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.
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.
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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-
- 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.
- 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.
- Bullate: This type of lichen forms extremely inflated squamules in the substratum. Example- Mobergia.
- Suffruticose: This type of lichen forms coralloid tufted cushions. Example- Peltula clavata.
- Lobate: In this type of lichen, the thallus becomes radially striate with marginal, partially raised lobes. Example- Caloplaca sp.
- Endolithic: This type of lichen grows inside solid rock, only the fruiting bodies exposed to air. Example- Caloplaca sp.
- Pulvinate: In this type of lichen, the thallus possess swelling at the base. Example- Euopsis pulvinata.
- 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:
- 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
- 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
- Cylindrical: These lichens have cylindrical, often hollow, branched structures that resemble tiny, delicate stems or rods. They can be erect or spreading. Example- Cladonia
- Coralloid: These lichens have a structure that resembles coral or fungi, with branched, irregular, and often densely clustered growths. Example- Cladonia
- 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.
- 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.
Internal Structure of Thallus of Lichens
Types Based on Algal Distribution
Lichens can be categorized into two types based on the distribution of algal members within the thallus: homoisomerous (or homomerous) and heteromerous.
- Homoisomerous Lichens:
- Structure: In homoisomerous lichens, fungal hyphae and algal cells are evenly distributed throughout the thallus. The algal partners typically belong to Cyanophyta.
- Examples: This structure is common in crustose lichens, where both partners intermingle to form a thin outer protective layer. Examples include Leptogium and Collema.
- Heteromerous Lichens:
- Structure: The thallus is divided into four distinct layers: upper cortex, algal zone, medulla, and lower cortex.
- The algal cells are confined to the algal zone.
- Examples: This structure is found in foliose and fruticose lichens. Examples include Physcia and Parmelia.
Detailed Internal Structure of Heteromerous Lichens
(a) Upper Cortex:
It is a thick, outer protective layer made up of compactly arranged, interwoven fungal hyphae oriented perpendicular to the surface. Intercellular spaces, if present, are filled with gelatinous substances.
(b) Algal Zone:
It is located just below the upper cortex; it contains algal cells entangled by loosely interwoven fungal hyphae. Common algal partners may be from Cyanophyta (e.g., Gloeocapsa, Nostoc, Rivularia) or Chlorophyta (e.g., Chlorella, Cystococcus, Pleurococcus). This layer may be continuous or patchy and serves photosynthetic functions.
(c) Medulla:
It is situated below the algal zone, consisting of loosely interwoven, thick-walled fungal hyphae with large spaces between them.
(d) Lower Cortex:
The lowermost layer composed of compactly arranged hyphae that are oriented either perpendicular or parallel to the surface. Some hyphae extend downward to penetrate the substrate, aiding in anchorage and are known as rhizines. The internal structure of Usnea, fruticose lichen, is slightly different due to its cylindrical cross-section. The layers from outside to inside include the cortex, medulla (composed of algal cells and fungal mycelium), and a central chondroid axis (composed of compactly arranged fungal mycelia).
Vegetative Structure of Lichens
Lichens have a unique vegetative structures and all of them has a vital importance in taxonomic studies.
- Breathing pores– Interwoven hyphae are present in some area of the cortex in Oropogon sp., as a result, pore like structures are formed through which gaseous exchange takes place.
- Cyphellae- These are the large circular depression confined to the lower cortex formed only in the foliose lichen. Gaseous exchange is the main function of cyphellae.
- Pseudocyphellae- These are the tiny pores formed at the surface of several foliose and fruticose lichens. The openings are filled by network of short cells.
- Cephalodia- These are the small darken coloured structure found in some species of lichen that contains cyanobacteria symbionts. Cephalodia can occur within the tissue of lichen, or on its upper or lower surface.
- Isidia- These are small corticated outgrowths present on the upper surface providing wider region for photosynthesis. It is formed on the green algal cells and fungal hyphae. Its main function is vegetative propagation.
- Soredia- It is a small, non-corticated and bud like outgrowth situated on the upper surface of lichen thallus. It is composed of a few algal cells closely enveloped by fungal hyphae. It is meant for vegetative propagation.
Reproduction in Lichens
Lichen reproduces by following methods-
Vegetative Reproduction in Lichens
It takes place by following methods:
- Fragmentation- The thallus of lichen breaks into small fragments and during favourable condition, each fragment develops into a new thallus. Example- Ramelia reticulata.
- By isidium and soredium- These are the most common methods of vegetative reproduction. These are the small protuberances produced on the upper surface of the thallus. They may either occur within definite pustule like compact structures or may rise so abundantly as to spread up like a thin layer of dust. Each soredium consists of a new cells surrounded by mass of hyphae.
Soredia and isidia are arised from the algal zone below the upper cortex. The cells of the algal zone divide actively and soon get surrounded by the fungal hyphae. Soredia are very light in weight and are easily disseminated by wind or rain wash. After falling on suitable substratum, they develop into new lichens. Example- Parmelia. - Phyllidia- These are separated leaf-like or scale-like dorsiventral part of the thallus of some foliose lichen. Example- Peltigera sp.
- Blastidia- These are yeast like small fragmented reproductive branch which becomes detached from the parent and grows into new plant. Example- Physcia.
- Schizidia- These are scale-like, splitted segments in the upper layer of lichen. This gives rise to new individuals. Example- Parmelia taylorensis.
- Hormocysts- When algal filaments and fungal hyphae grow together in a chain like system and break into clumps. Example- Lempholema.
- Goniocysts- These are formed when algal cells and its derivatives are wrapped in fungal hyphae in the form of an unsorralium- like structure which is found in Goniocystagia.
Asexual Reproduction in Lichens
Sporulation
Certain lichen may also reproduce asexually by means of Conidia, Oidia, and Pycniospores. In some cases, hyphae break down into small pieces known as Oidia while pycniospores are produced within the flask shaped structure known as pycnidia. Each pycnidium opens to the surface through a small pore known as ostiole. After falling on suitable substratum, pycnidiospore germinate and coming in contact with appropriate alga, they further develop into new lichen. Example- Cladonia.
Sexual Reproduction in Lichens
The sexual reproduction in Ascolichens and Basidiolichens is like class Ascomycetes and Basidiomycetes respectively. The male reproductive organ is called spermagonium and the female is known as carpogonium. They develop either on the same hyphae or on two different hyphae of the same mycelium.
Spermagonium– The spermagonia are flask-shaped structures embedded in the upper surface of the thallus. They open outside by a small pore known as ostiole. The fertile hyphae in the cavity of spermagonium abstract minute rounded cells at its tip. These male cells are called spermatia. In some lichens, the pycnia like structures also function as spermagonia.
Carpogonium– A carpogonium consists of two parts i.e. lower coiled multicellular portion called ascogonium and the upper, long, straight, thread like portion called trichogyne. The ascogonium lies deep in the medullary portion while trichogyne emerges out of the thallus and receives spermatia.
Fertilization in Lichens
On being disseminated, the spermatia have been found sticking to the protruding tip of trichogyne. This is the only evidence that spermatia function as male gametes. After fertilization, trichogyne withers, and the ascogonium produces freely branched acrogenous hyphae. These hyphae produce asci at their ends. All these structures are surrounded by the sterile hyphae. It results in the formation of fruiting body which is either apothecium or perithecium type.
Germination in Lichens
The ascospores may be simple or separate. They are very light in weight and easily disseminated to a long distance by wind. After falling on suitable substratum it germinates and produces fungal hyphae. The hyphae grow into a new lichen thallus.
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.
References
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- A Text Book of Botany Vol.-II, 2013, Dr. K.A. Siddiqui, Kitab Mahal, Allahabad. •
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