VIRUSES
Five kingdom classification of RH Whittaker has not
classified viruses, viroids and lichens in any group.
Lichens are already described earlier in this chapter.
The viruses did not find a place in any classification system
since they are not truly living, as living organisms are those
that have a cellular structure. The viruses are non-cellular
organisms that are characterised by having an inert
crystalline structure outside the living cell. An inert virus
outside the cell is called virion. These are obligate parasites.
They do not have a biosynthetic machinery. Once they infect
a living host cell, they take over the machinery of the host cell
to replicate themselves, killing the host.
Discoveries Related to Virus
(i) The name virus was given by Pasteur (virus means
venom or poisonous fluid). DJ Ivanowsky (1892)
recognised certain microbes as causative organism of
the mosaic disease of tobacco.
(ii) MW Beijerinck (1898) demonstrated that the
extract of the infected plants of tobacco could cause
infection in healthy plants. He called the fluid as
contagium vivum fluidum (infectious living fluid).
(iii) WM Stanley (1935) showed that viruses could be
crystallised and crystals consist largely of proteins.
Virus is a ultramicroscopic nucleoprotein entity. Size of
virus ranges from about 10 nm (foot and mouth virus of
cattle), 17 nm (alfalfa mosaic virus) to 1250 x 40 nm
(beet yellow virus), T MV (Tobacco Mosaic Virus) is
300 x 15 nm, 400 nm (parrot fever virus), x 15 nm
( Pseudomonas).
The architecture of viruses is mainly of following types
(i) Spherical, e.g., Alfalfa mosaic virus,
(ii) Cubical, e.g., Vaccinia
(iii) Helical (elongate body) e.g., T M V.
(iv) Polyhedral (short, broad body, rounded, polyhedral
shape), e.g., Poliomyelitis virus.
(v) Binal (cuboidal and helical parts both), e.g., many
bacteriophages like T 2 and T

Structure
A virus structure has mainly four parts
i. Envelope
It is the outer covering present in certain viruses. It is made
of protein Of viral origin, lipid and carbohydrate of host.
Spikes or outgrowths may or may not be present. Some
common enveloped viruses are HIV, Herpes virus, Vaccinia
virus.
ii. Capsid
It is a protein covering around the genetic material. Capsid
has protein subunits called capsomeres. T MV has
2130 capsomeres. The capsomeres are arranged helically or
in geometric forms.
iii. Nucleoid
It contains genetic material which is either DNA or RNA
but never both. Most of the animal viruses have
double-stranded DNA as genetic material, while majority
of plant viruses have single-stranded RNA as genetic
material. Some animal viruses may even have single or
double-stranded RNA.
iv. Enzymes
They are rarely present. Lysozyme is found in
bacteriophages. In some RNA viruses called retroviruses,
RNA polymerase, reverse transcriptase enzymes are present.
Classification
Based on the presence of DNA or RNA, viruses are divided
in two main groups
i. Deoxyvira or DNA viruses
These are generally animal viruses with a few important
ones having RNA. e.g., rabies virus, polio virus, retroviruses
including HIV or AIDS virus. The structural form of
deoxyvira are deoxyhelica, deoxycubica and deoxybinala.
ii. Ribovira or RNA viruses
These are generally plant viruses with a few containing
DNA. e.g., cauliflower mosaic virus. The structural form of
ribovira are ribohelica and ribocubica.

Types of Viruses
Viruses are classified by Holmes (1948) into three groups
based on their host types
i. Plants Viruses (Phytophagineae)
These viruses cause disease in plants. Examples of plant
viruses are Tobacco Mosaic Virus (T M V), Potato Mosaic
Virus (P M V), tomato leaf curl virus, etc.
ii. Animal Viruses (Zoophagineae)
These viruses cause disease in humans.
e.g., influenza virus, smallpox virus, poliomyelitis virus,
hepatitis virus, mumps virus, rhino viruses.
iii. Bacterial Viruses (Phagineae)
These are also known as bacteriophages and mainly infect
lower organisms (phagineae bacteria). Bacteriophages were
discovered by Edward Twort (1915) and d’ Herelle (1917)
independently.
For gxample, T 2 , T 4 , lambda, coliphages (bacteriophag«
of E.coli), cyanophages (blue-green algal viruses,
e.g., LPP-I, SM-I, N-1), phycophages (algal viruses),
mycophages (fungal viruses), zymophages (mycophages of
yeast).
The shape of bacteriophages is tadpole-like consisting of
head (icosahadral) and a tail. Head has a protein cover
capsid. Internally, the head encloses a highly folded
double-stranded DNA (approx. 50 in length).
A connector is present in between the head and the
The connector contains a neck and a collar. Collar possesses
several whiskers. The tail is the narrow cylindrical part.
has central hollow core or tube through which viral DNA’
injected into the host.
The core contains enzymes like lysozyme, etc. and
(formed
surrounded by
sheath proteins
2000 capsomeres). The tail ends in a basal plate or e
plate. The basal plate includes tail pins and tail
(spikes) that are generally involved in attachment to the
host cell.
Viral Reproduction
Viruses multiply after entering into living cells. The two
types of viral reproduction are lytic and lysogenic cycle.

LICHENS
Lichens are symbiotic associations between algae and fungi.
The algal component (autotrophic) is called phycobiont
and fungal component (heterotrophic) is mycobiont.
Lichens are found in habitat like walls, window panes,
barren rocks, cooled volcanic lava, tree bark, soil and
aquatic habitats.
Different forms of lichens are formed based on their habitat
such as saxicolous, lignicolous, corticolous, terricolous,
marine and freshwater.
Morph010kv
Lichens vary in colour like orange, brown, dark brown,
grey, yellowish green, etc.
Based on their morphology, lichens can be of various types
(i) Crustose These lichens are crust-like, closely
attached to the substratum, e.g., Graphis, Lecanora.
(ii) Leprose These are like minute scales attached
superficially to the substratum, e.g., Lepraria.
(iii) Foliose These are like twisted and crinkled leaf,
e.g., Parmelia, Physcia.
(iv) Fruticose These are branched, erect with bushy
appearance, e.g., Evernia, Ramalina, Usnea.
(v) Filamentous These lichens consist of chains of algal
cells wrapped around by fungal hyphae,
e.g., Racodium.
Internal Structure
The maior part of lichen body constitutes fungus and only 50/0
part is algal cotnponent. The body is divided as upper cortex,
algal zone (gonidial layer), middle medulla, lower cortex and
rhizines.
Only the algal zone contains photosynthetic partner. In about
98% lichens, the fungal partner belongs to Ascomycetes. In
the rest, the fungal partner belongs to Basidiomycetes and
Deuteromycetes.
Symbiotic Relationship
The algal partner or phycobiont plays following roles
(i) Nitrogen-fixation, if cyanobacterial type.
(ii) Photosynthesis
(iii) Provides vitamins and other growth substances.
The fungal partner or mycobiont is involved in
(i) Outer covering for protection.
(ii) Attachment to substratum.
(iii) Protection against harmful radiations.
Ou the basis of above mentioned roles of algal and fungal partner,
lichens show symbiotic relationship, i.e., symbiosis or mutualism.
Sometimes, mycobiont sends haustoria into algal part. It
prevents alga to secrete pectic substances or induces alga to
secrete nutrients. In this case, fungus is considered to be a
controlled parasite over the alga. This phenomenon of
controlled parasitism is called helotism.

Reproduction
Lichens may reproduce vegetatively or sexually. They
reproduce vegetatively by the following ways such as
fragmentation, death and decay, etc.
Lichens reproduce sexually by following ways
(i) Formation of ascomycetous fruiting body.
(ii) Formation of basidiocarp.
Sexual reproduction is not considered to be a common means
of reproduction to form a new lichen.
Importance of Lichens
Lichens have widespread importance for the mankind. Some
of the important uses of lichens are as follows
(i) Lichens contain a complex carbohydrate called
lichenin. It is used as food for many animals.
(ii) Many lichens are useful in producing medicines.
Usnea and Cladonia (antibiotics and usnic acid).
Cetraria islandica (laxative mucilage).
Parmelia (curing epilepsy).
Lobaria (treating lung diseases).
(iii) Lichens like Roccella tinctoria, Parmelia
omphalodes are used in making different types of
dyes. Litmus was also previously obtained from
Roccella montaingne.
(iv) Extracts of Lobaria pulmonaria and Cetraria
islandica are used in tanning leather.
(v) Some lichens like Usnea are used in brewing industry.
(vi) Lichens are best indicators of pollution. They do
not grow in polluted environments.
(vii) Lichens help in understanding the process cf
biological succession by the ecologists.

REPRODUCTION
Reproduction in fungi occurs by vegetative, asexual and
sexual methods.
Vegetative Reproduction
In this mode, a part or fragment of the somatic body
separates and forms a new individual. It occurs by
fission, budding, sclerotia and by
fragmentation,
rhizomorphs.
Asexual Reproduction
In asexual reproduction, a new individual is formed from a
specialised part of the parent. Gamete formation does not
occur. This commonly occurs through spores or small
single-celled or few celled propagules.
The asexual reproduction in fungi occurs by following
methods
(i) Zoospores In Phycomycetes, e.g., Albugo,
Saprolegnia, Achyla, etc.
(ii) Sporangiospores In Rhizopus, Mucor, etc.
(iii) Conidia In Penicillium.
(iv) Conidiosporangia In Phytophthora, Albugo.
(v) Chlamydospores In Rhizopus.
(vi) Arthospores In Mucor, Rhizopus.
Sexual Reproduction
All fungi reproduce asexually except the artificial group i.e.,
Fungi imperfecti (Deuteromycetes). The sex organs that
produce gametes
are called gametangia (sing.
gametangium). The sexual reproduction in fungi occurs by
oospores, ascospores and basidiospores.
The various spores produced in distinct structures are called
fruiting bodies.
The sexual cycle involves the following three stages
(i) Plasmogamy It is the first stage in which the
cytoplasm of two sex cells come close together but
do not fuse with each other. Only the protoplasm of
both cells fuse together. The resulting cell
is
dikaryon or binucleate. The phenomenon is called
dikaryotisation.
(ii) Karyogamy It is the stage in which two compatible
nuclei fuse together resulting into a diploid nucleus
is called synkaryon
(iii) Meiosis It is the stage in which the diploid nucleus
divides resulting in the formation of haploid nucleus
(reduction division).
There are various modes of sequal fusion in ferns. These are
as follows
(i) Plasmogametic copulation Fusion of motile
(flagellated) gametes takes place, e.g. Allomyces.
(ii) Gametangial contact The two gametangia are close
to each other but do not fuse. The non-motile male
gamete passes its nucleus into female gamete through
fertilisation tube, e.g. Phytophthora Albugo.
(iii) Gametangial copulation The two gametangia fuse to
form a zygospore e.g. Mucor, Rhizopus.
(iv) Spermatisation Numerous, minute, uninucleate
spore-like bodies, called spermatia, are transferred to
receptive hyphae cells e.g. Puccinia graminis.

FUNGAL STRUCTURE
The fungal body is an assemblage of long, extremely fine, almost
transparent threads called hyphae. Numerous hyphae
twined around one another to form mycelium (pl.mycelia).
Fungal Hyphae
Fungal hyphae are thin, tubular, transparent threads Or
filaments filled with protoplasm and covered by wall.
The hyphae in fungi are of following types
i. Aseptate Hyphae
In aseptate hyphae, cross walls or septae are not formed at
the time of nuclear division. Such hyphae are
multinucleate. If a mycelium contains aseptate and
multinucleate hyphae, it is called coenocytic hyphae.
ii. Septate Hyphae
In this type, cross walls or septae form after the nuclear
division. The cells may have one, two or many nuclei.
These have septal pores or cross walls in their hyphae, which
allow movement of substances between adjacent cells.
V Dolipore Septum
The central septal pore contains a barrel-shaped inflammation
in many Basidiomycetes. This kind of septum is called dolipore
septum. These pores may get partially plugged by membrane
bound bodies and crystalline structures called woronin bodies.
Fungal Tissues
In fungi, fungal tissue is formed by interweaving of fungal
hyphae called as plectenchyma. It can be further divided as
prosenchyma and pseudoparenchyma.
Prosenchyma is formed Of distinct hyphae running
together in parallel, while pseudoparenchyma is a false
parenchyma formed by close packing and fusion of hyphae.
Fungal Cell
Fungi are eukaryotic cells. A cell wall is present on the
outside, made of chitin and polysaccharide. They possess
all the eukaryotic cell organelles except plastids.
Plasmalemma bears coiled membranous outgrowths called
lomasomes lying below the cell wall. Near the hyphal tip
the cytoplasm contains small vesicles called chitosomes.
These contain cell wall materials. Food reserve is in the
form of glycogen and oil.
During cell division, the nuclear envelope may not dissolve
as in plant and animal cells.

(i) The interior of bacterial cell seems simple and
granular because the membrane bound organelles
are not present in it.
(ii) Mucilage, cell wall and plasma membrane are
together called cell envelope.
(iii) Mucilage or glycocalyx gives gummy or sticky trait
to cells.
Though it is not essential for bacterial survival,
glycocalyx has several functions such as
(a) Protection from dessication and toxins.
(b) Preventing host phagocytes to get attached to
the bacterium.
(c) Holding cells together.
(d) Storing chemicals for virulence.