Classification of Lichens (With Simple Diagram)

In this article we will discuss about classification of lichens based on Morphology, habitat, internal structure and fungal partner.

Table of Contents

Classification of Lichens

A lichen is not a single organism, it is a symbiotic association between a fungus and algae.
There is no natural classification system for lichens.
They are classified based on morphology, habitat, internal structure and fungal partner.

On the basis of their morphology, lichens are classified into three main groups: crustose, foliose and fruticose lichens

I. Crustose Lichens (Encrusting lichens)

These lichens found as a thin or thick crust over rocks, soil or tree barks.
They are very difficult to separate from their substrate.
The thalli may be fully or partially embedded, So that only the fruiting bodies visible above the surface.
E.g., Lecanora, Graphis, Rhizocarpon, Ochrolechia etc.

II. Foliose Lichens (Leafy lichens)

These lichens have a flat, expanded, leaf like thallus (commonly grayish or brownish in colour), that spreads out horizontally over the surface.
They attach to the substrate using rhizoid-like outgrowths called rhizines.
They can be easily removed without damaging the substrate.
E.g., Physcia, Parmelia, Gyrophora etc.

III. Fruticose Lichens (Shrubby Lichens)

These are the upright or hanging lichens.
They have a branched, bushy thallus that can hang from the substrate.
The thalli may be erect or pendant.
These are flat, cylindrical or ribbon like, well branched and resembling small shrubs.
These lichens are attached only at the base by a flat disc and can be easily removed by hand.
E.g., Cladonia rangiferina, Usnea barbata etc.

Muscicolour (Mosses): These lichens grow along with mosses. E.g., Cladonia.
Foliicolous (Leaves): These lichens are found on the surface of leaves. E.g., Calicium.
Terricolous (Soil surface or Ground): These lichens occur on the soil surface, particularly in hot climates with sufficient rain and dry summer. E.g., Cladonia, Florekeana, Lecidea, Collema etc.
Saxicolous (Rocks or Stones): These lichens grow on rocks and stones surface. E.g., Dermatocarpon, Xanthoria, Verrucaria etc.
Corticolous (Tree barks): These lichens grow on the barks of trees, primarily in subtropical and tropical regions. E.g., Parmelia, Usnea, Grpahis.
Lignicolous (Wood): They are grow directly on the wood. E.g., Calicium etc.
Marine Lichens: They are grow on siliceous rocky shores of sea. E.g., Verrucaria, Caloplaca etc.
Fresh water lichens: These lichens grow on siliceous rocks in fresh water. E.g., Epheba, Hymenelia etc.

On the basis of internal structure the lichens are divided into two groups: Homoiomerous lichens and Heteromerous lichens

I. Homoiomerous lichens

In these lichens, the fungi and algae are not differentiated into distinct layers of tissue. Instead, algal cells are randomly distributed throughout the thallus.
E.g., Collema and Leptogium

II. Heteromerous lichens

These lichens are differentiated into several layers of tissues.
A cross section of the heteromerous lichen shows four distinct zones: Upper cortex, Gonidial layer (Algal layer), Medulla and Lower cortex.

Upper Cortex: This layer forms the upper surface of the thallus. It is thick, protective and composed of tightly interwoven fungal hyphae, typically lacking intercellular spaces.
Gonidial layer: This layer consist of algal cells and they are surrounded by thin walled, loosely packed fungal hyphae.
Medulla: This layer constitutes the bulk of the thallus and made up of loosely packed, thick-walled fungal hyphae.
Lower Cortex: This layer consists of closely packed, dark coloured hyphae. From the lower cortex, rhizinae arise, which serve to attach and absorb nutrients.

The fungal partner primarily belongs to ascomycetes, with some basidiomycetes and rarely deuteromycetes.
On the basis of the fungal partner, lichens are classified into three groups: Ascolichens, Basidiolichens, and Deuterolichens.

I. Ascolichens

In this lichens, the fungal component is belongs to Ascomycetes.
The sexual reproduction of Ascolichens is similar to Ascomycotina.
They produce ascus with ascospores during the sexual reproduction.
More than 95% of the lichens fall into the Ascolichens
These lichens are further divided into two subgroups:
Gynocarpeae: Here, the fruiting body (ascocarp) is an apothecium. E.g., Parmelia.
Pyrenocarpeae: In this subgroup, the fungal fruiting body (ascocarp) is an perithecium type. E.g., Dermatocarpon.

II. Basidiolichens

In basidiolichens, the fungal component belongs to basidiomycetes.
The sexual reproduction of basidiolichens is similar to basidiomycotina
They produce basidia with basidiospores during sexual reproduction.
Only a few lichens (four genera reported so far) are classified as Basidiolichens.
E.g., Dictonema, Corella and Cora

III. Deuterolichens

They are also known as lichen imperfectii.
They have fungal partners from the Deuteromycotina division.
These lichens do not undergo sexual reproduction or should say that lichens with sterile thalli are constituted by this group.
E.g., Lepraria, Leprocaulo, Crysothrix.