The lichens which are broad like leaves with irregular surface, flat, broad, divided edges and many grooves are called foliose lichen. They are also called foliar lichens. Rhizine is produced from its lower surface. Such as Xanthoria calcicola, Peltigera canina, Parmelia Cetraria, Parmotrema tincotorum, Collema tenax, Flavoparmelia caperata, Zanthoria peltigera, Physcia caesia etc.

Those lichens which are intimately attached to the habitat and form a thin, flat and hard covering are called crustose lichens. They are also called lichens. Like- Graphis scripta, Lecanara muralis, Strigula angulul, Cryptothecia rubrocinta, Diploicia canescens, Calicium indicum, Pyrenula micheneri, Arthonia radiata, Trypethelium virens, Xylographa vitiligo etc.

This is the simplest lichen. A single hyphae of the fungus envelops the alga. The fungus looks like a molecule or powder in the container. For example – Lapraria incana.

1. Structural classification
In 1984, scientists Harsworth and Hill divided lichens into five groups based on structure.
(i) Leprose lichen : This is the simplest lichen. A single hyphae of the fungus envelops the alga. The fungus looks like a molecule or powder in the container. For example – Lapraria incana.
(ii) Crustose lichen : Those lichens which are intimately attached to the habitat and form a thin, flat and hard covering are called crustose lichens. They are also called lichens. Like- Graphis scripta, Lecanara muralis, Strigula angulul, Cryptothecia rubrocinta, Diploicia canescens, Calicium indicum, Pyrenula micheneri, Arthonia radiata, Trypethelium virens, Xylographa vitiligo etc.
(iii) Foliose lichen : The lichens which are broad like leaves with irregular surface, flat, broad, divided edges and many grooves are called foliose lichen. They are also called foliar lichens. Rhizine is produced from its lower surface. Such as Xanthoria calcicola, Peltigera canina, Parmelia Cetraria, Parmotrema tincotorum, Collema tenax, Flavoparmelia caperata, Zanthoria peltigera, Physcia caesia etc.
(iv) Fruticose lichen : Cylindrical, filamentous and branched hanging lichens with attractive and complex structures are called fruticose lichens. They are also called flowering lichens. Such as Cladonia leporina, Usnea barbata, Alectonia sarmentosa, Letharia columbiana, Evernia prunastri etc.
(v) Filamentous lichen : The lichen in which the algal part is filamentous is called filamentous lichen. Eg Ephebe lanata, Racoleus trichophorus etc.
2. Habitat classification
(i) Corticolous : This lichen grows on the bark or stems of trees in tropical regions. Like- Graphis scripta, Evernia prunastri, Parmelia sulcata, Usnea barbata etc.
(ii) Terricolous : They grow in moist and upland soils. For example – Collema tenax, Cladonia leporina, Lecidea atrobrunnea, Cora pavonia etc.
(iii) Sexicolous : They grow on rocks or rocks in temperate regions. Such as Xanthoria calcicola, Aspicilia contorta, Dermatocarpon miniatum etc.
(iv) Lignicolous: They grow on wet wood. Such as Calicium lenticulare, Chaenotheca trichialis, Cyphelium notarisii, Piptoporus betulinus etc.
(v) Foliicolous : They grow on the leaves of ferns or succulent plants. Like Porina epiphylla, Strigula angulul etc.
(vi) Omnicolous : They grow on leather, wood, iron etc. For example, Lecanora dispersa.
3. Classification based on lichen forming fungi
(i) Ascolichen : If the lichen forming fungus belongs to the class of Ascomycetes then it is called ascolichen. E.g. Rhizocarpon macrosporum.
(ii) Basidiolichen : If the lichen forming fungus belongs to the class Basidiomycetes then it is called Basidiolichen. For example – Clavaria mucida.
(iii) Deuterolichen : If the lichen-forming fungi belong to the class of Deuteromycetes, they are called deuterolichens. Such as Racodium rupestre, Leparia incana etc.
4. Structural classification
(i) Homoiomerous : Lichens which contain algae and fungi equally are called homoiomeric. For example – Collema tenax.
(ii) Heteromerous : Those lichens which have algae and fungi unequally are called heteromerous lichens. For example- Parmelia sulcata.

Lichens can collect nutrients very quickly from air and rainwater. It rapidly absorbs sulfur dioxide, heavy metals, radioactive materials etc. Lichens die as a result of absorbing these pollutants. Therefore lichen numbers are low in air pollution areas. A low number of lichens can confirm that the environment is slowly becoming polluted. That is, lichen works as an indicator of environmental pollution.

1. Lichens are dual organisms. Because, it is made up of two organisms, Algae and Fungi.
2. It is autophagous and self-sufficient.
3. It is heterozygous, flattened, monoecious and non-flowering.
4. The fungus forms the thallus structure and the algae are encased inside the structure.
5. It is grey, grey-green, white, yellow, fawn, green, red, pink or black in colour.
6. The algal member accounts for 5–10% of the mass of the lichen thallus.
7. They can range from tiny to several feet.
8. Lichens can grow in all environments of the world. Hence they are called Cosmopolitan plants.
9. They can be homomerous and heteromerous.
10. They have rhizines like rhizoids. Rhizine absorbs water.
11. Their reproductive function is usually controlled by fungi.
12. Lichens are highly sensitive to air pollution.
13. They reproduce in organ, asexual and sexual ways.
14. In sterile sterile media, lichens can grow where no organisms can grow.
15. It acts as a precursor to soil formation in hard rock.

Lichen is called Cosmopolitan plant. Lichens can grow in environments where no other organisms can grow. Lichens grow on barren soil, sand, stones, walls, wood shavings, tree bark, living leaves, etc. It can grow in flat mountain ranges, tundra regions and desert regions.

In lichens, the algal member belongs to the class Myxophyceae and the fungal member belongs to the class Ascomycetes. The algal member of the lichen is called the phycobiont or photobiont and the fungal member the mycobiont. In the coexistence of algae and fungi, fungi are more advantageous than algae. This position is called helotism as the algae live as slaves to the fungi. This symbiosis is called partial parasitism as the fungus collects nutrients by means of absorbents called haustoria. Some botanists have identified the position of fungi in lichens as controlled parasites on algae.

Lichen is derived from the Latin word Leichen meaning algae-like fungus. The new thallus formed by the coexistence of algae and fungi is called lichen. Lichens are called mitozoites.
In 1803, scientist Eric Acharius first described lichens and is considered the discoverer of lichens. Among the modern researchers, V. Ahmadjian has done the most research on lichens and is called the father of modern lichenology. 17,000 species of lichens of 400 genera have been found in the world and scientists estimate that their number will be 20,000.

The word Symbiosis is formed from the Greek words Sym meaning together and bios meaning living. If two organisms of different species living together do not harm each other, but only benefit, then it is called Symbiosis or Mutualism. Algae and fungi form mythos. The algal member is called the phycobiont or photobiont and the fungal member is the mycobiont. Both algae and fungi benefit from mitosis. Mythology explained.
How Algae Benefit
1. Contains water vapor from lichens.
2. The fungus takes in the CO2 produced by respiration.
3. Absorbs mineral salts from fungi.
4. Taking shelter in the body of the fungus.
How fungi benefit
1. Algae consume food produced in the process of photosynthesis.
2. Can breed easily. Dr. Siddique Publications
3. Can overcome adverse environment.
4. Can live in any environment of the world.