Lichens

Lichens

Contents: 

1. INTRODUCTION

2. GENERAL ACCOUNTS

3. INTERNAL STRUCTURE OF THALLUS

4. LIFE CYCLE

5. FERTILIZATION

6. ROLE OF LICHENS AS INDICATOR

7. REFERENCES

1. Introduction:

The lichens are an association consisting of a phycobiont (al algae) and a mycobiont (a fungus) growing together to form a plant body of recognizable structure and appearance. There are 18,000 to 20,000 species of lichens that are known.

2.General account of lichens

2.1 Occurrence

Lichens are present everywhere we can say that they are ubiquitous. They are present between all regions of the Arctic and Antarctic. Lichens are found on exposed areas like bark of trees and on wood , directly on the ground and shores of sea. Lichens are also present on the rocks that are present in the fresh water. These are also present in the polar regions of frozen substrata.

2.2 Components of lichens

Lichens have two components. It consist of fungi(mycobionts) and algae(photobionts)

2.3 Morphology of lichens

The body of the lichen is thallus like. Its color may range from grey to grayish-green. But in some species the color may be red, orange, yellow or brown. On the basis of morphology and attachment it classified into three types. These types are given below,

2.3(i) Crustose

This type of lichen is flat, thin and crust like. These are present closely to the substratum, for example graphis, verrucaria etc. Its thalli look like crustose and they attached to the substratum firmly and cannot separate from it.

2.3(ii) Foliose

In this type the thallus is leaf like and flat, lobed, branched. They have attaching organ that is called rhizines and it helps to attach with the substratum. For example physcia, gryophora etc.

2.3(iii) Fruticose

In Fruticose the thallus is branched or cylindrical and erect. It is bushy in appearance. These are attached to the substratum through their basal portion. Examples are Usnea and Bryoria etc.

3. Internal structure of thallus

On the basis of the algal cells the lichen thalli are two types. These two types are given below.

3.1 Homiomerous

In these thalli the algal cells are more or less distributed in the thallus e.g., collema. A thin protective layer is formed by the fungal hyphae. Crustose has such type of thalli.

3.2 Heteromerous

In heteromerous the algal cells are present in distinct layer. Fungal hyphae form the main bulk of thallus. This type of thalli is present in Fruticose and foliose lichens e.g., physcia, parmelia etc. The tissue zones are present in it these are following.

3.2(i) Upper cortex

It is upper dense layer and formed by the interwoven fungal hyphae. It is water retaining layer. In many lichens the parenchyma like tissues are present that are formed by the fungal hyphae.

3.2(ii) Algal layer

Algal layer is present below the upper cortex and it is a distinct algal layer. The fungal hyphae enveloped these algal cells. This part is photosynthetic.

3.2(iii) Medulla

The medulla is present just below the algal layer. It is the central portion of the thallus. Loosely arranged hyphae is present in it that are longitudinally oriented. These are act as conducting cells. The medulla forms the bulk of the thallus.

3.2(iv) Lower cortex

The lower surface of the thallus is bounded by a dense layer of fungal layer. It consists of lower cortex. Those hyphae run parallel to the surface of hyphae. The attaching organs that called rhizomes arise from the lower cortex. It play very important role firstly it conduct water. But the main function is anchor the lichens to substratum.

4. Life cycle

Lichen starts its life cycle when the mycobiont combines with the phycobiont. So the fungal filament changes into the algal cells and it provides the nourishment to the thallus and also provides the structure. It is a unique feature of lichens that they reproduce when the alga and fungus disperse together.

4.1 Reproduction

Lichens reproduce by vegetative, asexual and sexual reproduction.

4.1(i) Vegetative reproduction

Lichens are reproducing vegetatively by following methods.

Fragmentation

This type of reproduction occurs in foliose and fruticose lichens. In this type of reproduction the thalli broken into smaller units or parts due to some mechanical stress or due to death of the older part of thalli. These separated parts have an ability to reproduce in new lichen thallus.

a)     Isidia

Isidia are papilla like outgrowths that are present on the upper surface of the thallus.

 Each of it consists of fungal hyphae and algal cells that are cortical layer. These are reproductive propagules. Carried away in the favourable environment and then it grow into the new thalli. This type of reproduction is found in the parmelia, bryoria etc.

Soredia

It is bud like outgrowth of the algal cells. These cells surround by the hyphae or fungal hyphae and it does not covered by the cortex. It exposed by rupturing the cortex. So it appears as a special structure that is called sorelia on the thallus. The mature soredia are dispersing by the animals, wind, rain drops etc. this type of reproduction occur in the parmelia, cladonia ssp. etc.

Blastidia

Some types of lichens can reproduce by the blastidia such as opuntiella etc. In which yeast like a special structure is formed that is called blastidia. This structure is detached from the body and forms new thalli.

4.1(ii) Asexual reproduction

Asexual reproduction occurs by the sporulation.

a)      By sporulation

Lichens can also reproduce by the sporulation. They reproduce by means of conidia, oidia and pycniospores. In some lichens the hyphae of the lichens are break down into small pieces and this is known oidia. But on the other hand pycniospores are produced in a specialized flask shaped structure that is called pycnidia. The pycnidia open through a specialized pore that is called ostiole. After falling on a substratum these spores germinate and then make a contact with the algae. Then it develops into new lichen.

4.1(iii) Sexual reproduction

Sexual reproductions occur in ascolichens and in basidiolichens. In sexual reproduction the male reproductive organ called as spermogonium and the female organ is called as carpogonium.

a)      Spermogonium

It is flask shaped structure. It is embeded in the upper surface of the thallus. It is open outside through a pore that is called ostiole. The fertile hyphae produce small rounded shaped cells at its tip. These cells are called spermatia. An interesting thing is that in some lichens pycnidia like structure act as spermogonia.

 

a)      Carpogonium

It consists of two parts. The first lower part is called as ascogonium and the other upper long part is called as trichogyne. The ascogonia lie deep in the medullary portion of the thallus. On the other hand trichogyne emerges out from the thallus and receives spermatia.

5. Fertilization

Fertilization occurs. After fertilization trichogyne withers. The ascogonium produced freely branched hyphae which produce asci on their tips. All structures surrounded by sterile hyphae after fertilization. As a result the formation of a fruiting body which is either apothecium or perithecium type.

6. Importance of lichens as indicator

Lichens play very important and beneficial role.

6.1 Monitoring of climate change and air pollution

·         Biological parameter

Many scientist and researchers used lichens and other microorganisms for measuring the environmental quality by using the biological parameters. These are called as indicators of climate change. They observed that the change occur in the life cycle of the animals and plants due to pollution and climate change. And the scientist and researchers used lichens to measure the pollution. Lichens are very sensitive to SO2 therefore it used for measuring the air quality.

·         Lichens as Bio-Indicators

Lichens are used as bio-indicators. Lichens are very sensitive to environmental parameters such as temperature, humidity, wind and air pollution etc. They absorb nutrient and water from the environment passively because they do not have vascular system. Composition of the lichens is very useful to gain information about climate change. Morphology and physiology of the lichens is change due to the change of climate and pollution so we can measure the change easily.  Lichens are also very important for nitrogen fixation and for nutrient cycling.

·         Heavy metal deposition

Lichens absorbs nutrient from the environment for their biological processes. Lichens trap small particles and of soil and rock, and absorbs some heavy metals through their ion exchange method. So this is the very unique character of the lichens. By using the lichens we can also measure or monitor the pollution parameters.

·         Gaseous air pollution

Lichens are also very important to measuring the gaseous air pollution. It is used as indicator of air pollution especially on the basis of the acidifying and Sulphur based pollutants. Lichens do not have any outer protective layer like in vascular plants. So it is very sensitive to pollution. A scientist his name was Giordani, in province of Italy (Genova) carried out the case study. He told about the apithetic lichens that are very sensitive to the rain fall and air, temperature etc. He also studied about the Sulphur dioxide that is also constraint in the urban areas.

7. REFERENCES

Ferry, B. W., Baddeley, M. S., & Hawksworth, D. L. (Eds.). (1973). Air pollution and lichens (p. 389). London: Athlone Press.

Hale, M. E. (1967). The biology of lichens. The biology of lichens.

Hale, M. E., Raiborn, C. A., & Prather-Kinsey, J. (1979). How to know the lichens. WC Brown Company.

Nimis, P. L., Scheidegger, C., & Wolseley, P. A. (2002). Monitoring with lichens—monitoring lichens. In Monitoring with Lichens—Monitoring Lichens (pp. 1-4). Springer, Dordrecht.