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              STUDY OF LICHEN DIVERSITY 

Introduction:

             Lichen appears to be a single plant but is formed due to intimate association of two different plants, one of which is a fungus and the other an alga.

             The fungal component of lichen is known as "mycobiont" and the algal component are is called as "phycobiont".They grow slowly and hence their food requirements are rather low. The water and minerals are absorbed by capillary action through certain hyphae.

          Organic food is manufactured by the autotropic alga. This food diffuses from algal cells to the saprophytic fungus. The  fungus in a lichen body secretes enzymes. These are responsible for increasing the permeability of the algal cells, which diffuse out sugars and other substances from their cell walls for the benefit of the fungus. Fungus provides protection, water and minerals to algae. Both partners are benefited means 'Symbiotic relationship'. 

Occurrence:

        Lichens are ubiquitous plants except near industrial smoky areas. They are world wide in distribution, growing from polar Regions to the equator in a variety of situations. They are commonly seen on rocks, tree trunks, roofs, fences and on the ground. A few marine lichens are also known.

External structue of thallus:

          There is considerable variation in the colours and forms of lichens. They vary greatly in colour ranging from grayish green to white, orange, yellow, yellowish -green, brown, brown and black.

       They differ in their size, ranging from few millimeters to several feet.

        On the basis of their forms, lichens are divided into three types-

1) Crustose:

       These occur as incrustations on rocks or from thin closely adherent growth on bark. They are so closely attached to the substratum on their undersurface that they cannot be removed from it entirely without injuring the thallus.

        The thallus may be partially or wholly imbedded so that sometimes only fruiting bodies are visible above the surface of the substratum.

e.g-  Graphis, Leconora, Rhizocarpon, Lecidia. 

2) Foliose:

          These are flat and leaf-like and apparently look like dried thalli of liverworts (flat and lobed).

The foliose (leaf-like) lichens are the most common types that grow on the trunks of trees or on rocks in the shady woods. 

        The margins are irregular and they are attached to the substratum of certain points by means of Central outgrowths called rhizines. Rhizins are formed from the fungal hyphae.

e.g  Parmelia, physcia, peltidea, Gyrophora etc. 

3) Fruticose:

            The thallus is slender and much branched. The branches may be cylindrical to ribbon shaped. The thallus grows either erect or hanging from Rocky areas, leaves and branches of trees.

       

           It is attached to the substratum by a basal disc, which is formed by compactly placed fungul hyphae. It has no differntiation into upper and lower surfaces. Fruiting bodies called apothecia are well developed.

e.g Usnea, Cladonia, Evernia etc.