What Are Lichens? Definition, Classification, Structure

What are Lichens

Lichens are a small group of curious plants. They are made up of algal and fungal components, livings together in an intimate symbiotic relationship. The algal component is known as phycobiont (phy kos = alga, bios = life) and the fungal component as mycobiont (mykes = fungu (bios = life). The plant body of lichens neither resembles algae nor fungi. 

Thus, lichen is an association of a fungus and an algal photosynthetic symbionts, resulting in a stable thallus of specific structure. Phycobionts generally belongs to cyanophyceae or some times to chlorophy ceae. The alga is unicellular. The phycobiont is generally an ascomycete but in rare cases it is a basidiomycete.

Lichens were first discovered by Tulasne in 1892. The relationship between the two partners is a matter of controversy. Some hold it to be a typical case of symbiosis whereas others consider it to be parasitism. However, it is now considered to ba a case of helotism, a type of symbiotic association where the fangus has a upper hand. The lichens grow on a variety of habitats and are common on rocks, bark of trees, etc. Many of them grow under extreme condition of cold, humidity and drought. They are most conspicuous in the Alpine and Arctic Tundra where they are dominant form of vegetation. In India lichens are common in temperate and Alpine regions of Himalaya, hilly region of peninsular India and along the sea cost.

There are about 400 genera and 1600 species of lichens, widely distributed in most part of the world. Some common species are: Cladonia aggregata, Graphics duplicata, Gyrophora cylindrica, Haematomma, puniceum, Phystia aspera, Usnea, aspera and Usnea dischotoma.

Classification of Lichens

A) On the basis of their general growth, type of thallus and their mode of occurrence. Lichens are generally four types.

1. Crustose lichens (Encrusting Lichens)
2. Foliose lichens (leafy lichens)
3. Fruticose lichens (Shrubby lichens)
4. Leprose lichen

B) On the basis of the nature of the fungal element the lichens are divided into three groups.

1. Ascolichens if the fungal component is a ascomycetous. They are further divided into two sub groups –

• Pyrenocarpeate: Includes those lichens in which the ascocarp is a perithecium e.g.: Dermatocarpon.
• Gymnocarpear: Includes those lichens in which the ascoarp is an apotheciam. e.g. Parmelia.

2. Basidiolichens: If the fungal component is a basidiomycetous. e.g. cora, Corella, Dictyonema.

3. Deuterolichens: (Hymenolichens) Fructifications are absent in this group of lichens or should say that lichens with sterile thalli are constituted by this group. e.g. Lepraria, Leprocaulo, Crysothrix.

Structure of 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 constitutents.

Morphology

On the basis of growth forms, and nature of attachment to the substratum lichens are divided into following four types.

(1) Crustose lichens (Encrusting Iichens).

1. These lichens occur as thin or thick crust over rocks, soil or tree barks. 
2. It is very difficult to separate them from substratum.
3. The thalli may be wholly or partially embedded so that only fruiting bodies are visible above the surface of the substratum.
4. Common examples are Lecanora, Graphis, Rhizocarpon, Ochrolechia etc.

Lichens: A crustose

(2) Foliose lichens (leafy lichens) 

1. These lichens are variously lobed leafy structures. 
2. They are attached to the substratum by rhizoid like outgrowth called the rhizines.
3. The thallus is generally greyish or brownish in colour.
4. Common examples are Xanthoria, Parmelia, Physcia, Anaptychia etc.

Lichens: A foliose lichen

(3) Fruticose lichens (Shrubby lichens)

1. These are the upright or hanging lichens. (pendant forms) 
2. These are attached only at the base by a flat disc. 
3. These are cylindrical, flat or ribbon like, well branched and resemble with little shrubs e.g., Cladonia, Usnea, Alectoria etc.

Lichens: A fruiticose Iichen

(4) Leprose lichen:

1. A fourth type of lichen called leprose has also been differentiated.
2. It has some fungal hyphae surrounding one or more algal cells. 
3. A distinct fungal layer envelopes the algal cells all over.
4. It appears as a powdery mass over the substratum e.g., Leparia incana.

 A leprose lichen

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.

1. Heteromerous lichens
2. Homoiomerous Iichens

Heteromerous Lichens

1. A transverse section of the hetermerous (foliose) lichen can be divided into following distinct zones:

Lichens: Transverse section of heteromerous (foliose) Iichen thallus

• Upper cortex: It is the upper- most protective layer made up of compactly interworm fungal hyphae. The compactly interwoven hyphe produce a tissue like layer (Plectenchyma and pseudoparenchyma) called the upper cortex. The intercellular spaces are absent, if present, they are filled with gelatinous substances. In some species of foliose Iichens this layer is interrupted in different places. These interruptions or areas are known as breathing pores and serve for aerations. In addition to these certain other structures are also present for gaseous exchange. These are called cyphellae.
• 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 are having reproductive function.
• Medulla: It is present just below the algal cells and is made of loosely interwoven hypnal of fungus. Medulla forms the middle portion of the thallus.
• Lower cortex: Like the upper cortex, it is the lower-most layer. In some lichens the layer absent e.g., Lobaria pulmonaira. This layer gives rise to bundles of hyphae (rhizines) which penetrate the substratum to function as anchoring organs.

2. Different types of lichens particulary the foliose and fruticose remain attached to the substratum by a variety of structures such as itiizinose 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, Letharia). Hapters (short, penetrating branches. e.g. Alectoria) and medullary hyphae.

3. 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.

Homoiomerous Lichens

1. In some lichens for example, Collema, Leptogium, the thallus shows a simple structure with little differentiation.
2. The algae cells and fungal hyphae are uniformly distributed.
3. Both algal cells and fungal hyphae are enveloped in a gelatinous matrix.
4. Such type of lichens are called homoiomerous.

Lichens: Transverse section of homoiomerous lichen

Special structures Associated with Lichens

I. Soredia:

• They are small bud like out growth occuring on the upper surface or margin of the thallus as greyish powder.
• The soredia are separable portion of the thallus consisting of one or more algal cells surrounded by the fungal hyphale.
• A soredium may develop within definite pustule like compact structure called soralium.
• Each soredium develops into a new thallus.

II. Isidia:

• They also occur on the upper surface of the thalli as coral – like simple or branched growhts.
• They consist of an external cortical layer and an internal algal layer.
• The algal element within the isidia is the same as that of the parent thallus.

III.Cephallodia:

• They are external or internal gall like out growths, generally of dark colour.
• They consists of fungal hyphal enclosing algal cells different from those of the thallus.
• The Cephallodia are either, as flat orbicular discs or as coralloid branches or as irregular warts and tubers e.g. Lecanora, Lobaria and Peltigera respectively.

IV. Cephellae:

• They occure on the lower surface of the thallus quite commonly in the genus Stricta, as small hollow circular, white depressions with its base resting on the medulla.
• It's margin formed from the ruptured cortex projecting slightly inwards.

A-D Lichens: Asexual reproductive structures: A, B, Soredium, C. Cephalodium, D. Isidium

Reproductive Structures

A) Vegetative and Asexual

I. Fragmentation

• It commonly occurs by injury.
• Each fragment is capable to give rise to a new thallus.

II. Soredia

• After detached from the thallus, each soredium may develop into a new thallus.
• Examples are Usnea, Parmedia

III. Isidia

• Each detached isidium may develop into a new thallus.
• Common example is Peltigera sp.

IV. Oidia

• Hyphe of certain lichens break up into oidia.
• Each oidium germinate into new fungal hyphae and produces a lichen when comes in contact with suitable alga.

V. Pycniospores

• Many lichens produce large number of small spore like structures, the pycniospaces.
• Pycnidiospores are formed within flask-shaped pycnidium, immersed within the thallus.
• The hyphae lining the cavity of the pycnidium produce many pycniospores that are discharged through the astiole.

Physcia. V.s. Pycnidium to show pycniospores

B. Sexual Structures

In lichens the process of sexual reproduction is performed only by the fungal component. The fungal component of most of the lichens belong's to the class Ascomycetes. Hence the sexual reproductive structures and reproduction is similar to that of ascomycetous fungi.

The female sex organs

• The female sex organs are known as carpogonium.
• A carpogonium is differentiated into a basal coiled ascogonium and an elongated multicelleslar trichogyne.
• The ascogonium remains embedded within the algal layer of the thallus.
• The trichogyne projects over the surface of the thallus.

The Male sex organs

• The male sex organs are flask – shaped spermogonia.
• They form spermatia which function as male germetes.
• The Spermogonium usually developes close to carpogonium
• This enables spermatia to adhere to the projected part of sticky trichogyne.
• On dissolution of the walls between the spermatium and trichogyne, the nucleus of spermatium migrates into carpogonium through trichogune.
• The male nucleus fuses with the female nucleus.

A-B, Lichen : Reproductive structures ; A. Spermogonium, B. Carpogonium

Apothecia, Perithecia and Ascospores

1. Sexual reproduction results in the formation of apothecia or perithecia.
2. The fruiting bodies are small cuplike or disclike and may be embedded in or raised above the surface of the thallus by short or long stalks. 
3. The structure of the wall of an apothecium is similar to that of the thallus, it consists of an upper and lower cortical layer with medulla in between.
4. The algal components may or may not be present in the vegetative part of the apothecium. 
5. The bottom of the cup or the surface of the disc is the fertile part of apothecium and is lined by the hymenium.
6. The hymenium consists of asci and paraphyses growing vertically. Paraphyees contain a reddish oily substance in them and never projects beyond asci.
7. Each ascus contains eight ascospores. The ascospores become two called when they disseminate.
8. Ascospares when come in contact of suitable alga, produce, the lichen thallus.

A-B – Lichen: Structure of fruiting body; A. L.S. of apthecium B. A part of hymenium showing asci.

Summary

A lichen is a composite organism that arises from algae or cyanobacteria (or both) living among filaments of a fungus in a symbiotic relationship. The combined life form has properties that are very different from the properties of its component organisms. Lichens come in many colors, sizes, and forms. The combined life form has properties that are very different from the properties of its component organisms. Lichens come in many colors, sizes, and forms. Lichens may have tiny, leafless branches (fruticose), flat leaf-like structures (foliose), flakes that lie on the surface like peeling paint (crustose), or other growth forms. A macrolichen is a lichen that is either bush-like or leafy; Other lichens are termed microlichens. Here, "macro" and "micro" do not refer to size, but to the growth form. Lichens do not have roots that absorb water and nutrients as plants do but like plants they produce their own food by photosynthesis using sunlight energy, from carbon dioxide, water and minerals in their environment. When they grow on plants, they do not live as parasites and only use the plants as a substrate.

Some lichens have a portion of their thallus lifted off the substrate to form 'squamules'. They are otherwise similar to crustose lichens in that they possess an upper cortex but no lower cortex. Foliose Lichens have an upper and lower cortex. They are generally raised to some extent above the substrate but connected to it by rhizines (specialised root-like hyphae). They are easier to remove from their substrate when collecting because of this. Leprose lichens are an odd group of lichens which have never been observed to produce fruiting bodies. Because knowledge of the form of the fruiting bodies is essential to the identification of fungi, these lichens have not yet been identified properly, or at least not yet given full scientific names. These fungi not only lack an inner cortex, but also lack an outer one, i.e. no cortex, only an algal cell layer and sometimes a weakly defined medulla.

Glossary

• Apothecium (plural apothecia): One type of fruiting structure produced by the fungal component of the lichen. An apothecium is cup- or disc-shaped (compare with perithecium) and contains the spores, which allow for sexual reproduction.
• Cilia: Linear or thread-like appendages projecting from the thallus or apothecia margins, Cilia are the black, hair-like appendages pictured here along the margins of powder-edged ruffle lichen (Parmotrema stuppeum) thallus.
• Cortex: The protective outer wall of the thallus, composed entirely of fungal tissue. Lichens may have two cortices (upper and lower), a single cortex or no cortex at all, depending on growth form. Below the cortex is the photobiont.
• Crustose: A lichen growth form distinguished by the thallus being tightly adhered to the substrate at all points. Crustose lichens do not have a lower cortex, exposing the hyphae to the substrate. It is impossible to remove a crustose lichen from its substrate without impacting the substrate in some way.
• Cyphella (plural cyphellae): Small depressions or pits in the thallus cortex that are lined with cells (compare with pseudocyphella).
• Foliose: A lichen growth form distinguished by a relatively flat, leaf-like thallus. Foliose lichens have an upper and lower cortex, making it easy to identify an upper and lower thallus surface. 
• Fruticose: A lichen growth form distinguished by a tufted, hanging or stalked thallus. Fruticose lichens have a single, continuous cortex that wraps around the thallus branches, making it difficult to discern an upper and lower surface. 
• Hyphae: Fungal filaments loosely distributed below the photobiont on the interior of the thallus.
• Isidium (plural isidia): A structure that projects from the thallus and contains both fungal and algal components. An isidia can detach from thallus and therefore serves in vegetative reproduction. 
• Perithecium (plural perithecia): One type of fruiting structure produced by the fungal component of the lichen. A perithecium is flask-shaped (compare with apothecium) and often embedded the thallus, making it somewhat inconspicuous. A small hole at the top of the perithecium releases spores, which allow for sexual reproduction.
• Photobiont: The photosynthetic component of a lichen, either green algae or cyanobacteria, located beneath the cortex.
• Pseudocyphella (plural pseudocyphellae): Small depressions or pits in the thallus associated with cracks in the cortex. The cracks in the cortex are not lined with cells, distinguishing these features from cypselae. 
• Rhizines: Linear or narrow root-like appendages that protrude from the lower thallus surface (compare with cilia) and attach to the substrate.
• Soredia: A powdery or granular structure released from cracks in the thallus cortex. A soredia is essentially the photobiont (algal component) wrapped in fungal hyphae and therefore serves in vegetative, or asexual, reproduction. 
• Squamulose: A lichen growth form distinguished by small, overlapping thallus units or scales. Squamulose lichens are not as tightly appressed to the substrate as crustose lichens but are more appressed than foliose lichens. These lichens have an upper cortex but may or may not have a lower cortex.
• Thallus: The lichen body, which contains both a fungal and algal (photobiont) component.