lichens

• The thallus is small, thin, flattened, leathery structure very closely attached to the substratum like stones, soil, rocks and barks.
• It is very difficult to separate them from the substratum without damaging it.
• The fruiting bodies of lichens are present on the upper surface.
• E.g., Lecanora, Graphis, Rhizocarpon, Ochrolechia etc.

2. Foliose lichens

• The thallus is flat, broad leaf like, well branched with lobed or irregular margins.
• Some part of their thallus adheres more or less firmly to the substratum by rhizinae
• rhizinae serve as an organ for anchorage and absorption.
• The lower surface of the thallus maybe white or sooty (black powdery).
• The upper surface of the thallus is greyish green in colour and It has many fruit bodies.
• E.g., Xanthoria, Parmelia, Physcia, Anaptychia etc.

3. Fruticose lichens

• The thallus is well branched slender, ribbon like or cylindrical, either bushy and erect (Evernia, Cladonia) or pendent and tassel like (Usnea).
• There is no upper and lower surface in the thallus but are attached to the substratum by a definit basal portion forming a flattened disc.
• These lichens are attached with rock surfaces and branches and foliage of trees.
• It can be removes from the surface by hand.
• E.g., Cladonia, Usnea, Alectoria etc.

Internal Structure of Lichens

• The lichen thallus is anatomically composed of both fungal and algal components.
• A cross-section of lichen thallus is consist of four distinct zones: the Upper Cortex, Algal Layer, Medulla, and Lower Cortex.

I. Upper cortex

• This is the outermost protective layer, made up of tightly interwoven fungal hyphae.
• The intercellular spaces are typically absent; if they present, they are filled with gelatinous substances.
• In some species, this layer is interrupted with breathing pores and these pores help for aeration.

II. Algal zone or Gonidial layer

• This layer consists of algal cells and they are surrounded by thin walled loosely packed fungal hyphae.
• This part serves as the photosynthetic region of the thallus.
• This layer is also called gonidial layer because, earlier scientist belief that these cells play a role in reproduction.

III. Medulla

• This layer present just below the gonidial layer.
• It is less dense and made up of loosely interwoven fungal hyphae.
• The fungal hyphae in this zone are scattered and possess thick walls.

IV. Lower cortex

• The lower cortex is present below medulla and it is made up of densely packed hyphae.
• In some lichens, this Lower cortex is absent. E.g., Lobaria pulmonaria.
• In this lower cortex gives rise to bundles of hyphae, known as rhizines.
• This rhizines penetrate the substratum to serve as anchoring structures.

Structure of Ascolichens, Basidiolichens and Deuterolichens

I. Ascolichens

• Thallus can be crustose, foliose or fruticose.
• They exhibit distinct upper and lower surfaces.
• Mostly the thallus are heteromerous, So internally they have a distinct four layered structure: Upper Cortex, Algal Layer, Medulla and Lower Cortex

2. Basidiolichens

• Primarily they are crustose lichens
• Thallus are mostly Homoiomerous, So internally they lack a distinct layered structure.
• Algal cells are scattered throughout the thallus, interspersed with fungal hyphae.

3. Deuterolichens

• They can be crustose, foliose, or fruticose.
• They can be either heteromerous or homoiomerous.

Conclusion

• Ascolichens consist of Ascomycetes fungi and have a clearly four layered internal structure (heteromerous)
• Basidiolichens are formed from Basidiomycetes fungi and they lack distinct internal layers (homoiomerous)
• Deuterolichens, on the other hand, are made up of Deuteromycetes fungi and they exhibit a wide range of internal structures, with both heteromerous and homoiomerous types.