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FUNGI
Fungi are highly evolved forms of micro-organisms as compared to bacteria. Though generally considered as
lower forms of plants, mycologists (those who study fungi) include them under a separate kingdom Mycota.
The scientific study of fungi started only in the 17th century when microscope became available. Antonio
Micheli (1729), made an extensive study of fungi and discovered that spores were reproductive structures
produced by fungi, which on germination gave rise to fungus.
What are fungi?
Fungi may be defined as small, generally microscopic, eukaryotic, usually filamentous, branched, spore-bearing
organisms. Though the generalists still consider fungi as lower forms of plants, unlike plants they lack
chlorophyll and therefore cannot manufacture their own food. Fungi therefore depend for their food on other
hosts (plants and animals). Some live on dead hosts while others live on living hosts. When they feed on the
dead matter, they are doing a good job of clearing the ever accumulating organic waste in our natural
environment. They however are bad when they feed on living hosts because the living plants and animals that
support the life of fungi become unhealthy or develop diseases and cannot grow normally.
Some fungi like the mushrooms are large enough to be clearly seen; a step further but a little more difficult to
notice are the white or coloured mould fungi which appear on decaying fruit or jam, an old wood in damp
localities, or on leather shoes in the hot, moist weather of the rainy season. But beyond these limits there is still a
vast mass of forms of fungi, often unrecognised as such. These include the bulk of parasites that cause damage to
crops. They can be seen only by special methods of examination (microscopes).
What are fungi?
Fungi are small, generally microscopic, eukaryotic, usually filamentous, branched, spore-bearing organisms that
lack chlorophyll. The filamentous vegetative body is called a mycelium. The mycelium branches out in all
directions. The individual branches of mycelium are called hyphae, and are tubular filamentous structures that
are generally uniform in thickness.
In some fungi the mycelium consists of many cells containing one or two nuclei per cell. In others the mycelium
contains many nuclei which may or be partitioned by cross walls (septa). Growth of mycelium occurs at the tip
of the hyphae. Fungi reproduce chiefly by means of spores
Characteristics of Fungi:
1. Fungi are eukaryotic: eu=true; they have true nuclues
2. The majority are multi-cellular (yeast is single celled)
3. Lack chlorophyll
4. Nutritionally they are heterotrophic, they digest food outside the body and absorb it. They are saprophytes,
parasites and some are mutualistic (symbionts)
5. Their basic body plan constitutes a mass of netlike filaments called hyphae. The entire mass is called the
mycelium. Most fungal hyphae are divided into cells by cross walls called septa. Some fungi are aseptate,
lacking cross walls. They are said to be coenocytic. Some types of fungi contain lateral hyphae that absorb
nutrients from other organisms, called haustoria.
6. Reproduction is accomplished by the release of sexual or asexual spores. For many species of fungi sexual
reproduction is a contingency used when environmental conditions are difficult. Under favorable conditions
asexual spores are produced by the millions and dispersed over a large area.
Place of fungi in the overall classification of living things
Since long ago living things have been divided into plants and animals, based on the following contrasting
characters.
Characters
1. Mobility
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Animals
mobile
Plants
stationery
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2. Food
3. Size
not self-manufactured
definite size
self manufactured
no definite size
The above classification however proved unsatisfactory when with the development of simple microscope by
Leeuwenhoek in 1675 and compound microscope by Hooke in 1820, the hitherto unknown world of microorganisms was discovered. Some organisms(e.g. Euglena) showed characters of both plants and animals. Where
to put an organism which was motile(animal character) and green (plant character)? To solve this confusion,
Haeckel, a German biologist in 1866 proposed a third kingdom Protista (protiston=very first), in addition to the
plant and animal kingdoms. Protista includes organisms that don’t have the development of tissues e.g. algae,
fungi, bacteria, protozoa etc. Later Protists themselves were divided into lower protists (Prokaryotes) and higher
protists (Eukaryotes). Prokaryotes that included bacteria and blue-green algae lacked a definite nucleus.
Eukaryotes which include fungi have a well-defined nucleus.
Whittaker in 1969 proposed a five kingdom classification of living world. These are:
1. Monera (=Prokaryota)
2. Protista (unicellular eukaryotes)
3. Plantae or Plant kingdom
4. The Fungal kingdom (Mycota)
5. Animalia or Animal kingdom
It can be thus seen from the above descriptions that there are different versions of classification. Old publications
still consider fungi under the Plant Kingdom. They describe under division Thallophyta that includes the lowest
and most primitive forms of plant. The plant body of every thallophyte is an undifferentiated mass of cells,
called thallus. Organs like root, stem and leaves are never found in this group. However, for our purpose, it will
be considered under the fungal kingdom, Mycota.
General characters of fungi
Habitat: Fungi are found everywhere - in plants, animals, water, air , soil, rocks, dead plants and animal bodies,
food products, paper and even on plastics. Their ability to live in diverse habitat is because of their potential to
utilise any kind of substrate as food and survive under any environment.
Body of fungus: All fungi, with very few exceptions, originate from spores. Spores are roughly similar to seeds.
These spores germinate, requiring for the purpose very much the same conditions as ordinary seeds, such as
moisture, suitable temperature, and so on. The result of germination is usually the protrusion of one or more fine
filaments, known as ‘germ-tubes’ or ‘infection thread’ from the spores. This grow in length at the tip to form a
long tubular filament known as hypha (pl. hyphae). Branches are given off in all directions and the resulting
branched mass of hyphae are known as the ‘mycelium’ and this forms the vegetative part of the fungus which is
concerned with growth and the absorption of food.
The hyphae may be segmented into a row of cells, by forming transverse walls (septa) across the filament. When
hypha is divided into compartments (cells) by means of cross walls (septa, sing. septum), it is known as septate;
when hypha is not divided by septa, it is known as aseptate or coenocytic.
Each cell is a hollow structure, bounded by walls of a pliable (soft), transparent substance, which in some cases
is composed of the chemical compound cellulose, best known as forming the bulk of cotton fibres. In most cases,
however, the wall largely consists of a substance closely resembling chitin, the material which forms the hard
covering of insects.
Within the walls, the cell is filled with protoplasm. The protoplasm does not fill the cell all through with a
uniform mass. Here and there little hollows or spaces, called ‘vacuoles’, occur containing water with dissolved
substances, the whole forming the cell sap. Suspended in the protoplasm are the nuclei. In certain cases two,
three or more have been found. In segmented filaments, each cell usually contains one nucleus only. Though
divided by septa, there are perforations in the septa through which cytoplasmic strands including nuclei migrate
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from one cell to other. In the lower fungi, the hyphae is ordinarily unsegmented. In such cases, the nuclei are
distributed irregularly in the protoplasm.
The food of fungi (Nutrition)
Since the fungi cannot manufacture their own food, they depend on other hosts for ready food. Therefore fungi
are heterotrophic organisms. They have absorptive or holophytic type of nutrition. The substrate is dissolved
and then absorbed. For this, the fungi produce extra-cellular enzymes which degrade the insoluble substrate into
smaller fragments and finally into soluble units which are then absorbed by the hyphae.
According to the manner in which they obtain their food, fungi are divided into great classes: Saprophytes and
Parasites.
Saprophytes obtain their food from dead tissues of animals or plants, or of substances derived from them.
Hence they are found in large numbers in soil, living on every bit of rotting leaf, twig or manure. They grow as
moulds on fruits, jam, stale bread and leather. Every bit of old timer in the forest is liable to be permeated by
their hyphae. In company with bacteria they play a useful part in nature as scavengers.
Parasites obtain their food from the living tissues of animals and plants. The great majority of parasitic fungi
feed on living plants (hosts). When they feed on those plants that we grow as crops, they cause heavy yield loses.
Fungal parasitism varies from facultative parasitism to obligate parasitism. The facultative parasites can grow
saprophytically in the absence of host while obligate parasites (downy mildews, powdery mildews, and rusts)
grow only on their specific hosts.
Parasitic fungi may be further divided into two types, ‘ectoparasites’ and ‘endoparasites’ according to their
situation on their host plants. Ecto-parasites include those forms that grow on the surface of the leaves, stems or
other parts of the affected plant, and obtain their food through the outer cell walls without penetrating deeply
into the tissues. They feed usually by means of special outgrowths from the hyphae, known as suckers or
‘haustoria’. These arise in hypha in contact with the epidermis, and penetrate wholly or partly through the
toughened outer walls of the epidermal cells. All the body of the fungus lies outside the plant, excepting these
haustoria, and there are even several cases where the haustoria have not been detected and it seems as if the food
is absorbed directly form the outer cells across the unbroken cell wall.
Endoparasites, on the other hand, penetrate into the plant and develop their vegetative mycelium within the
tissues. Sometimes the internal mycelium is confined to a small part of the plant, as in the case of numerous leafspotting fungi, where each little patch of invaded leaf tissue shows as a discoloured spot owing to the death of
cells.
When a single spore of one of these leaf-spotting fungi germinates on a suitable leaf, the germ-tube penetrates
into the leaf, grows, branches and the resulting hyphae spread all around in the tissues. Their food is obtained
from the cell sap either by means of suckers or haustoria or by absorption through the walls in case of fungi that
do not have haustoria. When a certain amount of food has been obtained and a certain number of leaf cells
killed, the fungus ceases the vegetative growth and reaches the reproductive stage. In other endoparasites, the
growth may extend to a considerable part of the plants (e.g. Phytophthora infestans) and reproduction may occur
while vegetative growth is still going on. The formation of spores within the tissues would be of little use in
disseminating the fungus, so the endoparasites practically all send out special sporophores to the surface of the
plant, and form their spores in the outside air. In some cases a second kind of spore, usually a sexually-formed
resting spore, is developed within the tissues, and this is only set free and germinates when they have rotted. The
use of this second spore form is generally to carry the fungus over some unfavourable season, so that
germination will only occur when conditions are again suitable for the general life of the fungus.
Reproduction
Fungi multiply by non-sexual (vegetative, and asexual reproduction) or sexual methods of reproduction. The
non-sexual methods (fragmentation, budding, fission, chlamydospores, oidia, zoospores, aplanospores, conidia
etc.) serve to disseminate it to far off places. Sexual reproduction occurs during conditions which are not
favourable for the fungus growth.
Non-sexual or Asexual Reproduction
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a) Fragmentation. A small bit of the broken hypha establishes a new colony. Fragmentation occurs frequently
in nature and is employed in the laboratory to keep the fungus growing by transforming small portions to new
culture tubes.
b) Budding. A small portion of the cell wall bulges out like a weak zone of the cycle tube, and a daughter
nucleus migrates into it. The bud is pinched off by constriction at the point of the origin of the bud. Sometimes,
budding is so quick that a chain of cells is formed due to non-detachment of the daughter cells.
c) Fission. This occurs in fission-yeast only. It is characteristic of bacteria. The cell divides in transverse plane
into two cells.
d) Chlamydospores. These are thick-walled, resistant spores which are formed to tide over the adverse
environment. These are formed by formation of thick walls around cells. Chlamydospores are not detached form
the hyphae. When hyphae die, these remain viable.
e) Arthrospores or oidia. The cells of the hyphae at the distal end round off and separate in basipetal
succession (from apex towards the base of the hypha). On germination, the arthrospores give rise to new fungus
colonies.
f) Sporangiospores and conidia. Spores formed internally, inside a sac-like structure called sporangia, are
called sporangiospores. Conidia are formed externally on the tip or sides of specialised hyphae
(conidiophore). The sporangiospores may be flagellate (having flagella) or non-flagellate. The flagella are hairlike appendages used for locomotion. The flagellate spores are called zoospores and are formed mostly by
aquatic or soil fungi. The zoospores are uniflagellate or biflagellate and this character forms the basis of
separation of the four classes of the sub-division Mastigomycotina.
The non-flagellate spores are called aplanospores and are produced by terrestrial fungi. The sporangia, usually
are borne on specialised hyphae called sporangiophores, which in some fungi are branched in such a
characteristic way that the genera are identified by their sporangiophores e.g. downy mildews. Similarly conidia
are borne on conidiophores; they may be characteristically branched and useful in taxonomy. The conidiophores
lie externally or formed inside flask-shaped or globular structures called pycnidium or flat, disc-shaped
acervulus.
Sexual reproduction
a) Gametogamy (Fusion between gametes). Gametes are naked sex cells, which copulate to form a zygote. If
the two gametes are similar in size they are called isogametes and their copulation, isogamy. Copulation
between two dissimilar gametes, one smaller (male) and the other bigger (female), is called anisogamy. The
fusion between a male, motile gamete and a non-motile female(egg) lying in the oogonium, is called
heterogamy.
b) Gametangiogamy. (fusion between gametangia). When the gametangia are similar in shape and size then
these are designated as + and - gametangia rather than as male and female. When the gametangia are of different
shapes, these are called heterogametangia - the male is usually smaller and clubshaped, while the female is
bigger and globular. Fusion between two similar gametangia results in a zygote which is called zygospore. The
zygote formed by the fusion between morphologically distinct gametangia is called a oospore and the process,
oogamy. The plasmogamy between them is brought about by (I) gametangial copulation or (ii) gametangial
contact.
i) Gametangial contact. The antheridium (and occasionally also the oogonia) is not differentiated into definite
protoplasts or gametes but are represented only by their nuclei. The male nuclei (never the cytoplasm), migrate
into the oogonium through a pore dissolved at the point of contact or through a fertilization tube formed by the
antheridium, example: Pythium, Phytophthora.
ii) Gametangial copulation is of two types. In one (e.g. Mucor or yeast), the entire gametangia fuse, the
intervening wall disappears and their contents come to lie in the common cell formed by their fusion. In other
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type (e.g. Rhizophidium), the contents of the male gametangium migrate into the female gametangium through a
pore or a fertilization tube. The male gametangium is left empty.
c) Spermatization. Spermatia (sing. spermatium), which are minute, male gametes, are formed like conidia on
spermatiophores. The spermatiophores may be formed exogenously or arranged inside a spermogonium e.g.
Puccinia. The spermatium, when comes in contact with the female gametangium through wind, insects, water,
sugary exudates etc., releases the male nucleus into the female gametangium through a pore.
d) Somatogamy. In some higher fungi, sex organs are not formed. The somatic cells, as such, act as gametangia.
Their fusion is governed by homothallism and heterothallism. Thus somatogamy may occur between cells of the
same hypha - (in homothallic genus) and between cells of different thalli (in a heterothallic fungus).
REFERENCES:
1.
Dube, H.C. (1978). A Textbook of Fungi, Bacteria & Viruses. Vani Educational Books.
2.
Singh, R.S. (1983). Plant Disease. Oxford & IBH Publishing Co., N. Delhi.
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