What are Lichens?

Lichen fungus is known to be a strange group of plants made up of two different organisms: an alga (phycobiont: in Latin phycos-alga; bios-life) and a fungus (mycobiont: in Greek mycos-fungus; bios-life). Algal and fungal components coexist in a truly symbiotic relationship. Simon Schwendener was the first to discover the true nature of lichens. He gave the algal component the name Phycobiont and the fungal component the name Mycobiont.

A lichen is a hybrid of one or two fungus species and an alga or cyanobacterium (blue-green alga) that produces a form distinct from the symbionts. Although lichens appear to be single plantlike organisms, under a microscope, the associations are seen to be made up of millions of cells of algae (called the phycobiont) woven into a matrix made of fungi filaments (called the mycobiont). Many mycobionts belong to a single Ascomycota group known as the Lecanoromycetes, which are distinguished by an open, often button-shaped fruit known as an apothecium.

Although lichens were long thought to be made up of a single fungus species and a single phycobiont, new research suggests that many macrolichens also have specific basidiomycete yeasts in their cortex. There are many different types of phycobionts, but half of the lichen associations contain Trebouxia, a single-celled green alga. About 15 species of cyanobacteria, including members of the genera Calothrix, Gloeocapsa, and Nostoc, act as photobionts in lichen associations.

Lichen Classification

Lichens come in a variety of forms. They are classified into different types based on their unique characteristics.

Based on their Growth

1. Crustose Lichens

Crustose lichens are flat, thin, and lack distinguishable lobes. They are usually found attached to stones, rocks, barks, and tree trunks. The best crustose lichens are Haematomma puniceum and Graphic scripta.


2. Foliose Lichens

Foliose lichens are more appealing than other types of lichens. They are flat, broad, smooth, and leaf-like structures that resemble crinkled and twisted leaves. It has a clear upper and lower surface. This type of lichen is commonly found attached to rocks and twigs via the rhizoid. Foliose lichens include Cetraria, Cluiudhuria, Parmelia, and Xanthoria.


3. Fruticose Lichens

These are the most common lichens, and they are thin and freely branched. Fruticose lichens are larger and more visually appealing growths that protrude from tree branches, foliage, and rocks. The most common fruticose lichens are Cladonia, Ramalina, and Usnea.


Based on their Habitat:

  1. Lignicolous is a term used to describe a person who has a These lichens can be found in the woods.

  2. Corticolous lichens are those that live on the bark of trees.

  3. Saxicolous lichens are those that live on stones or rocks.

  4. The sea. These lichens can be found growing on siliceous rocks near the sea's edge.

  5. Water that is not salty. These lichens are found growing on hard siliceous rocks, particularly near freshwater.

  6. Terricolous- These lichens grow on the soil and are thus referred to as terrestrial lichens.


Based on their Internal Structure:

  1. Heteromerous lichens

  2. Homoiomerous lichens.


Based on their Fungal Partner:

  1. Ascolichens

  2. Basidiolichens

  3. Hymenolichens


Lichen Algae

The lichen fungus is classified as a fungus, and the fungal partners are classified as Ascomycota and Basidiomycota. Lichens can also be classified according to their morphology. There are three major types of lichens, though there are others. Crustose lichens are lichens that are tightly attached to the substrate, giving them a crusty appearance. Foliose lichens have leaf-like lobes, can only be attached at one point in the growth form, and have a second cortex below the medulla. Finally, fruticose lichens are known to have rounded structures and a branched appearance in general. Figure 2 depicts an example of each type of lichen.

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General Characteristics of Lichens

Let’s discuss lichen characteristics:

  • In general, the major portion of the thallus is occupied by the fungal component and this fungal component produces its own reproductive structures.

  • The algal partner is basically responsible for making the food by the process of photosynthesis. The food diffuses out as well as it is absorbed by the fungal partner.

  • Owing to their symbiotic relationship, lichens can live in a variety of habitats as well as climatic conditions including extreme environments.

  • Lichens can be classified into the following types based on their growing medium.

  • Follicolous (grows on tree barks), Corticolous (grows on tree barks) (grows on leaves surfaces),

  • Saxicolous is a type of saxicolous plant (grows on rock surfaces)

  • Terribly impressive (grows on soil)

  • Musically inclined (grows on mosses)

  • Lichen growth forms are commonly seen on surfaces, forming greyish, greenish, or orange areas. They are divided into three major types based on their morphology and size, namely,

  • Crustose is a sugar (crust like)

  • Foliose is an abbreviation for Foliose (leaf like)

  • Fructose (shrubby)

  • Crustose lichens are known as microlichens whereas foliose and fruticose lichens are known as macrolichens.

  • The thallus is the main plant body of the lichen. The thallus is the vegetative portion, which is similar to that of mosses and liverworts.

  • Mycobionts (Ascomycete or Basidiomycete) form a close symbiotic relationship with phycobionts (green algae or blue-green algae). After association, both phycobionts and mycobionts lose their distinct identities and are referred to as lichens. Lichens now function as a single organism, both morphologically and physiologically.

  • Lichens reproduce in a way that is completely different from that of fungi and algae. Lichens reproduce vegetatively by the formation of special propagules known as diaspores. Soredia and isidia are the most common lichen diaspores.

  • The lichens' fungal partner reproduces sexually. In lichens, sexual reproduction begins with the formation of fruiting bodies, which is followed by the formation of spores known as ascospores. Ascospores are dormant spores that can survive in harsh environments.

FAQs (Frequently Asked Questions)

Question 1: Is Lichen a Parasite?

Answer: Lichens have properties that are distinct from those of their constituent organisms. Lichens come in a variety of colours, sizes, and shapes, and they can appear plant-like at times, but they are not plants. They do not live as parasites on plants, but rather use the plant's surface as a substrate.

Question 2: Is Lichen a Natural Taxonomic Group?

Answer: Lichens are a biological group that does not have formal taxonomic status among living organisms. Although the mycobiont and phycobiont have Latin names, the lichen that results from their interaction does not.