LICHENS

Lichens are the group of plants made up of algal and fungal components, living together in an intimate symbiotic relation ship.

Algal component is known as Phycobiont and fungal component is known as Mycobiont.

The Term Lichen was first used by Theophrastus to denote a superficial growth on the bark of olive trees

Based on the type of habitat colonised the lichens are grouped as

Corticolous- grows on the bark of the trees Eg: Usnea, Graphis, Parmelia

Saxicolous: Grows on rocks Eg: Dermatocarpa, verrucaria

Lingicolous: Grows on wood Eg: Calicium, Chaenotheca

Terricolous: Grow on the ground Eg: Lecidea, Cladonia

The two components of lichens Phycobiont and Mycobiont

The algal components belongs to Chlorophyceae or Myxophyceae where as the fungal component belongs to Basidiomycotina or Ascomycotina

Nature of Association

The relation is considered to be symbiotic in which both algae and fungi get benefitted. It is believed that algae synthesized the organic food materials sufficient for both algal and fungal components. Water and minerals are made available to algae by the fungal component.

Although the association of both the components of lichens is symbiotic, the fungal component shows predominance over the algal component, and the latter is a subordinate patner. Such a association is known as Helotisim.

Structure

The plant body of lichens is a thallus, which is irregular inshape. It is usually grey or greyish green in colour, but some species are yellow, red, orange or brown in colour.

External Morphology

Lichens are broadly grouped into three types, on the basis of their morphology

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 embeded in the substrtum and cannot be removed from the substratum without injuring the thallus. Some times are fruit bodies are visible above the surface of the substratum.

Eg; Graphis, Verrucaria, Haematomma and Leconora, Rhizocarpon, Caloplaca                                

Foliose Lichens

These lichens are flat with leaf like and lobed thallus

They are attached to the substratum with the help of rhizoid like rhizines

Eg: Parmelia, Physcia, Peltigera, Gyrophora, Cetraria, Collema,Leptogium

Fruticose Lichens

These are brush like having cylindrical or star shaped branched thallus.

The branches may grow erect or hand from the substratum

The plant body is attached to the substratum with the help of basal mucilagenous disc.

Eg: Alectonia, Cladonia, Usnea, Ramalina

Internal Structure

Crustose lichens:Anatomically crustose lichens do not show much differentiation.

In T.S  the lichen thallu shows cortex , an algal layer and medulla.

The cortex is made up of fungal hyphae, beneath which is the algal layer composed of algae and fungi in close association.

Below the algal layer is the medulla, made up of a loose tissue of branching hyphae .

The lower cortex  is not distinguishable

Foliose lichen thallus

On the basis  of distribution of algal cells among the fungal tissue, two types of foliose thalli are recognised.

They are

1) Homoisomerous : The algae are more or less uniformly distributed through out the thallus. Such forms are only few.

The algae is usually gelatinous and belongs to cyanobacteria.

The outer protective layer of the thallus is formed by the fungi.

Eg: Collema,Leptogium

Heteromerous type

The algal cells form a distinct layer within the thallus. Bulk of the thallus is made up of fungal hyphae.The thallus is differentiated in to four distinct regions

1. Upper cortex

2. Alagal Layer

3. Medulla

4.  Lower Cortex

Upper cortex:

It is the outer most thich and protective zone of the thallus

It is made up of compactly interwoen fungal hypahe

The hyphae are arranged at right angle to the surface of the thallus

There are usually no intercellular spaces between the hyphae and if present are filled with gelatinous material.

In some lichens the upper cortex is interrupted at intervals by pores which are meant for aeriation and are known as breathing ppores.

Eg: Parmelia

Algal Layer:

Beneath the upper cortex , is the algal layer also known as gonidial layer.

It consists of looseely interwoven fungal hyphae, intermingled with algal cells of a green algae (usually Chlorella, Pleurococcus, cystococcus) or Myxophyceae (Usually Gloecocapsa, Nostoc, Rivularia)

Medulla

It is the central part of the thallus, comprised of loosely interwoen fungal hyphae with larger spaces btween them.

The hyphae are thick walled, oriented in different directions.

Lower Cortex:

Below the medulla is the lower cortex consisting of densely compacted hypahe.

Some of these hyphae become specialized  and extended downward from the lower surface of the thallus and help in attachment of thallus to the substratum .

These specialized hyphae are known as rhizines. Rhizines may be simple or branched.

Fruticose lichens:

The lower cortex does not occur in fruticose lichens due to their 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

Lichens reproduce  both by vegetative , asexual and sexual  methods.

Vegetative methods: Common mode of reproduction

1.Fragmentation: Small fragments of thallus are formed by accidental breaking or due to death and decay of older parts.Each fragment develops into a new thallus, provided the fragment contains both algal and fungal components.

2. Soredium:These are small bud like outgrowths which develop on the upper surface of the thallus. A soredium contains one or few algal cells closely enveloped by a weft of fungal hyphae.Both algal and fungal components are the same as in the parent thallus.

Soredia forma granular layer of greyish white colour on the surface of the thallus.

They detached from the thallus by the impact of wind  or rain.some times the soredia develo in an organized manner in special pustule-like areas. Then they are know as Soralia. Eg: Parmelia, Physcia. The soredia germinate on suitable substratum develop  into new thallus

3. Isidium:

These are small coral like outgrowths which develop on the upper surface of the thallus. Each isidium has an outer cortical layer of fungal hyphae, enclosing the algal cells.

Isidia vary in their shape

Rod shaped-Parmelia sexatilis

Coralloid shaped-Peltigera

Cigar shaped-Usnea compasia

Scale like- Collema crispum

Isida are mainly meant for increasing the phtosynthetic area of the lichen thallus. However when get detached from the thallus , they behave as reproductive structures.

The isidium germinates under favourable conditions and form new thallus.

Cephalodium:Some lichens are diphycophilous and show three membered symbiosis(Two algal components+one fungus)

In such lichens one of the algal partner is segregated into special external or internal swellings called cephalodia.

The cephalodia are small , hard, dark coloured, gall like swellings and contain alwayas a different algal component than the lichen thallus.

For Ex: in Peltigera aphthosa, the cephalodium contains a blue green alga, but the algal component in the thallus is a green algae.

Thus one phycobiont confined to cephalodium while the other one is present in the thallus. There is no organic connection between them.

Eg: Lobaria, Solorina, Peltigera

Cephalodium

Asexual Reproduction

Some lichens develop flask shaped structures called Pycnidia immersed in the thallus

The opening of the pycnidium is called Ostiole. The interior of the pycnidium is lined by a number of hyphae, which act as conidiophores or pycnidiophores.

The conidiophores cuts of a chain of small cells called conidia.

The conidia on being dispersed , germinate under favorable conditions by sending hyphae in all directions.

These hyphae when come in contact with a suitable algae, develop into a new lichen body.

Sexual Reproduction

In lichens sexual reproduction is exhibited by the mycobiont .

Phycobiont has no contribution to the sexual process .

The mode of sexual reproduction in Ascolichens is similar to that of Ascomycotina while that in Basidiolichens is similar to that of Basidiomycotina.

Sexual reproduction in Ascolichen

The female sex organs are known as carpogonia. A carpogonium is differentiated  into a basal coiled ascogonium and an elongated multicellular hypha called trichogyne. The ascogonium remain embeded with in the algal layer of the thallus, where as the trichogyne projects over the surface of the thallus.

The male sexs organs are flask shaped spermognia. They form spermatia at the tips of spermatiophores.

Spermatia act as male gametes. They come out of the spermogonium along with gummy fluid.

They are deposited on the trichogyne of the ascogonium.

On dissolution of the walls between the spermatium and trichogyne, the nucleus of spermatium migrates into the carpagonium through trichogyne. The male nucleus fuses with the female nucleus.

After fertilization many ascogenous hyphae develop from the basal portion of ascogonium. This is followed by crozier formation and the formation of asci and ascospores.

The asci bearing fruit body is called ascomata (Ascocarp). The ascocarp may be either an apothecium (Parmelia, Anaptyahia) or Perithecium (Dermatocarpon, Verrucaria, Peltigera)

Structure of Apothecium

The apothecia are small, elevated cup shaped  or disc shaped fruiting bodies found in the many lichens.

They vary in colour from reddish, reddish brown , yellow to black depending upon the species.

The bottom of the cup is lined by hymenium that consists of number of upright ascii ntersprersed with slender paraphyses containing a reddish oily food-substance.

The wall of apothecium is composed of the vegetative part of the thallus. If the vegetative part consists of both the algal and fungal components it is called Lecanorine type Eg: Lecanora, Parmelia, Physcia.

If the apothecium contains only the fungal component, it is called Lecideine type. Eg: Lecidea, Cladonia, Gyrophora