Brief introduction to Lichens

Dr. Pallavi

J.N.L. College Khagaul

• Lichens are complex life forms that exists in nature. They comprise of twodifferent organisms living in close proximity, fungal partner (mycobiont)and algal partner (phycobiont). The main body is provided by fungalfilaments while algal cells are dispersed in between these filaments.

• They are different then the normal symbiotic association as theymorphologically and functionally behave different then their individualcomponents.

• The algal partner is usually a green alga, suchas Trebouxia, Pseudotrebouxia, or Myrmecia, or is often a cyanobacterium,such as Nostoc or Scytonema. The fungal partner may be an Ascomycete(mostly) or Basidiomycete.

• Lichens are cosmopolitan and grow best in moderate temperature,sufficient, humidity and direct sunlight. However, they can withstandextremes of climate (cold, heat and drought) and thus are foundeverywhere ranging from hot deserts to chilly mountains.

• 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., Graphis, Usnea, Parmeliaetc.

• 3. Terricolous. Grow on soil (terrestrial) e.g., Cladonia florekeana, Lecidea, Collema etc.

• 4. Lignicolous. Grow directly on wood e.g., Calicicum etc.

• the lichens can be divided into following 2 sub-classes on the basis of their fungal partner:

• (A) Ascolichens. Fungus belongs to Ascomycetes.

• (B)Basidiolichens. Fungus belongs to Basidiomycetes. There are only 3 genera which belong to Basidiolichens. These are: Cora, Corella, Dictyonema.

Thallus Organization in 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.

• External Form:

• 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.

• (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.

• 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 .

• Internal Structure:

• 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:

• (1) Upper cortex,

• (2) Gonidial layer,

• (3) Medulla and

• (4) Lower cortex.

• (1) Upper 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.

• (2) Gonidial layer:

• 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.

• (3) Medulla:

• 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.

• (4) Lower cortex:

• 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.

• 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.

Reproduction in lichens(A) Sexual reproduction

• In lichens, only the fungal partners is able to reproduce sexually. Since, the algal component found in lichens can be found free-living as well and could then reproduce sexually however, within a lichen sexual reproduction of the photobiont is suppressed.

• The sexually reproducing lichens are either ascomycetes or basidiomycetes. Ascomycetes produce their sexual propagules (called ascospores) and basidiomycetes produce their sexual propagules (called basidiospores). If new thalli are to be formed then these spores must capture the compatible photobionts, on coming in contact with the suitable algal species, new lichen is able to develop.

Asexual reproduction• 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.

• 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 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. Sorediaarise 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.

• 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.

Importance of lichens

• Early Colonisers-They grow on rocks and facilitate the process of soil formation thus are always the first step of ecological succession

• Food-In tundra, Cladonia rangifera (Reindeer Moss) constitutes the staple food of reindeer, caribou, musk ox, etc. Certaria islandica (Iceland Moss) is used as a food article in Iceland, Sweden and Norway. Lecanora esculenta is regarded as bread of heaven by Jews. In Bellary district, Parmelia (Rock Flower) is a table delicacy. Endocarpon miniatum (Stone Mushroom) is a vegetable in Japan.

• Air pollution indicator as can not survive high level of Sulphur in the air.

• Source of litmus used in acid base indicator in chemistry eg: Rocella tinctoriay.

• Lichens add significantly to the aesthetics of the ecosystems in which they occur. The lovely orange and yellow colors of Caloplaca and Xanthoria lichens add much to the ambience of rocky seashores and tundras.

• Medicines:

• Usnic acid got from Usnea and Cladonia has antibiotic properties. It is used in preparation of ointment for bums and wounds. A number of drugs were obtained in older times from lichens.