Lichens: Characteristics, Types, Structure

Lichens are group of organisms of composite nature, having two dissimilar organisms (algae and fungus) bounded in a symbiotic relationship.

Algae or cyanobacteria and fungus forms beneficial symbiotic relationship to form lichen. The algal component is called phycobiont and the fungal component is called mycobiont.

The algal component is usually green-algae (chlorophyta) or blue-green algae (Cyanophyta). Usually the fungal component is formed by Basidiomycetes or Ascomycetes.

Lichens

The fungal partner occupies the major portion of the thallus and produces its own reproductive structures.

The algal partner manufactures the food through photosynthesis which probably diffuses out and is absorbed by the fungal partner.

About 400 genera and 15,000 species of lichen are found all over the world.

General Characteristics of Lichens

• Lichens are the group of plants having composite thalloid structure consisting of algae and fungi bounded in a symbiotic relationship in which both the components are intertwined to form a single organism.
• Ascomycetes that forms mycobiont is called Ascolichen.
• Basidiomycetes that forms mycobiont is called Basidiolichen.
• Lichens are widely distributed in all over the world in varied habitat.
• Morphologically, they are of three types- Crustose lichen, foliose lichen and fruticose lichen.
• Lichen reproduces by vegetative, asexual and sexual methods.
• Only mycobiont is involved in sexual reproduction. It either reproduces by ascospores and basidiospores in ascolichen and basidiolichen respectively.
• The male sex organ is called spermogonium and the female sex organ is called carpogonium.
• They grow abundantly in forest areas and also considered as “pollution indicator.”

Forms/Types of Lichens

Morphologically, they are of three types-

a. Crustose Lichens

These are the lichens that form a crust closely attached to substratum and may be partly or wholly embedded in it is called crustose lichen. 

In these lichens, the thallus is a flat, dorsiventral structure, closely attached to the substratum, e.g. Parmelia acetabulum. Water loss is restricted primarily to the upper exposed surface. 

The following sub-types can be distinguished-

1. Leprose or powdery: The thallus has powdery appearance and is the simplest amongst all the types. It lacks distinct thallus organization and the phycobiont and mycobiont is indistinguishable. Example-  Lepraria incana.
2. Peltate: This type of lichen is often developed in lichens colonizing soil or rock surfaces in hot, arid regions of the world. Example-  Petula radicata.
3. Bullate: This type of lichen forms extremely inflated squamules in the substratum. Example- Mobergia.
4. Suffruticose: This type of lichen forms coralloid tufted cushions. Example- Peltula clavata.
5. Lobate: In this type of lichen, the thallus becomes radially striate with marginal, partially raised lobes. Example- Caloplaca sp.
6. Endolithic: This type of lichen grows inside solid rock, only the fruiting bodies exposed to air. Example- Caloplaca sp.
7. Pulvinate: In this type of lichen, the thallus possess swelling at the base. Example- Euopsis pulvinata.
8. Effigurate: This type of lichen possesses radially arranged prolonged marginal lobes. Example- Acarospora.

b. Foliose Lichens

Foliose lichens are leaf-like, flat and only partially attached to the substratum. Typically they have a dorsiventral organization with the distinct upper and lower surfaces. Often the thallus is divided into lobes that show various degree of branching. They are attached to the substratum by a specialized structure called rhizines. Example- Parmelia.

The different sub-types of foliose lichen are as follows- 

Laciniate lichens- These are the foliose lichens that are lobate and vary considerably in size. The lobes can be radially arranged or overlapping. Example- Parmelia sp., Peltigera etc.

Umbilicatelichens – It has a circular thalli consisting either of one single, unbranched lobe or multi-lobate thalli with limited branching patterns. All are attached to the substratum by a central umbilicus from the lower surface. Example- Lasallia papulosa.

c. Fruticose Lichens

The thallus lobes of fruticose lichens are hair-like, strap shaped or shrubby and the lobes may be flat or cylindrical. 

The sub-types of fruticose lichens are:

1. Shrubby: These lichens have a well-defined, bushy structure with cylindrical, twig-like branches. They often look like small shrubs or bushes and can be pendulous or upright. Example- Usnea
2. Hanging: These lichens hang down in long, hair-like strands, often appearing like curtains or beards. They are typically found in hanging forms, growing downwards from branches or rocks. Example- Usnea
3. Cylindrical: These lichens have cylindrical, often hollow, branched structures that resemble tiny, delicate stems or rods. They can be erect or spreading. Example- Cladonia
4. Coralloid: These lichens have a structure that resembles coral or fungi, with branched, irregular, and often densely clustered growths. Example- Cladonia
5.  Leafy: Some fruticose lichens have a flattened, leaf-like appearance but are still attached to the substrate with a central holdfast, giving them a somewhat foliose look. Example Peltigera.
6. Hairy: These lichens have long, hair-like structures that can be straight or twisted, often covering a large surface area. They may appear woolly or velvety. Example- Alectoria.

Aside from these three types of lichen, there are other types also which are discussed as follows-

• Gelatinous lichens– It is mostly formed of cyanophyta with only a little mycobiont portion. The gelatinous appearance is from the appearance is from the mucilage community formed on the exterior of blue-green algae cells. Example- Collena migrescens
• Filamentous lichens– It has a growth form like a mass of thin, stingy, non-branching hairs or filaments. Filaments of the algal partner are surrounded by the sheath of fungal filaments. Example- Ephebe.
• Byssoid lichen– It is lichen with a wispy growth form, having the appearance of teased wool. Example Coenogonium implexus.
• Squamulose lichen– It is lichen that is composed of overlapping scales called squamules. If they appear leafy, they may look like foliose lichen but the lower cortex is absent as foliose lichens do. Example- Cladonia 
• Placoid lichen- Here the entire surface of the thallus is radially striate and contains raised marginal tissue. Example- Caloplaca. 

Economic Importance of Lichens

Lichens are of high economic importance. The economic importance is as follows-

Ecological Significance

Pioneer colonizers: Lichens are said to be the pioneers in establishing vegetation on bare rocky areas. They are the first members to colonize the barren rocky areas. During development, they bring about the disintegration of rock stones by forming acids. Thus they play an important role in nature in the formation of soil.

Role in environmental pollution: Lichens are very sensitive to atmospheric pollutants such as sulfur dioxide. They are unable to grow in towns, cities and around industrial sites such in towns, cities and around industrial sites such as oil refineries. So, the lichens can be used as bio-indicators of air pollution.

Food and fodder

The lichens serve as important source of food for invertebrates. A large number of animals such as mites, caterpillars, termites, etc. feed partly or completely on lichens. Lichens are rich in polysaccharides, certain enzymes and vitamins and thus lichens are also being used as food by humans during famines.

Cladonia rangiferina also known as reindeer moss is the main food for reindeers in polar countries.

Parmelia are used as curry powder in India. In France, lichens are used in confectionaries for making chocolates and pastries. Cetraria islandica is used as fodder for horses.

Source of medicines

Lichens are used to cure jaundice, fever, diarrhoea, epilepsy, hydrophobia and various skin diseases. Laboria pulmonaria and Cetraria islandica are used in respiratory diseases and tuberculosis.

Usnea barbata is used in uterine aliments.

Cladonia sp. is used for the treatment of whooping cough.

A yellow substance called usnic acid is obtained from Usnea sp. and Cladonia sp. is used as antibiotic and used in the treatment of various infections. It is effective against gram-positive bacteria. Some lichen compounds such as lichenin, isolichenin, have anti-tumor properties. 

Many antiseptic creams such as Usno and Evosin are made from lichen.

In industries

Some lichens are used in leather industry. Lichens are also used in preparing natural dyes. Orchill, a blue dye obtained from Leconara is used to dye woollen articles and silk fabrics. A brown dye obtained from Parmelia sp. is also used in dyeing.

Lichens are also used in cosmetic industries. Some lichen contains aromatic substances in their thallus and are used to produce various cosmetics.

Cetraria islandica contain carbohydrates in the form of lichenin. In Sweden and Russia, this lichen is used to prepare alcohol.

Lecanora esculenta yields a large amount of calcium oxalate. 

Natural products

Lichens are also known to produce various natural products like Salazinic acid, Lecanolic acid etc.

Poisons

Some lichens are poisonous due to presence of various substances in them. Letharia vulphina also known as wolf moss is poisonous due to presence of vulpinic acid. Cetraria juniperina is poisonous due to the presence of pinastrinic acid.