LICHENS
Lichens are a small group of plants made up of algal and fungal components, living tigether in an intimate symbiotic relationship. Lichen is an autotrophic, thallophytic composite organism which consists of both algae and fungi. The algal component is known as phycobiont. In this symbiotic assiciation or relationship both the partners derive mutual benefit. The fungus derives food from the algal cells and inturn protects the alga from unfavourable conditions, specially from drought. Only the fungal component is involved in in sexual reproduction. The growth of lichens is very slow. Direct light, moderate or cod tempearture, moisture and pure atmosphere favours the growth of lichens. Polluted, smooky atmosphere as found in industrial area is nit favourable for the growth of lichens. Lichens arew orld wide in distribution. These are found growing in a variety of habitats from arcitic to the Antarcitic and all the regions in between. In India , Lichens are much more common in Eastern Himalayas as compared to that in the Western Himalayas. The algal component belongs to chlorophyceae or Myxophyceae (Cyanobacteria), where as the fungal component belongs to Basidiomycotina or Ascomycotina.
Types of Lichens
Lichens are classified into different types
I. On the basis of the structure of the thallus, lichens have been classified in to Three broad types
• Crustose Lichens
• Foliose Lichens
• Fruticose Lichens
II. On the basis of the type of fungal component, Lichens have been classified in to Three broad types.
• Ascilichens
• Basidiolichens
• Deuterolichens
I. On the basis of the structure of the thallus :
1. Crustose Lichens:
The thallus is thin, flat and crust like. The thalli are appressed to the substratum forming thin flat crusts. The thalli are partly or wholly embedded in tthe substrartum and cannot be removed from the substratum without injuring the thallus. Sometimes only the fruiting bodies are visible above the surface of the substratum.
Examples: Graphis, Haematomma, Verrucaria, Lecanora
2. Foliose Lichens:
These lichens are flat with leaf like and lobed thallus. They are attached to the substratum with thw help of rhizoid like rhizines.
Examples: Parmelia, Peltigera, Physica
3. Fruticose Lichens:
These are bush like having cylindrical or strap shaped branched thallus. The branches may grow errect or hang from the substratum with the help of a basal mucilage disc.
Examples: Usnea, Cladonia, Ramalina
II. On the basis of the type of fungal component:
1. Ascolichens:
The fungal component of these Lichens is a member of the class Ascomycetes.
Example: Dermatocarpon, Parmelia
2. Basidiolichens:
The fungal component of these Lichens is a member of the class Basidiomycetes.
Example: Corella, Dictyonema
3. Deuterolichens:
The fungal component of these Lichens is a member of the class Basidiomycetes.
Anatomy or Internal Structure of Lichens:
The internal organisation of lichens depends upon the type of Lichen based upon its structure.
1. Crustose Lichens :
• Anatomically there is much differentiation in Crustose Lichens.
• In transverse setcion, the lichen thallus shows cortex, an algal layer and medulla.
• The cortex is made up of fungal hyphae.
• Following cortex an algal layer madee up of algae are present.
• Medulla is the final layer made up of a loose tissue of branching hyphae.
• The lower cortex is not distinguishable.
2. Foliose Lichens:
• On the basis of distribution of algal cells among the fungal tissue, two types of foliose thalli are recognised.
• They are Homiomerous and Heteromerous type.
• Homiomerous Type: The algae are more or less uniformly distributed throughout the thallus. The algae are usually gealtinous and belongs to Cyanobacteria. The outer protective layer of he thallus is formed by the fungi.
Examples: Collema, Leptogium.
• Heteromerous Type: The algal cells form a distinct layer with in the thallus. Bulk of the thallus is made up of fungal hyphae. Thallus is differentiated into four distinct regions namely, Upper Cortex, Algal layer, Medulla and Lower Cortex. The upper cortex is made up of completely interwoven hyphae. The Algal layer or gonidial layer is consisting of loosely interwoven fungal hyphae, intermingled with algal cells of a green alga. Medulla which is the central part of the thallus comprised of loosely interwoven fungal hyphae with larger spaces between them. Lower cortex is present below the medulla madeup of densely compacted hyphae present at the lower surface of the thallus.
3. Fruticose Lichens:
• The lower cortex doesnot occur in fruticose lichens due to cylindrical structure and medulla forms the central part of the axis.
• The external layer of a lichen thallus or surface is termed the cortex. Beneath it is a layer of fungus enmeshed algal cells called the algal layer. Below the algal layer is a region of cottony, loosely woven fungal hyphae free from algal cells, the medulla.
Reproduction:
The main modes of the reproduction found in Lichens.
1. Vegetative reproduction:
Lichens reproduce vegetatively by mean sof Fragmentation, Isidia and Soredia.
2. Asexual reproduction:
Only fungal component of the Lichen can be reproduced asexually. In Ascolichens, asexual reproduction commonly takes place by ascospores and the fruiting body or fructification is known as the Ascocarp. In some Ascolichens, the fruiting body appears as a cup shaped structure commonly called apothecium. Each Ascus usually contains eight ascospores. Sometimes in some lochens ascogonium takes the shape of Perithecium. In Basidiolichens asexual reproduction takes place by means of basidiospores. In some lichens asexual reproduction is fullfilled by pycniospores or Pycnidia. These Pycniospores germinate by giving rise to hyphal branches in all directions and when any such branch comes in contact with algal cells, a lichen body is formed.
3. Sexual reproduction:
Sexual reproduction in Ascolichens and bsidiolichens resemble to that of Ascomycetes and Basidiomycetes members respectively. Ascolichens are the most common lichens containing male and female sex organs like spermatia and carpogonia (Ascogonia) respectively. Fertilisation if followed by the formation of ascogeenous hyphae from the basal portion of the ascogonium. The ascocarp is lined with a paliside like layer of paraphyses. During weather ascospores germinate and give rise to germtubes which on coming in contact with algal element produce the new thallus.
Lichens are a small group of plants made up of algal and fungal components, living tigether in an intimate symbiotic relationship. Lichen is an autotrophic, thallophytic composite organism which consists of both algae and fungi. The algal component is known as phycobiont. In this symbiotic assiciation or relationship both the partners derive mutual benefit. The fungus derives food from the algal cells and inturn protects the alga from unfavourable conditions, specially from drought. Only the fungal component is involved in in sexual reproduction. The growth of lichens is very slow. Direct light, moderate or cod tempearture, moisture and pure atmosphere favours the growth of lichens. Polluted, smooky atmosphere as found in industrial area is nit favourable for the growth of lichens. Lichens arew orld wide in distribution. These are found growing in a variety of habitats from arcitic to the Antarcitic and all the regions in between. In India , Lichens are much more common in Eastern Himalayas as compared to that in the Western Himalayas. The algal component belongs to chlorophyceae or Myxophyceae (Cyanobacteria), where as the fungal component belongs to Basidiomycotina or Ascomycotina.
Types of Lichens
Lichens are classified into different types
I. On the basis of the structure of the thallus, lichens have been classified in to Three broad types
• Crustose Lichens
• Foliose Lichens
• Fruticose Lichens
II. On the basis of the type of fungal component, Lichens have been classified in to Three broad types.
• Ascilichens
• Basidiolichens
• Deuterolichens
I. On the basis of the structure of the thallus :
1. Crustose Lichens:
The thallus is thin, flat and crust like. The thalli are appressed to the substratum forming thin flat crusts. The thalli are partly or wholly embedded in tthe substrartum and cannot be removed from the substratum without injuring the thallus. Sometimes only the fruiting bodies are visible above the surface of the substratum.
Examples: Graphis, Haematomma, Verrucaria, Lecanora
2. Foliose Lichens:
These lichens are flat with leaf like and lobed thallus. They are attached to the substratum with thw help of rhizoid like rhizines.
Examples: Parmelia, Peltigera, Physica
3. Fruticose Lichens:
These are bush like having cylindrical or strap shaped branched thallus. The branches may grow errect or hang from the substratum with the help of a basal mucilage disc.
Examples: Usnea, Cladonia, Ramalina
II. On the basis of the type of fungal component:
1. Ascolichens:
The fungal component of these Lichens is a member of the class Ascomycetes.
Example: Dermatocarpon, Parmelia
2. Basidiolichens:
The fungal component of these Lichens is a member of the class Basidiomycetes.
Example: Corella, Dictyonema
3. Deuterolichens:
The fungal component of these Lichens is a member of the class Basidiomycetes.
Anatomy or Internal Structure of Lichens:
The internal organisation of lichens depends upon the type of Lichen based upon its structure.
1. Crustose Lichens :
• Anatomically there is much differentiation in Crustose Lichens.
• In transverse setcion, the lichen thallus shows cortex, an algal layer and medulla.
• The cortex is made up of fungal hyphae.
• Following cortex an algal layer madee up of algae are present.
• Medulla is the final layer made up of a loose tissue of branching hyphae.
• The lower cortex is not distinguishable.
2. Foliose Lichens:
• On the basis of distribution of algal cells among the fungal tissue, two types of foliose thalli are recognised.
• They are Homiomerous and Heteromerous type.
• Homiomerous Type: The algae are more or less uniformly distributed throughout the thallus. The algae are usually gealtinous and belongs to Cyanobacteria. The outer protective layer of he thallus is formed by the fungi.
Examples: Collema, Leptogium.
• Heteromerous Type: The algal cells form a distinct layer with in the thallus. Bulk of the thallus is made up of fungal hyphae. Thallus is differentiated into four distinct regions namely, Upper Cortex, Algal layer, Medulla and Lower Cortex. The upper cortex is made up of completely interwoven hyphae. The Algal layer or gonidial layer is consisting of loosely interwoven fungal hyphae, intermingled with algal cells of a green alga. Medulla which is the central part of the thallus comprised of loosely interwoven fungal hyphae with larger spaces between them. Lower cortex is present below the medulla madeup of densely compacted hyphae present at the lower surface of the thallus.
3. Fruticose Lichens:
• The lower cortex doesnot occur in fruticose lichens due to cylindrical structure and medulla forms the central part of the axis.
• The external layer of a lichen thallus or surface is termed the cortex. Beneath it is a layer of fungus enmeshed algal cells called the algal layer. Below the algal layer is a region of cottony, loosely woven fungal hyphae free from algal cells, the medulla.
Reproduction:
The main modes of the reproduction found in Lichens.
1. Vegetative reproduction:
Lichens reproduce vegetatively by mean sof Fragmentation, Isidia and Soredia.
2. Asexual reproduction:
Only fungal component of the Lichen can be reproduced asexually. In Ascolichens, asexual reproduction commonly takes place by ascospores and the fruiting body or fructification is known as the Ascocarp. In some Ascolichens, the fruiting body appears as a cup shaped structure commonly called apothecium. Each Ascus usually contains eight ascospores. Sometimes in some lochens ascogonium takes the shape of Perithecium. In Basidiolichens asexual reproduction takes place by means of basidiospores. In some lichens asexual reproduction is fullfilled by pycniospores or Pycnidia. These Pycniospores germinate by giving rise to hyphal branches in all directions and when any such branch comes in contact with algal cells, a lichen body is formed.
3. Sexual reproduction:
Sexual reproduction in Ascolichens and bsidiolichens resemble to that of Ascomycetes and Basidiomycetes members respectively. Ascolichens are the most common lichens containing male and female sex organs like spermatia and carpogonia (Ascogonia) respectively. Fertilisation if followed by the formation of ascogeenous hyphae from the basal portion of the ascogonium. The ascocarp is lined with a paliside like layer of paraphyses. During weather ascospores germinate and give rise to germtubes which on coming in contact with algal element produce the new thallus.
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