Class 11 Biology – Lichens Notes
Complete notes on lichens, their structure, types and economic importance for Class 11 Biology.
Introduction to Lichens
Lichens are a distinct group of plants formed by the intimate association of two organisms — a fungus and an alga.
The two organisms live in such a close association with each other that they appear to be a single organism.
The fungal component of a lichen is called the mycobiont, whereas the algal component is called the phycobiont.
The major portion of the lichen thallus is composed of the fungal component, while the algal component is generally embedded within it.
The phycobiont synthesizes organic food for itself as well as for the fungus.
In return, the mycobiont protects the alga, absorbs water and minerals for the lichen, and attaches the thallus to the substratum.
Occurrence and General Characteristics
There are about 500 genera and approximately 20,000 species of lichens.
Lichens may grow on:
- Walls and roofs of houses
- Leaves
- Tree bark
- Bare rocks
- Dead wood
- Soil
Generally, lichens are xerophytic in nature.
Their plant body is a thallus. The thallus may be grey, green, orange, red, brown or black in colour and is often irregular in shape.
Types of Lichens
According to the form of their vegetative body, lichens are classified into three major types:
1. Crustose (Crustaceous) Lichens
Crustose lichens are also known as encrusting lichens.
Their thallus is flat and may be thin or thick, without distinct lobes.
The thallus is often partially or completely embedded in the substratum from which it cannot be separated without damaging it.
Examples: Lecanora, Graphis and related crustose forms.
2. Foliose (Foliaceous) Lichens
Foliose lichens possess a flat and leaf-like lobed thallus.
They are attached to rocks and twigs by means of rhizoid-like outgrowths known as rhizinae.
These rhizinae develop from the lower surface of the thallus.
Examples: Parmelia, Peltigera and Collema.
3. Fruticose (Fruticulose) Lichens
Fruticose lichens have a much-branched, cylindrical or ribbon-like thallus.
They are attached to the substratum by their basal portions, which consist of strands of densely packed hyphae.
Examples: Usnea, Cladonia and Ramalina.
Comparison of the Three Types of Lichens
| Type | Main Character | Attachment | Examples |
|---|---|---|---|
| Crustose | Flat, crust-like thallus without distinct lobes | Closely attached or embedded in substratum | Lecanora, Graphis |
| Foliose | Flat, leaf-like and lobed thallus | Attached by rhizinae | Parmelia, Peltigera, Collema |
| Fruticose | Branched, cylindrical or ribbon-like thallus | Attached by basal portions | Usnea, Cladonia, Ramalina |
Economic Importance of Lichens
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Ecological Value
Lichens are regarded as pioneer plants in ecological succession because they can grow on barren and naked rocky surfaces where many other plants cannot grow.
They gradually disintegrate rocks and contribute to the accumulation of minerals and organic matter.
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Food and Fodder
Some species of lichens such as Parmelia and Lecanora are used as food in different parts of the world.
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Medicinal Uses
Some lichens have traditionally been used for medicinal purposes.
- Peltigera and Cladonia species have been used in the treatment of fever.
- Usnea and Cladonia species have been used in preparations for burns and wounds.
- Peltigera has traditionally been associated with treatment of liver disease and hydrophobia.
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Dyes
Species of Evernia prunastri, Parmelia and Ochrolechia yield dyes used for colouring fabrics and paints.
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Perfumes
Lichens such as Evernia prunastri and Lobularia are used in perfumery and in the manufacture of scented incense and other cosmetic perfumes.
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Brewing and Distilling
In Sweden and Russia, Cetraria islandica and Cladonia rangiferina have been used in the manufacture of alcohol.
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Bio-indicators of Air Pollution
Lichens are used as bio-indicators of air pollution because they cannot survive well in areas where air pollutants occur in large amounts.
Quick Revision
- Lichen is an association between a fungus and an alga.
- The fungal component is called the mycobiont.
- The algal component is called the phycobiont.
- The phycobiont prepares food through photosynthesis.
- The mycobiont provides protection, water, minerals and attachment.
- The three main forms are crustose, foliose and fruticose.
- Lichens act as pioneer organisms during ecological succession.
- Lichens can be used as bio-indicators of air pollution.
Important Questions
- What are lichens?
- What are the fungal and algal components of a lichen called?
- Explain the relationship between mycobiont and phycobiont.
- Classify lichens on the basis of their vegetative body.
- Write the characteristics of crustose lichens.
- Describe foliose lichens with examples.
- Describe fruticose lichens with examples.
- Explain the ecological importance of lichens.
- Write the economic importance of lichens.
- Why are lichens called bio-indicators of air pollution?
Discussion
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