In this essay we will learn about Lichens. After reading this essay you will learn about:- 1. Meaning of Lichens 2. Habit and Habitat of Lichens 3. Classification 4. Special Structures 5. Nature of Association 6. Reproduction 7. Ecology 8. Physiology.
Essay # Meaning of Lichens:
Lichens constitute a small group of thallophytic and autotrophic plants. They form a unique combination of two completely different individuals, of which one belongs to algae and the other to fungi.
The algal component is called phycobiont (Gr. plxykos = alga + bios = life) and the fungal component is known as mycobiont (Gr. mykes = fungus + bios = life). The two components remain in close contact and appear to be a single plant. Therefore, lichens are also known as composite or dual organism.
There are about 400 genera and 1600 species of lichens yet known. Each individual of a species contains the same alga and fungus.
The algal component mostly belongs to Chlorophyceae (green algae) and in a few lichens it belongs to Cyanophyceae (cyanobacteria, blue green algae). About 21 genera of algae (20 of green algae and remaining genus Hetero-coccus belongs to Xanthophyceae) and 12 genera of blue green algae are found as phycobionts.
Trebouxia, Protococcus, Cystococcus. Cladophora, Trentepohlia, Coccomyxa (green algae), Nostoc, Stigonema, Scytonema, Rivularia, Gloeocapsa. (blue green algae) are the common phycobionts. Species of Trebouxia (green algae), Nostoc, Scytonema (blue green algae) are the most common.
Mycobiont in the lichens mostly belongs to Ascomycetes (sac fungi). However, a few lichens also show Basidiomycetes (club fungi) fungal partner. As algae and fungi both contain cell wall which is the characteristic feature of plant cell, lichens are also considered as plants.
Theophrastus (371-284 BC) was the first scientist to introduce the term lichen in his book History of Plants for extra outgrowth on the tree barks.
Morisson (1699) called lichen as muscofungus. Schwendener (1867) was the first scientist to establish the dual nature of lichens. Erich Acharius (1757-1819), a swedish doctor is credited as the founder of the systematic study of lichens because he was the first scientist to divide cryptogams into six families including lichens.
Bonnier (1886-89) successfully synthesized a lichen by growing fungal spores with algae. Debary (1870), Reinke (1872), Crombie (1885), Chopra (1934), Plessel (1963), Ahmadjian (1960, 1962, 1982) and many other workers studied lichens in detail. The branch of botany which is concerned with the study of lichens is called lichenology and a scientist who studies the lichens is called a lichenologist.
Essay # Habit and Habitat of Lichens:
Lichens are cosmopolitan in distribution and grow best in moderate temperature, sufficient, humidity and direct sunlight. They can withstand extremes of climate (cold, heat and drought) and thus are found everywhere ranging from hot deserts to chilly mountains.
They are commonly seen growing on bare rocks, old walls, high mountains, and even in alpine and arctic tundra’s. Cladonia rangiferina, commonly known as the ‘reindeer moss’, grows luxuriantly in tundra’s.
Some lichens like Usnea hang from the smaller branches of trees. Lichens grow very slowly. They are most sensitive to air pollution (sulphur dioxide) and probably that is the reason why they are not found near cities. In India, lichens are very common in Eastern Himalayas, Darjeeling and Gangtok.
Lichens are divided into following categories on the basis of their habitat:
1. Saxicolous. Grow on stones or rocks e.g., Dermatocarpon, Xanthoria, Verrucaria, etc.
2. Corticolous. Grow on bark of trees e.g., Grpahis, Usnea, Parmelia etc.
3. Terricolous. Grow on soil (terrestrial) e.g., Cladonia florekeana, Lecidea, Collema etc.
4. Lignicolous. Grow directly on wood e.g., Calicicum etc.
5. Marine. Grow on siliceous rocky shores of sea e.g., Caloplaca, Verrucaria etc.
6. Freshwater. Grow on hard siliceous rocks in freshwater e.g„ Epheba, Hymenelia etc.
Essay # Classification of Lichens:
The systematic position of lichens is a controversial matter because it is a combination of two different members of two different groups. Bessey (1950), Martin (1950) and Alexopoulos (1956) included the lichens with true fungi.
However, Bessey (1950) included them in Order Leconorales of Ascomycetes. Later on Smith (1955) and some other workers opined that lichens should be treated as a distinct group. Bold (1957) had given the name Mycophycophyta, a name which represents the dual nature of the organisms.
Zahlbruckner (1907, 1926) divided the lichens into following 3 sub-classes on the basis of their fungal partner:
(A) Ascolichens. Fungus belongs to Ascomycetes.
It is further divided into two series depending on the fruiting body:
Series 1. Gymnocarpeae. Fruiting body is an apothecium i.e., in the form of a more or less open disc, e.g., Parmelia.
Series 2. Pyrenocarpeae. Fruting body is perithecium i.e., a closed structure e.g., Dermatocarpon.
(B) Basidiolichens. Fungus belongs to Basidiomycetes. There are only 3 genera which belong to Basidiolichens. These are: Cora, Corella, Dictyonema (Fig. 1).
(C) Hymenolichens. Fungus belongs to Deuteromycetes lichens are also known as imperfect lichens.
Thalloid lichens are green or bluish-green in colour. Some species may have yellow, red, orange or brown pigments. They are usually dull in appearance because of the translucent fungal covering over the algal constituents.
On the basis of growth forms, and nature of attachment to the substratum lichens are divided into following three types:
(1) Crustose lichens (encrusting lichens).
These lichens occur as thin or thick crust over rocks, soil or tree barks. It is very difficult to separate them from substratum. The thalli may be wholly or partially embedded so that only fruiting bodies are visible above the surface of the substratum e.g., Lecanora, Graphis, Rhizocarpon, Ochrolechia etc. (Fig. 2).
(2) Foliose lichens (leafy lichens).
These lichens are variously lobed, leafy structures attached to the substratum by rhizoid like outgrowth called the rhizines e.g., Xanthoria, Parmelia, Physcia, Anaptychia etc. (Fig. 3).
Fruticose lichens (Shrubby lichens).
These are the upright or hanging lichens (pendant forms) attached only at the base by a flat disc. These are cylindrical, flat or ribbon like, well branched and resemble with little shrubs e.g., Cladonia, Usnea, Alectoria etc. (Fig. 4).
A fourth type of lichen called leprose has also been differentiated. It has some fungal hyphae surrounding one or more algal cells. A distinct fungal layer envelops the algal cells all over. It appears as a powdery mass over the substratum e.g., Leparia incana (Fig. 5).
Internally the thallus is composed of algal and fungal components. Such type of thallus is known as consortium.
On the basis of internal structure the lichens are divided into two groups:
(A) Heteromerous lichens
(B) Homoiomerous lichens.
T. S. Heteromerous Lichens:
A transverse section of the heteromerous (foliose) lichen can be divided into following 4 distinct zones (Fig. 6):
(1) Upper cortex,
(2) Gonidial layer,
(3) Medulla and
(4) Lower cortex.
It is the upper-most protective layer made up of compactly interwoven fungal hyphae. The compactly interwoven hyphae produce a tissue like layer (plectenchyma or pseudo parenchyma) called the upper cortex. The intercellular spaces are absent, if present, they are filled with gelatinous substances.
In some species of foliose lichens this layer is interrupted at different places. These interruptions or areas are known as breathing pores and serve for aeration. In addition to these certain other structures are also present for gaseous exchange. These are called cyphellae.
This layer consists of loosely interwoven hyphae intermingled with algal cells. This region is the photosynthetic region of the thallus. This layer is also called gonidial layer because of the earlier concept that these cells have reproductive function.
It is present just below the algal cells and is made of loosely interwoven hyphae of fungus. Medulla forms the middle portion of the thallus.
Like the upper cortex, it is the lower-most layer. In some lichens the layer is absent e.g., Lobaria pulmonaria. This layer gives rise to bundles of hyphae (rhizines) which penetrate the substratum to function as anchoring organs.
Different types of lichens particulary the foliose and fruticose remain attached to the substratum by a variety of structures such as rhizinose strand (thick strands e.g., Buellia pulchella), hyphal nets (fungal hyphae forming net like structures, e.g., Psora decipiens), hypothallus (thick, black, spongy, algal free tissue e.g., Anzia), holdfast (basal, algae free region, e.g., Usnea, Let liana), hapters (short, penetrating branches e.g., Alectoria) and medullary hyphae.
The above structure of a lichen shows that the algae cells are restricted or confined to form a distinct layer. Such type of lichens are called heteromerous (Fig. 6).
T. S. Homoiomerous Lichens:
In some lichens for example, Collema, Leptogium, the thallus shows a simple structure with little differentiation. The algae cells and fungal hyphae are uniformly distributed. Such type of lichens is called homoiomerous. (Fig. 7).
Special Structures Associated with Lichens:
(1) Cyphellae and Pseudocyphellae:
If seen with naked eye these structures appear as cup-like white spots on the lower surface of the thallus. But under the microscope they appear as small, hollow, circular, white cavities. From these cavities medulla is exposed and hyphae protrude out. If these cavities are of a definite form with a distinct border, these are known as cyphellae (Fig. 8) e.g., Stricta.
If the distinct border is absent and there is only distinct roundish opening in the cortex, looser hyphal medullary tissue comes out in the form of discrete patches, these are known as pseudocyphellae e.g., Alectoria, Bryoria, Coelocaulon etc. The function of these structures is to allow free passage of air to the algal cells.
These are dark coloured, small wart or gall like abnormal structures which develop on the upper surface or within the thallus e.g., Peltigera aphthosa, Lobaria pulmonaria etc. The cephalodium contains the same fungal hyphae but the algal component (generally Cyanophceae e.g., Nostoc) is always different from the parent thallus.
Such lichens, having three membered symbiosis (2 algae + 1 fungus), are called diphycophilous lichens. The cephalodia help in retaining the moisture in thallus (Fig. 9A, B).
Reproduction in Lichens:
Lichens reproduce by all the three methods of reproduction—Vegetative, asexual and sexual.
(1) Vegetative Reproduction:
(a) By Fragmentation.
Death and decay of older parts of the thallus produce smaller pieces which give rise to new thallus. Sometimes the broken pieces (fragments) develop into new thalli, provided they contain both the algal and fungal components.
(b) By Soredia.
It is the most common method of vegetative reproduction. These are small protuberances, produced on the upper surface by the thallus. They may either occur within definite pustule-like compact structures called soralium (Fig. 10 D) or may arise so abundantly as to spread up like a thin greyish layer of dust. Each soredium consists of a few algae cells surrounded by a mass of hyphae. (Fig. 10 A-C).
Soredia arise 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 a new lichen e.g., Parmelia, Bryoria etc.
(c) By Isidia:
These are the stalked, un-detachable outgrowths produced by the thallus on its upper surface (Fig. 11). Like soredia, the isidia are also composed of both fungal and alga, components but differ from them in being covered with a definite cortex.
The algal component is of the same kind as in thallus. The isidia may be rod shaped (Parmelia sexualities), coralloid (Umblicaria postulata), cigar shaped (Usnea comosa) or scale shaped (collema crispum).
The main function of the isidia is to increase the photosynthetic surface of the thallus. Sometimes these also act as organs of vegetative propagation.
(d) By lobules:
Some dorsiventral outgrowths are produced on the margins of the thallus of Parmelia and Peltigera lichens. These structures are known as lobules and act as organs of vegetative propagation.
Some lichens by forming propagules of different kinds such as phyllidia (leaf or scale like dorsiventral portions e.g., Peltigera), blastidia (yeast like, segmented e.g., Physcia), schizidia (scale like e.g., Parmelia), hormocyst (algae hyphal and fungal hyphae grow together in a chain like manner e.g., Lempholema), goniocysts (unsorallium like structures), etc.
(2) Asexual Reproduction:
Certain lichens may also reproduce asexually by means of conidia (e.g., Arthonia), oidia and Pycniospores or pycnidiospores (Fig. 12 A, B). However, it is of rare occurrence. In some cases hyphae break down into small pieces known as oidia while pycniospores are produced within the flask shaped structures known as pycnidia (Fig. 12).
Each pycnidium opens to the surface through a small pore known as ostiole. The pycnidial wall is made up of sterile fungal hyphae. Inside the pycnidia fertile hyphae obstruct sexual spores (pycnidiospores) at their tips. After falling on suitable substratum pycnidiospores germinate and coming in contact with appropriate alga, they develop further into a new lichen.
(3) Sexual Reproduction:
The sexual reproduction in Ascolichens and Basidiolichens is like class Ascomycetes and Basidiomycetes respectively. Ascolichens have been studied in more detail from this point of view. The male reproductive organ is called the spermogonium and the female is known as carpogonium. They develop either on the same hypha or on two different hyphae of the same mycelium.
The spermogonia 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 the spermogonium abstract minute rounded cells at its tip. These male cells are known as spcrmatia. In some species of lichens, however, the pycnidia like structures also function as spermogonia (Fig. 7).
A carpogonium consists of two parts (Fig. 13) 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.
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. However, Morean and Morean (1928) opined that there is never any fertilization of the protruding trichogyne by spermatia.
After fertilization trichogyne withers. The ascogonium produces freely branched acrogenous hyphae. These hyphae produce asci at their ends. All the structures after fertilization (i.e., developing asci, ascogenous hyphae and ascogonium) are surrounded by the sterile hyphae. It results in the formation of fruiting body which is either a apothecium or perithecium type.
Structure of Apothecium:
It is round and cup-shaped structure (Fig. 14 A). If the apothecium consists only the fungal component, it is known as lecideine type (e.g., Lecidea, Cladonia, Gyrophora) and if it consists bath algal and fungal components it is known as lecanorine type (e.g., Lecanora, Parmelia).
It can be divided into two parts (Fig. 14 A):
(1) Disc of the Apothecium:
(a) Hymenium (Thecium). It is the upper-most fertile layer of apothecium consisting of a closely packed, palisade like layer of sac-like asci and sterile hair like fungal hyphae known as paraphyses. This layer is also called hymenial layer or hymenium. Each ascus contains 8 ascospores (Fig. 14 B). Ascospores are of various shapes and size, multicellular uni- or bitunicate and uni- or multinucleate.
The sterile tissue that separates the asci is called hamathecium. As many as four types of hamathecial tissues are identified in an ascocarp.
Arise from the base of ascocarp and grow upwards (Fig. 15 A).
They are formed by stretching of the tissues of ascocarp before the development of asci (Fig. 15 B).
Arise from the roof of ascoscarp and grow downwards (Eig. 15 C).
Arise in the ostiolar canal and protrude outside the ostiole (Fig. 15 D).
The region consists of the closely interwoven sterile hyphae. It is present just below the fertile layer.
(2) Margin of Apothecium:
This part surrounds the disc and also forms the edge of the apothecium.
Germination of ascospores:
The ascospores may be simple or septate. 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 hypha grows into a new lichen thallus, if it comes in contact with an appropriate algal component.
Essay # Nature of Association of Lichens:
The nature of association of both the components of a lichen is quite controversial.
Following three different explanations have been given for the nature of association:
(1) Mutualism or Symbiosis:
According to some botanists the association in lichens is of symbiotic type because both the components (alga as well as fungus are mutually benefited). The fungal component absorbs water and minerals from the substratum as well as absorbs moisture and provides protection to the algal partner. In return the fungal component derives food from the algal cells.
The fungal component in the lichen association is the dominating partner. The algal component lives as a prisoner or as a subordinate partner. Some workers have suggested the term helotism for such type of association.
Workers like Fink (1913) have suggested that the fungus lives as a parasite on the algal partner. According to Geitler (1937), fungal hyphae give out haustoria and appresoria to absorb the food material from the algal cells but the algal partner is able to survive as an independent individual, if separated artificially from the fungal partner.
Economic Importance of Lichens:
Many different uses of lichens can be studied under the following heads:
(1) Useful Aspects:
(a) Ecological significance:
(i) Pioneer colonizers:
Lichens are said to be the pioneers in establishing vegetation on bare rocky areas (lithosere). They are the first members to colonize the barren rocky area. During development they bring about the disintegration of rock stones (biological weathering) by forming acids e.g., oxalic acid, carbonic acid etc. Thus, they play an important role in nature in the formation of soil (a phenomenon called pedogenesis).
(ii) Role in environmental pollution:
Lichens are very sensitive to atmospheric pollutants such as sulphur dioxide. They are unable to grow in towns, cities and around industrial sites such as oil refineries and brickworks. So, the lichens can be used as reliable biological indicators of pollution. By studying lichens on trees, a qualitative scale has been devised for the estimation of mean SO2 level in a given season. Thus lichens are used as pollution monitors.
(b) Food and Fodder:
The lichens serve as important source of food for invertebrates. A large number of animals for example, mites, caterpillars, termites, snails, slugs etc. feed partly or completely on lichens. Lichens as food have also been used by man during famines. They are rich in polysaccharides, certain enzymes and some vitamins.
Cetraria islandica (Iceland moss) is taken as food in Sweden, Norway, Scandinavian countries, Iceland etc. Lecanora esculenta is used as food in Israel and Umbilicaria esculenta in Japan. Species of Parmelia (known as rathapu or ‘rock flower’ in Telgu) are used as curry powder in India. In France the lichens are used in confectionary for making chocolates and pastries.
Cladonia rangiferina (Reindeer moss) is the main food for reindeers (a kind of deer) in polar countries. Cetraria islandica is also used as fooder for horses. Species of Stereocaulon, Evernia, Parmelia and Lecanora are also used as fodder.
(c) Source of Medicines:
Since very early times the lichens are used to cure jaundice, fever, diarrhoea, epilepsy, hydrophobia and various skin diseases. Various lichens are of great medicinal
A yellow substance usnic acid is obtained from species of Usnea and Cladonia. It is a broad spectrum antibiotic and is used in the treatment of various infections. It is effective against gram positive bacteria. Some lichen compounds e.g., lichenin, isolichenin have anti-tumour properties.
Protolichesterinic acid, a compound obtained from some lichens, is used in preparation of anti-cancer drugs. Erythrin obtained from Roccella montagnei, is used to cure angina. Many antiseptic creams such as Usno and Evosin are available in the market and are well known for their antitumour, spasmolytic and antiviral activities.
(d) In Industry:
(i) Tanning and dyeing:
Some lichens are used in leather industry. Cetraria islandica and Lobaria pulmortaria show the astringent property. This astringent substance is extracted from the thallus and is used in tannin industry. Lichens are also used in preparing natural dyes. Orchil, a blue dye obtained from Roccella and Leconara, is used to dye woollen articles and silk fabrics.
It is purified as orcum and used as a a biological stain. A brown dye is obtained from Parmelia spp. whereas Ochrolechia spp. yield a red dye. Litmus used as a acid-base indicator, is also a dye and is obtained from Roccella tinctoria and Lasallia pustulata.
(ii) Cosmetics and perfumes:
Evernia, Ramalina, Pseudorina are reported to have perfumed volatile oils. Due to the aromatic substances present in the thallus, the lichens are used in the preparation of various cosmetic articles, perfumery goods, dhoop, hawan samagris etc.
(iii) Brewing and distillation:
Some species of lichen for example, Cetraria islandica contain carbohydrates in the form of lichenin. In Sweden and Russia alcohol is produced from these lichens. These lichens are also used in confectionary.
Lecanora esculenta is found in lime stone deserts and yields large amount of calcium oxalate crystals. These are 60% of its dry weight.
(e) Natural products:
Lichens are known to produce over 550 natural products. Some important natural products are:
(f) Poison from Lichens:
Some lichens are poisonous due to presence of various substances in them:
(ii) Harmful Effects:
(a) Lichens growing on young fruit trees and sandal trees are harmful to the plant.
(b) During hot season some species of lichens (e.g., Usnea barbarata) become so dry and inflammable that they often help in spreading forest fire.
(c) Some lichens act as allergens.
(d) The commercial value of glass and marble stone is reduced because of itching of their surface by lichens.
(e) Some lichens e.g., Cladonia rangifera, Cetraria islandica accumulate large quantities of radioactive strontium (Sr90 ) and caesium (Cs137 ) from atomic fall-outs. These may be incorporated in the food chain, lichen → reindeer → man, leading to their accumulation in human tissues.
Ecology of Lichens:
The ecology of lichens refers to the place they hold in ecosystem. It is estimated that mainly 8% of the earth’s terrestrial surface is covered by lichens. Ecologically the lichens are very important because they are pioneer colonizers and helps in soil formation.
(a) Pioneer Colonizers:
They can grow on those surfaces where nothing else can grow for e.g., the only plants growing on a bare rock will be crustose lichens:
It is due to the facts that:
1. Lichens can tolerate irregular and extended period of severe desiccation. They are poikilohydric (pioilo-variable, hydric-related to water) meaning that they have little control over the status of their dehydration. They can tolerate irregular and extended periods of severe desiccation.
Like some mosses, liverworts, ferns and a few resurrection plants, upon desiccation lichens enter a metabolic suspension of static (known as cryptobiosis) in which the cells of lichens symbionts are dehydrated to a degree that halts most biochemical activity.
2. In this crypto-biotic state lichens can survive under extremes of temperature, radiation and drought in the harsh environments they often inhabit.
3. Lichens do not have roots and do not need to top continuous supply of water.
4. Lichens are smaller in size and have slow growth rate.
(b) Soil Formation:
During their growth, lichens slowly decompose substrate by forming acid. Crustose lichens patienty collect around and beneath itself tiny amounts of moisture, minerals and organic matter. When freezing temperature come, the water collected by lichens expands as it form ice and the expanding action again helps in the weathering of rocks.
Over a period of perhaps many years, even centuries, the lichens gather an extremely thin layer of soil around it. As the lichens grow, the soil-producing process speeds up and takes place over an large area. Eventually other more complex plants, perhaps foliose or fruticose lichens, mosses, ferns or even some flowering plants may take root in the modest soil and replace crustose lichens.
Physiology of Lichens:
Physiology of lichens refers to the study of the physical and chemical processes which go on in these living organisms. The basic physiology of lichens is that what determines their efficiency at growing in extreme environments. Because they lack roots, they are able to grow on rock surfaces and bark, relying on absorbing substances throughout their upper surface, which lacks a protective outer cuticle.
Minute pores are often present and the upper layer contains polysaccharides (carbohydrates), which attract water. The algae rest just below the surface also contains light-screening compounds to protect them under high level of ultraviolet radiation.
Green algae produce the sugar alcohol rabitol and cyanobacteria produce alucose (sugar), which are passed over to the fungus and rapidly converted to sugar alcohol mannitoal. This process ensures that lichens have the extraordinary ability to maintain themselves during very long dry periods, where they hardly metabolize at all.
This synthesis of chemical assenal and calcium oxalate crystals helps the lichens to survive under extreme conditions.
Some lichens contain upto 30 percent dry weight of organic compounds, which act as stress metabolites, others have antibiotic activity and they act as a deterrent to organisms that may prey on the slow-growing linchens. These chemicals also play an important role in keeping some lichen tissues dry to allow gaseous exchange and carbon fixation.