Lichens
The lichens are a distinct group of plants produced by the intimate association of two organisms, a fungus and an algae. The two organisms live in close association with each other so much so that they appear to be a single organism. The fungus taking part in the formation of lichens is called mycobiont and the algae component is called phycobiont. The phycobiont synthesizes organic food for itself and for the fungus.
There are about 500 genera and 20000 species (Wilson 1992) of lichens. They grow on the walls and roofs of houses, leaves, tree bark, dead wood or even in soil. Generally, they are xerophytic in nature. The plant body is thallus and grey green, orange, red, brown or black in colour. The thallus is irregular in shape. They have been classified into three types:
1) Crustose (Crustaceous) lichens
They are also called encrusting lichens. The thallus is flat, thin or thick without any distinct lobes. The thallus is often partially or completely embedded in the substratum from which it cannot be separated without damaging it. Eg: Graphis, Lecidea, Lecanora etc.
2) Foliose (Foliaceous) lichens
These lichens occur as flat and leaf like lobed thalli, attached to the rocks and twigs by means of rhizoid like outgrowths, called rhizines. These rhizines develop from the lower surface of the thalli. Eg: Parmelia, Peltigera, Collema etc.
3) Fruticose (Fructicose) lichens
Fruticose lichens have a much branched, cylindrical or ribbon-like thallus. These are attached to the substratum by their basal portions composed of strands of densely packed hyphae. Eg: Usnea, Cladonia, Ramalina etc.
Economic importance of lichens
- Ecological value: lichens are the pioneer plants in ecological succession because they grow on barren naked, rocky surfaces where other plants cannot grow.
- Food and fodder: Some species of lichens like Parmelia, Lecanora are used as food in different parts of the world.
- Medicines: lichens like Pertusaria & Cladonia species are used in the treatment of fever.
- Dyes: Evernia prunastri, Parmelia species dyes which are used for coloring fabrics and paints.
- Brewing and distilling: In Sweden and Russia, Cetraria islandica and Cladonia rangiferina are used to manufacture the alcohol.
- Bio-indicators of air pollution: lichens are used as bio-indicator for air pollution because they cannot survive in those where air pollutants occur in large amount.
Harmful aspects of lichens
- 1) lichens growing on young fruit trees and sandal trees are harmful to the plants.
- 3) The commercial value of glass and marble stone is reduced because of etching of their surface by lichens.
- 4) During hot season, some species of lichens become so dry and inflammable that they often help in spreading forest fire.