What are Lichens?
Lichen is not a single organism. Rather, it is a symbiosis between different organisms - a fungus and an alga or cyanobacterium. Lichens are a small group of curious plants, whose body is made up of an algal and a fungal component, both of which live together in an intimate relationship. The plant body neither resembles to algae nor to fungi. However, reproductive organs of lichens are fungal in character. Both algal and fungal partners are benefited
by the relationship. This type of relationship is known as symbiotic.
The fungus derives nutrition from the alga which in turn is protected by the fungus. The fungal hyphae form a close network resembling a tissue like mass with the algal cells entrenched in it. The algal component of lichen is known as phycobiontiandthe fungal component is called mycobiont. The phycobiont is chiefly a green alga or cyanobacterium, whereas the mycobiont is usually a member of Ascomycotina or Basidiomycotina.
Distribution of Lichens
There are about 400 genera and 15,000 species of lichens, widely distributed in most parts of the world. They
usually grow on the barks of trees, dry kindling of wood, bare rocks and other similar situations. Generally, they
are xerophytes in nature and thus can survive long periods of drought.
Types of Lichens
Foliose Lichen
The plant body of lichens is thalloid and irregular in shape. It is usually grey or greyish green in colour. On the basis of their external morphology, lichens are divided into the following three categories.
i) Crustose lichen
ii) Foliose lichen
iii) Fruticose lichen.
Crustose lichen
The crustose lichens have very poorly defined thallus and are closely attached to the substratum providing a crust-like appearance. The small thalli may be partially or wholly embedded so that sometimes only fruiting bodies are visible above the surface of the substratum . Example : Graphis.
Foliose lichen
Foliose lichens have flat, leaf-like, expanded body. They are attached to the substratum with the help of rhizoid-like rhizome. Example : Parmelia.
Fruticose lichen
Fruticose lichens are shrubby lichens with a well developed, cylindrical and branched thallus. They grow erect or hang from the substratum . Examples : Cladonia, Usnea.
Economic Importance of Litchens
Colorful Lichens
Lichens are wonderful gift of nature. They are of significant ecological importance as pioneers in colonization of rocky habitat. They are the first plants to survive on dry bare rocks. They are also very good indicators of air pollution. Lichens are used as food since ancient times. They are important constituents of the food in north Polar Tundra and eastern Siberian regions. Cladonia rangifera (reindeer moss) serves as a common food in Tundra regions for animals, specially reindeer and musk ox.
Dried lichens are fed to horses and swans. Several lichens are used as food by mites, snails, caterpillars, slugs, termites, etc. Lichens also be indebted their repute as source of medicines due to the presence of lichenin and some other astringent substances, which proved very effective in the treatment of ailments such as jaundice, fever, epilepsy, hydrophobia and coetaneous diseases.
They are also important constituents of several ayurvedic medicines. Dyes obtained from lichens have been used since ancient times for coloring fabrics. Litmus, an important acid-base indicator dye in chemical laboratories, is obtained from Roccella montagnei. Some lichens are used as tanning agent in leather industries, others in the manufacture of hawan matters, dhup and other perfumeries because of their agreeable smell. Several lichens are harmful as they cause a considerable damage by etching of glass surfaces and marble stones. Lichens show a great ability to concentrate nutrients from very dilute sources and indiscriminately absorb many toxic substances from the atmosphere.