Lichens are composite organism (symbionts) of algae & fungi. Their alga & fungus components are respectively called as Phycobiont and Mycobiont. The phycobionts belong to cyanophyceae & chlorophyceae and mycobionts belong to Ascomycetes and Basidiomycetes. Lichens in India are found in Himalaya regions, moist shady regions, on the bark of trees and on the rocks with bryophyte plants.The vegetative part of Lichen is thallus.

The thallus is flat, leaf like and lobed. It resembles the dried liverworts of bryophate. The thallus has distinct dorsal and ventral surfaces. The dorsal surface is dark brown or light green whereas the ventral side is black in colour. The thallus is attached to the substratum! with the help of ventral rootlets called rhizines. The rhizines are single, branched or unbranched hyphae.The internal structure of the thallus is complex.

Internally, it has apper cortex, algae layer, medulla and lower cortex.Nutrition to lichen is provided by alga. The alga synthesize sugar by photosynthesis. This sugar is used by fungus. Hence, there is symbiotic nutrition. Vegetative reproduction in lichen takes place by fission & special structures called soredia & insidia.Sexual reproduction in lichen depends on fungus. If the associated fungus is member of Ascomycetes, the sexual reproduction is by ascospores.

The ascospores are formed in asci (singular – ascus) which are confined in funnel body called Apothecium Male reproductive organs are called sporogonium and female reproductive organs are called as carpogonium.

Types of Lichens:

They are of three types on the basis of shape & structure viz.

1.Crustose Linchens – Their thallus is flat and they grow lightly appressed to a substrate forming a biological layer of the adhering organism. They adhere very closely to the substratum at all points with the help of rhizines. The shape of the thallus may be round or oval or branched. The colour of the thallus is brown or red or yellow or black etc.

2.Foliose Lichens – These lichens are flat & lobed like leaf. The thallus is attached to the substratum with the help of rhizines which may be branched or unbranched. Some leafy lichens are attached with the help of hapteron. In some lichens, the rhizines have disc- like ends which secrete mucilase that help in attachement.
Example – Gyrophare, Parmelia, Collema etc

3.Fructose Lichens –  The Thallus of these lichens is highly branched & they are bush-like. They are Cylendrical or ribbon-like and branched.

The branches are hairy or leafy. Upper & lower surface are not distinct in the Thallus. The Thallus is attached to the substratum with the help of Flate disc. The Thallus is brownish green, yellowish green, Bluish-green or black.

Example – Usnea, Cladonia etc.

A. Useful Lichens:

•  Lichens develop as pioneer vegetation & begin the process of succession.
• Lichens are highly sensitive to pollution & have zero tolerance to SO₂ pollution. Hence, they are indicator of pollution.
• Some Liches are used as food for insects & catterpilar larvae.
• Liches are also used for treatment of various diseases. Such as Peltigera canina Lichen is used to treat hydrophobia. Parmelia saxatilis is also called as skull lichen and it is used to treat epilepsy.
• Thallus of lichen release special aroma (fragrance), hence is used in dhup batti (incense) & havan (Ritual).
• Some dried lichens are also used as spice.
• Lichen rock is popular as “Pathar ke bhool”.

B. Harmful Lichens:

• During summer the dried lichens become inflammable.
• They catch fire & result in forest fire.
• Some lichens are highly toxic & may be lethal.
• In humid & moist places, the lichens grow on the doors, windows, walls, floor etc & damage the building.