In this article we will discuss about:- 1. Definition of Lichens 2. Structure of Lichens 3. Relationships 4. Reproduction.
Definition of Lichens:
Lichens are dual organisms or entities which contain a permanent association of a fungus or mycobiont and an alga or phycobiont. The fungal partner is usually an Ascomycota and sometimes, a basidiomycete. The algal partner is mostly a green alga or a cyanobacterium (blue-green alga).
The term lichen was coined by Theophrastus (370-285 B.C.). There are about 400 genera and 15,000 species of lichens.
Lichens often grow in most inhospitable and uninhabited places like barren rocks, cooled volcanic lava, icy tundra or alpines, sand dunes, roofs, walls, window panes, tree bark, leaves, etc. They commonly live under humid and exposed conditions but can tolerate extreme desiccation.
However, lichens cannot tolerate air pollution, especially due to sulphur dioxide. Lichens are perennial. Their growth is slow. Some lichens of arctic region are believed to be 4500 years old. Lichens have greyish, yellowish, greenish, orange, dark brown or blackish colouration.
Structure of Lichens:
In shape, the lichens are of three types:
Crust-like closely appressed to the substratum and attached to it at several places, e.g., Graphis, Lecanora, Rhizocarpon, Haematoma.
The body of the lichen is flat, broad, lobed and leaf-like which is attached to the substratum at one or a few places, e.g., Parmelia, Peltigera. Foliose lichen Cora (= Dictyonema) pavonia resembles bracket fungi in appearance.
The lichen is branched like a bush and attached to the substratum by means of disc, e.g., Cladonia, Usnea, Evernia. The bulk of lichen body is formed by fungal partner or mycobiont. It includes the surface, medulla (or interior) and rhizines (attaching devices). The algal partner or phycobiont constitutes hardly 5% of the lichen body. It is generally restricted to a narrow zone (algal zone) below the surface.
Relationships of Lichens:
The fungus performs three functions:
(i) Body structure and covering
(iii) Absorption of water and minerals. It can absorb water from wet air (atmosphere), dew and rain.
Minerals are picked up both from substratum and atmosphere. Special chemicals are excreted by the fungus partner of the lichen to dissolve minerals from the substratum. The major function of alga is photosynthesis.
The cyanobacterial alga additionally takes part in nitrogen fixation. The alga picks up water and mineral salts from the fungus while the fungus obtains part of the food manufactured by the alga. Therefore, in a lichen the association between alga and fungus is that of mutual benefit (mutualism) popularly called symbiosis.
However, at times the fungus is found to:
(i) Send haustoria into algal cells
(ii) Induce alga to secrete organic substances and
(iii) Prevent the alga to develop pectic covering.
Therefore, some workers believe that the fungus is a controlled parasite over the alga. The phenomenon is called helotism. Despite the knowledge about the algal and fungal components of a lichen and their mutual relationships, it has not been possible till now to create an artificial lichen.
Reproduction of Lichens:
Lichens multiply by four methods:
(i) Progressive death and decay resulting in the separation of a lichen into two or more parts,
(ii) Fragmentation caused by mechanical injury due to wind, trampling or animal bites,
(iii) Isidia are superficial outgrowths of the lichens which are primarily meant for increasing surface area and photosynthetic activity. At times, they are broken off. Each isidium is capable of forming new lichen because it has a core of algal cells surrounded by a sheath of fungal hyphae.
(iv) Soredia. They are microscopic lichen propagules which are produced in large numbers inside sori called pustules. Soredia are dispersed by air currents. After falling on a suitable substratum each soredium gives rise to a lichen because it has a few algal cells surrounded incompletely by a weft of fungus.