Lichens: Introduction and Classification | Botany

In this article we will discuss about:- 1. Introduction to Lichens 2. Classification of Lichens 3. Features 4. Nature of the Association (Physiology) 5. Habitat 6. Distribution 7. Ecological Importance.

Introduction to Lichens:

The lichens are a small group of curious plants with about 18,000 species. We call them curious because they are composite or dual organisms. For a long time the lichens were considered as individual plants and were lumped together with the mosses.

Their true nature was discovered in the middle of the nineteenth century. During the 1860s it was found that the thallus of every lichen is composed of two quite different organisms, a fungus and an alga, forming a self-supporting combination.

The fungal component is called the mycobiont and the algal component is known as the phycobiont. The two live in intimate association. So much so that they appear to be a single plant. The fungal component forms the bulk of the lichen thallus.

The algal component occupies only a small part comprising about 5-10 percent of the mass of the thallus. The lichen thallus thus is in nature. The fungal hyphae form a close network resembling a tissue-like mass with the algal cells often embedded in it.

Another extraordinary thing about the lichens is that the new individual resulting from this association differs considerably from either of the partners in form and habit. It also has a physiology of its own.

The lungi obtain food either saprophytically from dead organic matter or else parasitically from the living bodies of host organisms. Algae synthesize their own food from carbon dioxide and water. In a lichen, both the components live on the food manufactured by the algal component.

Lichens thus are biotrophic in their mode of nutrition as they obtain their food from the living algal associate. Probably food materials from the alga diffuse out and are absorbed by the fungus. The reproductive organs, however, are entirely fungal in character.

Every individual of a given lichen species contains the same alga and the same fungus. This dual nature of the lichen thallus has been confirmed experimentally. People have been successful in building up a new lichen individual out of a fungus and its appropriate alga.

The algal component in the association may belong to the Cyanophyceae or to the simple Chlorophyceae. It may be filamentous or non-filamentous. In the majority of the lichens, the alga is unicellular.

About 26 genera of algae are involved. Of these, eight are blue-green, seventeen green algae and one yellow green. The common blue-greens are Nostoc, Stigonema, Rivularia and Gloeocapsa. About 80 percent of all lichen thalli contain green algae.

Of these, unicellular green alga Trebouxia is the commonest.

On the basis of the algal component, the lichens may thus be divided into two groups namely:

(i) Cyanophycophilous with blue-green alga phycobionts, and

(ii) Chlorophyco philous with green algal phycobionts.

The fungal partner is generally an Ascomycete. Only in 2 or 3 genera out of 400, the fungal component is a Basidiomycete. No lichenized Phycomycetes have been reported so far.

Classification of Lichens:

Many scientists classify the lichens on the basis of:

(i) The nature of the fungal element, and

(ii) The kind of the fructification.

On this basis, the lichens are divided into the following two groups:

1. Ascolichens in which the fungal component is an Ascomycete.

The Ascolichens are subdivided into two sub-groups:

(a) Gymnocarpeae in which the ascocarp is of an apothecium type.

(b) Pyrenocarpeae in which the ascocarp is perithecium type.

G.L.Chopra (1934) in his monograph on the “Lichens of Darjeeling and the Sikkim. Himalayas” described 80 species belonging to 38 genera of Ascolichens. The list includes one new species, three new varieties and a new genus Chaudhuria, with one species C. indica.

2. Basidiolichens:

In this group are placed the Lichens in which the fungal partner is a Basidiomycete.

The branch of Botany which is concerned with the study of lichens is called Lichenology. A scientist who has specialized in the study of lichens is called a Lithenologist.

Features of Lichens:

1. The Lichens are dual or composite organism.

2. The thallus-like plant body is made up of a Fungus and an Alga living in closest association. The nature of association between the two partners is the best example of symbiosis in the plant kingdom.

3. Fungus provides the body of the organism whereas Alga synthesizes carbohydrate food for itself and the Fungus.

4. The Lichen thalli are generally of three kinds, crustose, foliose and fruticose.

5. Internally the thallus particularly in the foliose lichens consists of four regions namely, the upper cortex, the algal layer, the medulla and the lower cortex.

6. Asexual reproduction by asexual spores and sexual reproduction are entirely the functions of the fungal partner.

7. The carpogonium is a coiled, multicellular filament.

8. It consists of a coiled, multicellular ascogonium and a straight, multicellular trichogyne.

9. There is a pore in the centre of each septum between the cells of the trichogyne.

10. The antheridia which are flask-shaped receptacles are sunk in the upper surface of the thallus.

11. Male cells or spermatia are non-motile. Each has a cell wall around it.

12. The ascus fruit in many species is of apothecium type and in others of perithecium type.

13. The contents of each ascus are fashioned into eight haploid ascospores. The ascospores may be simple or septate.

14. Each ascospore under suitable conditions germinates to produce a fungal hyphae which if it comes in contact with a proper alga, develops into a new lichen. If the fungal hypha fails to find an appropriate alga, it perishes.

Nature of the Association (Physiology):

Two different hypotheses have been put forth to explain the nature of the association in this plant complex. According to one hypothesis, the alga is considered a mere victim of the fungus. In other words, the fungus lives as a parasite on the algal partner.

The parasitism, of course, is of a mild nature as it permits most of the algal cells to live.

The adherents of this hypothesis advance the following two facts in support of their view:

1. In some lichens the fungal hyphae give out haustoria or appressoria. These penetrate the algal cells to obtain nutrition for the fungus mycelium.

2. When the two components of the lichen thallus are artificially separated the alga is able to live as an independent individual. It grows and multiplies like an independent organism. But the fungus is unable to do so. It invariably dies.

This experimental evidence is cited as a positive proof of the fact that the fungus lives as a parasite on alga in the lichen thallus. The supporters of this hypothesis do not see any real benefit which the alga derives from the fungal partner.

They argue that the water and minerals in solution, the alga could get in the free living state as well by itself as in the association with the fungus.

The second hypothesis is that the two partners in the lichen thallus derive mutual benefit from their partnership. There are advantages on both sides. They hold that the fungus with its rhizoids absorbs water and minerals in solution from the substratum.

It also absorbs moisture from the fog and moist air. The water and minerals thus absorbed are passed on to the alga. The fungal hyphae are also somewhat gelatinous. They absorb water readily and hold it tenaciously.

In this way, they afford protection to the algal partner from fatal drying when the air humidity is very low. The alga is thus able to live in exposed places where it would have been impossible for it to grow in the free state.

In addition the fungal hyphae, which form the body of the lichen, provide shelter to the alga. They thus protect it from intense light, drought, and other adverse weather conditions.

The duty of alga is to synthesize the necessary carbohydrates with the help of its green chloroplasts and in the case of cyanophycophilous lichens, nitrogen as well. This it does both for itself and also for the fungus.

The alga, therefore, provides food to the fungus. In the course of providing food to the fungal partner, a few of the algal cells may be exhausted. They may perish. The bulk of them, however, live. They grow and multiply within the thallus.

According to this view, the nature of association between the two components of the lichen thallus appears to be a mutualistic one. The adherents of this hypothesis cite lichens as the best example of symbiosis (mutualism) in the plant world.

The high resolution radio autography experiment of Jacob and Ahmadjean (1971) in Cladonia lends support to this hypothesis. In this experiment, C¹⁴-labelled sodium bicarbonate was provided as a source of carbon dioxide for photosynthesis to a segment of the lichen thallus.

It appeared in the organic compounds first in the algal component and 15 minutes later in the hyphae of the fungal component. This evidence supports the movement of materials from the alga to the fungus in the lichen thallus.

The consensus of opinion now favours second hypothesis. The adherents of this view place the lichens in a class distinct from both the fungi and algae.

There is third view which suggests the via media between the two extremes. According to this view, the relationship between the fungus and the algal partner in the lichen thallus is an example of symbiosis but the fungus in this partnership has the upper hand.

The algal partner lives as a prisoner or as a subordinate partner. The association between the two partners is thus described as beneficial slavery for the alga. The supporters of this view suggest the term helotism to this kind of association.

Habitat of Lichens:

The lichens grow in a wide variety of habitats. They are commonly found growing on the walls and roofs of houses, leaves, tree bark, bare earth and even barren, unpromising rocky surfaces. Generally they are xerophytic in nature and thus can withstand long periods of drought.

Consequently, they thrive and multiply in habitats where other vegetation is non-existent such as sand dunes, deserts and bare rocks. Here they live happily under conditions of drought and apparent starvation. Usually they are attached firmly to the substratum.

However, they derive little of their inorganic requirements from the substratum. Rather they depend on the rain or spray. They absorb, whatever substances they require through the whole surface of the thallus from those dissolved in the rain water.

Many species are able to withstand low temperature. They are abundant on high mountain elevations, and grow in extremely cold regions such as the arctic tundras where conditions are unfavourable for the growth of other plants.

Many lichens grow in the tropics and subtropics. These mesophytic lichen species grow in the abundance of moisture. Some aquatic species occur only on the sea shore.

According to their habitat, Lichenologists divide the lichens into three categories, saxicolous, corticolous and terricolous. The saxicoles are predominantly stone or rock lovers. They grow on firm substratum in cold regions.

The corticoles are generally bark lovers. They are mostly confined to the tropics and subtropics where there is abundance of moisture. Here they grow on the leaves and bark of trees as epiphytes depending on their hosts for anchorage only.

The terricolous species are terrestrial and thus inhabit the soil. The factors favouring lichen growth are direct light, moderate or cold temperature, pure atmosphere, firm substratum and sufficient atmospheric moisture.

The conditions unsuited for the growth of lichens are scanty precipitation, hot and dry summer. The growth in lichens is very slow.

Distribution of Lichens:

Lichens are one of the most widely distributed groups of plants. They are found all over the globe from the Arctic to the Antarctic and all regions in between and in diverse habitats.

The factors which help in their worldwide distribution are:

1. Their symbiotic life.

2. Their prolific methods of vegetative propagation and efficient means of dispersal.

3. Resistance to extremes of temperature and moisture which enables them to grow at all places where life can be supported at all.

They are found far north and far south than any other plants of the arctic region. Here they along with mosses comprise the entire vegetation of the place. Far north lie the tundras of Iceland, Lapland, Greenland and Canada and other subarctic regions.

The lichens are also met with at higher altitudes on land surfaces than any other plants. Here they colonize the fresh rocky surfaces. The lichens are equally at home in the equatorial jungles.

In India they are very common all over the Himalayas particularly the Eastern side. They are also found in the higher hills of Peninsular India.

Ecological Importance of Lichens:

The lichens are of considerable ecological importance. They are slow but efficient soil farmers. They are the pioneer plants to grow on barren, naked, rocky surfaces where no other plants can grow. Crustaceous forms are usually the first to appear.

They are followed by the foliaceous and eventually the fruticose types. They live there happily for years under conditions of drought and apparent starvation. When there is occasional shower of ram the lichen thalli soak it up like a blotting paper and retain it to support a more active life over long dry spells.

Their other requirements are met with by the substances dissolved in rain water which is absorbed through the entire surface of the thallus. The continuous growth of the lichen thalli brings about gradual disintegration of the rocks immediately beneath.

The lichen thalli secrete certain organic acids which gradually dissolve and disintegrate the rocks to which they cling. The disintegration is also helped by the stresses and the strains caused by the expansion and the contraction of the gelatinous lichen thalli.

The rock particles together with the decaying and dead lichen thalli form a soil fertile enough for other plants to make appearance. The successors are the mosses. Sooner or later flowering plants begin to grow in this soil.