A general account on lichens
Lichens are dual organisms formed by the symbiotic association of a fungus and an alga. They are also known as composite plants.
The fungal partner is usually an Ascomycete, but sometimes a Basidiomycete. The algal partner may be a green alga or a blue green alga. A peculiar nutritional relationship exists between the facultative fungus and photoautotrophic alga. The fungus and alga together appear as a single plant. Lichen include about 400 genera and 16000 species. And the study of lichen is called Lichenology similarly the scientist associated with Lichenology is known as Lichenologists.
Lichens are a group of compositr plants. Each lichen consists of a fungus and an alga the fungul component of lichen is called mycobiont and the algal component is called phycobiont. The mycobiont and phycobiont live in intimate association and form a lichen. The association is called symbiotic association. In most genera of lichens, the mycobiont is a Ascomycetous fungus, eg. Parmelia, Dermatocarpon, etc. They are popularly known as ascolichens. In three genera of lichens, the fungul component is a basidiomycetous fungus. They are called Basidiolichens. Eg. Cora, Corella and Dictoyonema.
The algal component of lichen may be a green alga or blue green alga. About 26 species of algae exist as phycobionts in lichens. Most lichens have green alga as the phycobiont. The unicellular green alga Trebouxia is the most common phycobiont in lichens.
About eight genera of blue green algae are known to occur in lichens. The most important among them are Nostoc, Stigonema, Rivilaria and Gloecapsa.
There are three hypothesis to explain the nature of association of alga and fungus in lichens. They are given below.
1) Parasitic hypothesis
It states that fungus in lichens lives as a parasite on the alga. This is proved by three facts:
I) In some lichens, the fungal hyphae produces haustoria which enter the algal cells to draw nutrition for it.
II) If the alga and fungus are separated from a lichen, the alga grows and reproduces independently, but the fungus fails to do so. The fungus almost dies.
III) The algae could get water and nutrients even if they are free living. So they are independent organisms being prisoned in fungul hyphae for the benefit of the fungus. The fungus is unable to grow, if there is no alga.
2) Symbiotic hypothesis
It states that, in lichens both the mycobiont and phycobiont derive mutual benefit from their association. The mycobiont absorbs and retains water and minerals. Besides this, it forms a protective covering around the algal cells. It protects the alga from desiccation and higher light intensities.
The algal partner synthesises carbohydrate which is used by the fungus. Such an association of organisms in which both organisms are benefited, is called symbiosis or symbiotic association.
3) Beneficial slavery
This hypothesis states that the association between the alga and fungus is actually symbiosis, but the fungus takes the upper hand over the alga. Here, the alga seems to be a beneficial slave. This type of association is called heliotism.