Lichen: Classification,Structure,Reproduction and Economic Importance

In this article we will discuss about (1) Definition of Lichen (2) Classification of Lichen (3) Internal Structure of Lichen (4) Reproduction  (5) Economic Importance of Lichen

(1) Definition of Lichen

• Study  of lichen called lichenology.
• Lichens are composite thalloid structure containing algae and fungi.
• Lichens are distinct group of plant having two components i.e. algal components called Phycobiont and fungal component called Mycobiont.
• Algal component of lichen synthesize carbohydrates by photosynthesis and provides nutrition of fungi and themselves while fungal component helps in water absorption and water retention. Hence lichen is an example symbiosis.
• The term "Lichen" was first given by Theopharastus for superficial growth on bark of Olea europea (olive) tree.
• Erik Acharius – father of Lichenology.
• De Bary – gave this association the name Symbiosis.
• Crombie – gave the master-slave hypothesis for this association (also called husband-wife relation ship). It is also called helotism (Most accepted association now a days).
• In this association, the fungal component shows predominance over algal component and the later is a subordinate partner. Such type of association is known as helotism.

HABIT & HABITAT

• The lichen are mostly perennial, aerial, slow growing and long lived plant.
• Lichens are cosmopolitan. Lichens are even in areas which appear unsuitable for normal plant like bare hard rock and cold arctic region.
• Lichens are most sensitive to air pollution specially SO2-pollution.
• In India lichen are most common in eastern Himalaya as compare to western Himalaya Lichens are absent in planes.

• On the basis of habitat, lichen are of following type :

1. Saxicolous – Lichens growing on rocks. eg. – Dermatocarpon, Pornia
2. Corticolous – Lichens growing on tree bark eg. – Parmelia, Usnea
3. Terricolous – Lichens growing on soil eg. Cladonia, Collema.
4. Lignicolous – Lichens growing on wood eg. – Cyphelium.

• The lichen may also occur in fresh water eg. – "Hymenelia lacustris"
• Few marine species eg. "Caloplaca marina"

 (2) Classification of Lichen

Major part of lichens thalli are composed of fungal component.

I.    On the basis of their fungal component:

A. Ascolichen : - Fungal partner is the member of ascomycetes.

Algal partner is mostly member of green algae and rarely blue green algae.

Most of the lichens are Ascolichen

B.   Basidiolichen :  Fungal partner belongs to basidiomycetes.

In basidiolichen, algal partner is always a member of Myxophyceae (B.G.A.).

Example – Cora pavonia

Important Notes

• In 80% cases of lichen algal partner is member of green algae or chlorophyceae and in 20% cases blue green algae.
• 80% Lichen have Ascomycetes Fungus and 20% is Basidiomycetes.
• Important member of green algae  in lichens is Trebouxia  [most common unicellular green alga], Pleurococcus, Cladophora.
• Blue green algae – Nostoc, Scytonema, Anabaena, Gloeocapsa, Rivularia etc.

 II.  classification of the basis of types of thallus :

Lichens are of three types -

1.   Crustose lichen:

Thallus are flat. The thallus is closely adhered to the substratum and provides a crust like appearance.

These lichens are partially or completely embedded in to substratum.

These can't be separated from the substratum without breaking them.

Fruiting bodies are visible above the surface of the substratum.

e.g – Rhizocarpon, Graphis, Lecanora, Verrucaria, Haemotomma.

2.   Foliose lichen:

Thallus are flat leaf-like and lobed.

They are attached to the substratum with the help of rhizoid like rhizines.

These hariy structures are developed from lower side of thallus e.g. Parmelia, Peltiger, physcia and collema.

3. Fruiticose lichen

Thallus is well developed, shrub like, cylindrical and branched thallus.

They grow erect (Ex. Cladonia) or hang from substratum (Ex. Usnea).

They are attached to the substratum by their basal mucilagenous disc

Internal Structure

V.S. of foliose lichen -          

a.   Upper cortex – It is made up of compactly interwoven fungal hyphae. Intercellular spaces are absent. If present then filled with gelatinous substance. In some lichens (Parmelia) breathing pores are present on upper cortex.

b.   Algal layer – It occurs just below the upper cortex. This layer forms photosynthetic zone of thallus. In this layer algal cells are present and some fungal hyphae are randomly oriented. This layer also called "gonidial layer".

c.   Medulla – Beneath the algal zone there occurs medulla. It is made up of loosely interwoven  fungal hyphae with large spaces between them which are randomly oriented.

d.   Lower cortex – It is made up of compactly interwoven fungal hyphae some of these hyphae become specialized and extend downward from the lower surface of the cortex and helps in the attachment of thallus to the substratum. These specialized hyphae are known as rhizines.

Internal structure of curstose lichens is also more or less similar to foliose lichens.

The lower cortex does not occur in fruiticose lichens due to their cylindrical structure and medulla forms the central part of the axis.

Important Points :

(i)   Breathing pores – The upper surface of some lichen have loosely arranged areas called breathing pores. They help in gaseous exchange. eg. Parmelia

(ii)  Cyphellae – These are small, almost circular depression on the lower side of thallus (In foliose lichen). They are meant for exchange of gases. Cyphellae [cyphella-singular] are analogous to stomata of higher plants.

(4) Reproduction

These are of following type -

(i) Vegetative Reproduction

(a)  Fragmentation : The main thallus breaks into small pieces and each piece grows to form new lichen thallus.

(b)  Soredium :  Some small bud-like outgrowths, known as soredia, develop on the surface the thallus. A soredium contains one or few algal cells closely enveloped by a weft of fungal hyphae. They are detached from the thallus by the impact of wind or rain. The soredia germinate on suitable substratum and form new thalli.

(c)  Isidium : Isidia are small, stalked, greyish-black coral-like outgrowths which develop on the upper surface of the thallus. The isidium has an outer cortical layer enclosing the algal and fungal components. It is usually constricted at the base and is easily detachable from the parent thallus. It germinates under favourable conditions and forms new thallus.

In addition to propagation, isidia also help in increasing the photosynthetic surface of the thallus. They vary in shape may be rod like (e.g. Parmelia), coral-like (e.g. Peltigera), scale-like (e.g. Collema) or cigar like (e.g. Usnea).

(D) Cephalodium : These are small wart-like structures formed on the surface or inside of the thallus. One of the characteristic feature of cephalodium is that its algal and fungal components differ from that of the thallus. It is due to the fact that cephalodia develop on the younger parts of the thallus from soredia of some other species. Hence, the cephlodium may be regarded as sterile thallus of some other lichen. They retain moisture. In some species, the cephalodium contains the same fungal hyphae as in thallus but the algal component is always different.

(ii) Asexual Reproduction

(a)  Oidia : The fungal hyphae form small thin walled bodies called oidia. On coming in contact with suitable algal component they form new lichen thallus.

(b)  Conidia : In several lichens, the fungal component form conidia. Conidia produced on conidiophores.

(c)  Pycnidiospores : These spores are formed in flask shaped structures, called Pycnidium. Pycnidiospores form new fungal mycelium and Mycelium form new lichen in contact with suitable algal component.

(iii) Sexual Reproduction

• Sexual reproduction is the function of fungal component.
• The fungal component of most of the lichens belongs to the class ascomycetes.       
• Female sex organ is called "Carpogonium".
• Carpogonium differentiated into two parts – lower coiled part is called "ascogonium" and upper straight part is called "trichogyne".
• The ascogonium remains embedded with in the algal layer of the thallus, whereas the trichogyne projects over the surface of the thallus.
• Male sex organ is called "Spermogonium"
• It is flask shaped. It open out side by ostiole.
• Inside spermogonium non motile sperms or spermatia are formed.
• The spermatia are ooze out through ostiole in the form of slimy mass and attach with the trichogyne and wall of both soon dissolves.
• Nucleus of spermatia reaches in ascogonium.
• Many "Ascogenous hyphae" arises from the fertilized ascogonium.
• These cells are uni or binucleated and the terminal or penultimate (sub-terminal) binucleate cell of the ascogenous hyphe develops into an ascus. In ascus both nuclei fuse to form a diploid nucleus.
• In ascus one meiosis and one mitotic division take place and 8-ascospores are formed in each ascus by paraphysis.
• Asci (Ascogenous hyphae, ascus mother cells, ascospores) and some sterile hyphae form fruiting body called "apothecium" eg. Parmelia or Perithecium eg. Dermatocarpon, Verrucaria.
• Ascospores are liberated from ascus and on coming in contact with suitable algae form new lichen thallus.

(5) Economic Importance of Lichen

1.   Ecological importance :

• The lichens are pioneer of vegetation in lithosere (succession on rocky surface). 
• In Lithosere – sequence of different stages is –
• Lichen initiates biological weathering of rocks by forming acids like Carbonic acid, Oxalic acid.
• Biological weathering of rocks leads to the formation of soil i.e. helpful in pedogenesis. So Lichens are farmer of nature.

2.   As Food & Fooder : -

• Cladonia rangiferina (reindeer moss) in tundra region (adapted to low temperature habitat).
• Cetraria icelandica (iceland moss) used in iceland.
• Umbillicaria is used as food in Japan.
• Endocarpon miniatum is used as vegetable in Japan.
• Lecanora esculenta is used as delicious food in Israel
• Evernia used in bakery products by Egypt.
• Species of parmelia are called "rock flower" and used as food in South India.

3.   Lichen as a source of dye :

• Litmus (acid-base indicator) is obtained from Rochella montagnei, Lasallia pustulata and Rochella tinctoria.
• An orchil (blue dye) is obtained from Rochella and Lecanora and Purified form of orchil is "orcein" and used as biological stain (Histological studies).

4.   Lichen as source of medicine :

• "Usnic acid" is important broad specturm antibiotic and it is obtained from Usnea and Cladonia.
• Parmelia saxatilis is used against curing epilepsy.
• Peltigera canina is used aganist hydrophobia.
• Xanthoria parientina is used aganist jaundice.
• Loberia pulmonaria is used against lung infection.
• Cetraria is used as Laxative.
• The "protolichesterinic acid" obtained from some lichen. This acid have anticarcinogenic properties.

5.   Brewing and distillation :

The lichen contains carbohydrate in the form of "lichenin". So some lichen like Cetraria and Lecanora are used for alcoholic fermentation.

6.   In cosmetics and perfumes :

Some lichens have aromatic substance like Evernia, Ramalina are used for manufacturing soap.

7.   Poisonous lichen :

Cetraria juniperina and Letharia vulpina (wolf moss) are used as poison.

8.   Indicator of air pollution :

Lichens are very sensitive to SO2 and die at higher level of SO2.

So lichens are not found in industrial areas where atmosphere is polluted by smoke (specially SO2), So lichens are biological indicator of air pollution.

9.   Forest fire :

Usnea, popularly known as "old man's beard", is of inflammable nature. They are responsible for fires in the forests during summer season.