LICHEN

Prepared by Dr. Rukhshana Parveen

Assistant Professor, Department of Botany Gautum Buddha Mahila College, Gaya

Magadh University, Bodhgaya

The term Lichen was first used by Theophrastus (371-284 B.C.) Lichen are symbiotic relationship between an algae or cynobacteria & a fungi .The

algal component is known as phycobiont or photobiont (Greek, phykos= alga, bios= life) & fungal component known as mycobiant (Greek, mykos = fungus, bios = life ).

The two components interwined so as to form a single thallus. These are self reproducing functional unit.

Hawksworth et al. (1983) defined lichen as ‘As stable self supporting association of a mycobiont and a photobiont”.

Found growing in almost all parts of the terrestrial world. Based on the types of habitat colonised, Lichen are grouped into six

types:- Corticolous: - On the bark of trees. (Usnea, Graphis, Alectoria, species of

Parmelia) Lignicolous: - On the wood directly. (Calicium, Chaenotheca, Cyphelium) Saxicolous: - On rocky substrata. (Dermatocarpon, Pornia, Xanthora) Terricolous:- on the ground. (Lecidea granulose, Cladonia floerkeana, Collema

tenax) Marine: -On siliceous rocky shore of sea. (Caloplaca marina, Verrccaria mucosa) Freshwater: - On hard siliceous rocky in fresh water. (Hymenelia lacustris

Classification of Lichen

There is no natural system of classification of Lichen. Only on the basis of the structure of fruiting bodies of fungal components Zahlbruckner (1907, 1926) classified Lichens into two groups:-

1. Ascolichens:- The fungal component belongs to Ascomycetes. These are further divided into two sub groups.

Pyrenocarpeae=> Flask shaped fruiting body called Perithecium. (Dermatocarpon)

Gymnocarpeae=> Disc shaped fruiting body called Apothecium. (Parmelia)

2. Basidiolichens: - The fungal component belongs to Basidiomycetes. (Corella, Dictyonema) Poelt (1973), Hessen and Jahns(1973) also gave classification of lichen.

Alexopoulos and Mims (1979) classified Lichens into three groups:-

1. Basidiolichens=> The fungal component is a member of the class Basidiomycetes. (Agaric)

2. Deuterolichens=> The fungal component is a member of the class Deuteromycetes. (Fusarium)

3. Ascolichens=> The fungal component is a member of the class Ascomycetes. (Sarcoscypha)

Structure of Lichen

External structure

The plant body is thalloid and irregular in shape. Grey /greyish in colour but some species are yellow, red, orange or brown in colour. Based on external structure Lichens are divided into three types.

1. Crustose Lichen: These are encrusting lichens with thin and flat thallus, firm in texture. Thallus is very closely adhered to the substratum and provides crust like

appearance. Wholly or partially embedded in the substratum, only fruiting bodies are

visible above surface. Example: - Graphis, Haematomma, Lecanora, Lecidia, Rhizocarpon,

Strigula, Verrucaria, Buellia, Dimerella, Caloplaca.

2. Foliose Lichen : Lichens are flat with leaf like and lobed thallus. They are attached to the substratum with the help of rhizoid like structure

called rhizines Example: - Xanthoparmelia substrigosa Parmelia, Physcia, Peltigera,

Xanthoria, Gyrophora, Collema, Chaudhuria.

3. Fruticose Lichen:

These lichens are well developed, shrub like, cylindrical and branched thallus.

They grow erect or hang from the substratum. The plant body is attached to the substratum with help of a basal

mucilaginous disc. Example: - Usnea, Cladonia, Alectonia, Letharia, Bryoria.

4.Leprose lichen:

Single or small cluster of algal cells are enveloped by fungal hyphae forming simple thallus providing a powdery appearance.

No distinct fungal layer. Example:- Leprasia incana.

5.Filamentous Lichen:

The algal partner is filamentous, well developed and remains ensheathed or covered by only a few hyphae.

Example:- Coenogonium, Racodium, Cystocoleus.

Figure1. Types of Lichen Thalli

Intrenal Structure

The internal structure of Lichen is very complex. The thallus is composed of fungal and photobiont cells, so well united as to give the

impression that you are looking at just one organism.

On the basis of internal structure mainly, the distribution of algal component among fungal component two types of foliose lichen are recognized. 1. Homoiomerous type:-

The algae are more or less uniformly distributed. The algae are gelatinous and belong to Myxophyceae. The fungi form the inter protective layer of the thallus. Example:- Collema, Leptogium

Figure2. Homoiomerous Type

2. Heteromerous type:-

The thallus in cross-section shows upper cortex, algal layer, medulla and lower cortex.

Upper Cortex=> Outer most thick and protective zone of the thallus composed of compactly

interwoven fungal hyphae. Hyphae are arranged at right angle to the surface of the thallus. Have no inter cellular spaces between hyphae. If space present then filled with

gelatinous substance..Breathing pores are present in Parmelia for gaseous exchange. Algal Layer=> Found below upper cortex.

Also called as gonidial layer. Algal cells are embedded in between tangled network of loosely interwoven hyphae.

Medulla=> Central part of the thallus. Loosely interwoven fungal hyphae with large space between them.

Lower Cortex=> Composed of compactly arranged hyphae which run parallel or perpendicular to the surface of the thallus.

Some hyphae extended downward from lower cortex and form root-like bundles called rhizines, which help in anchoring the thallus to the substrate.

Structure of crustose and fructicose lichen is also similar but in fructicose only lower cortex absent due to their cylindrical structure.

Figure3. Heteromerous Type

Reprodcution in Lichen

Lichen reproduces by vegetative, asexual and sexual means. A. Vegetative reproduction

1. Fragmentation: On maturity the older portions of the thalli of lichens die and decay.

The thallus breaks into pieces accidentally and each piece develops into a new plant. Ramalina reticulata.

2. Isidia:

Isidia are small, stalked, greyish, black, coral like outgrowth which develop the surface of the thallus.

Isidium has outer cortical layer enclosing algal and fungal components.

Figure4. Lichen: - V.S. of thallus showing Isidia

3. Soredia:

Small buds like outgrowths called Soredia develop on the surface of the thallus which is greyish white in colour.

Soredium contains one or few algal cell enveloped by fungal hyphae. When detached, germinate to form new thallus.

Parmelia develop in organised manner called Soralia.

Figure5. Lichen: V.S. of thallus showing Soredia

4. Cephalodium:

These are small wart like structure formed on the surface of thallus.

The algal and fungal components are different in cephalodium than that present in thallus because it develop on younger part of thallus.

Figure6. Lichen: V.S. of thallus showing Cephalodia

B. Asexual spores

1. Conidia: Many lichens produce conidia in flask shaped pycnidia immersed in the thallus. Conidia on being dispersed, germinate under favourable conditions by sending out

hyphae.

Figure7. Lichen: V.S. of thallus passing through Pycnidium

2. Oidia: The hyphae of some lichen beak up into small bodies called oidia, which germinate

into new hyphae. c. Sexual reproduction

Sexual reproduction is exhibitied only by mycobiont. 1. In Ascolichen:-

Female sex organ is called Carpogonium. Basal coiled ascogonium (embedded within algal layer) and upper elongated multicellular tricogyne (projects over the surface.)

Figure8. Lichen: Female sex organ

Male sex organ is called Spergonia (flask shaped). It forms male gamete called Sermatia. It develop close to carpogonia . This enables spermatia to adhere to the projected part of sticky trichogyne on dissolution of walls, nucleus of spermatia migrates to female nucleus and fuse with it.

Figure9. Lichen: Male sex organ

Several branched ascogenous hyphae form base of fertilized ascogonium. Ascogeouns hyphae -> Ascus-> Two nuclei fuse to form diploid nucleus -> First

divide meiotically then mitotically forming eight haploid daughter nuclei. These eight haploid nuclei act as eight ascopore. Paraphysis is present. Somatic tissues and paraphysis form fruiting body. These fruiting bodies are of two types.

1. Apothecium ( Disc shaped ) eg. Parmelia

2. Perithecium(Flask shaped) eg. Dermatocarpon.

Ascopore =>Vary in shapes, size, structure and septation. These are Hyaline. Ascopore produce hypha, when these hypha found alga, then it form new lichen.

Figure10. Lichen: V.S. of Apothecium structure and portion of Hymenium

showing Ascospores

2. In Basidiolichen:- It is presumed that the bdiosres are formed in the same way as in other Basidiomycotina. Each basidium bears four sterigmata and four basidiospores along with the paraphyses. The basidia form the hymenium.

Economic Importance of Lichen Lichens are wonderful gift of nature. They are the source of food, medicines, flavours and stains.

Useful activities of lichen 1. As food and fodder

Lichens are important constituents of food in North Polar Tundra and Eastern Siberian region.

Species of Lecanora, Parmelia (as curry powder in India), Umbillicaria (used in Japan) and Cetraria are used as food.

Evernia prunastri are used for making breads in Egypt. In France lichen are used for making chocolates, pastries. Endocarpon miniatum used

as vegetables. Aspicilia calcarea, Lecanora saxicola are used as food by mites, snails, caterpillars

and termites. Lobaria pulmonaria, Evernia prunastri, Ramalina fraxinea and Ramalina fastigiata

are used as fodder for animals because of great nutritive value due to presence of Lichenin.

Cladonia rangifera serves as food for reindeer and musk ox. Dries lichens are fed to horses and swans.

2. As medicine Medicinal value of lichen is due to the presence of Lichenin and some

astringent substances. Some lichens are used in the treatment of bile, diarrhoea, fever, nervous

disorders, and hydrophobia and skin diseases. Parmelia is used in snake and scorpion bites. Cladonia, Cetraria and Pertusaria in intermittent fever. Cladonia pyxidata in whooping cough. Species of Usnea stop bleeding. Usnic acid obtain from Usnea and Cladonia used in infections. Cetraria islandica used in tuberculosis and other lung diseases.

3. As dyes Red and purple dye obtain from Ochrolechia androgyna, Ochrolechia tartaria Orchil a blue dye obtain from cetraria icelendica . Brown dye obtained from Parmelia omphalodes. Litmus an important acid base indicator dye in chemical laboratories obtains

from Roccella montagnei and Lasallia pustulata.

4. In tannin industry Cetraria icelandica, Lobaria pulmonaria used as tannin agent in leather

industries.

5. In cosmetic and perfumery Evernia and Ramalina are source of essential oils used in manufacturing of

cosmetic soaps. Lichens are used in hawan samagries, dhup, perfumeries because of pleasant

smell.

6.In fermentation and distillation process In Russia, Sweden and Siberia liquors are manufactured by fermentation and

distillation of lichen like Cladonia rangiferina and Ramalina fraxinea.

7. As culture media Lichens are important constituents of some culture media for the culture of

fungi and bacteria in laboratories.

8. Ecological importance Lichens are pioneer plants which help in colonisation of bare rocky habitat. Lichens serve as indicators of air pollution.

9. As nitrogen fixers

Lichens with with blue green algal phycobionts play an important role in provining fixed nitrogen in certain ecosystem.

Harmful activities of Lichen Lichens cause loss of the surface of glasses and marble stones. Long threads of pendant lichens like Usnea are inflammable, thus help in spreading

forest fire. Lichen like Cladonia cause total destruction to the colonies of mosses by direct

parasitic attack

Books Cited : -

1. Saxena, R.S. 2018. Text Book B.Sc. Botany-1. 2. Singh, A. P. and Pushkar, Kumar. Life Sciences. 3. Singh, R. K. 2018. Fundamentals of Botany. 4. Singh, V., Pande , S.N. and Jain 2010. Text Book of Botany.