Here is a term paper on ‘Fungi’. Find paragraphs, long and short term papers on ‘Fungi’ especially written for school and college students.
Term Paper on Fungi
Term Paper Contents:
- Term Paper on the General Consideration of Fungi
- Term Paper on the Origin of Fungi
- Term Paper on the Classification of Fungi
- Term Paper on the Evolution among Fungi
- Term Paper on the Reproduction in Fungi
- Term Paper on Evolution and Degeneration, of Sexuality in Fungi
- Term Paper on the Economic Importance of Fungi
Term Paper # 1. General Consideration of Fungi:
This great group of thallophytes is readily distinguished from other groups by the total lack of the photosynthetic pigment, chlorophyll resulting in the disability of manufacturing their own food. Hence, they are compelled to live either as parasites, or as saprophytes.
But, sometimes they live in close association with algae, as in the case of lichens; such a mode of life is known as symbiosis, in which the associates derive mutual benefit from each other. They also live in close association with the roots of certain higher plants, particularly forest trees. This combination of the fungus and the host is known as mycorrhiza.
This symbiotic relationship does not seem to be distinctly harmful to the tree, yet it is often thought to be a mild form of parasitism. The habitats of fungi are extremely diverse. They not only occur as parasites forming haustoria within the host cells, and saprophytes, but are often found as epiphytes and may even be subterranean.
Most fungi have a life history which consists of two phases:
(a) Vegetative phase, in which the fungus grows through the host or substratum and constructs its vegetative body, and
(b) Reproductive phase, in which the fungus produces spores or other reproductive structures by means of which it perpetuates itself.
The vegetative body, with the exception of the unicellular forms, consists of a more or less extensive, much-branched, filamentous, colourless or coloured structure, called the mycelium, in which a single filamentous branch is called a hypha. This mycelium originates as a result of germination of a spore, which, in the simplest case, is unicellular and uninucleate and may or may not be motile.
A spore germinates, by pushing out from any point or from a thin place (germ pore) on its wall, in a tube-like or a filament-like process, called the germ tube. Rapid elongation and branching follow and a mycelium is formed, the growth of the hypha being restricted at the tip.
The mycelium may be septate or aseptate. When aseptate, the protoplast is continuous throughout and contains numerous minute nuclei and vacuoles within it; such a mycelium is known as a coenocyte. The septate mycelium contains many hyphal cells and each cell contains a uninucleate, binucleate, or multinucleate protoplast.
Among higher fungi the life cycle consists of two phases:
(a) Monokaryon phase, in which the mycelium consists of hyphal cells which are regularly uninucleate, and
(b) Dikaryon phase, with binucleate cells.
In some cases, the nuclei of uninucleate and binucleate cells divide several times so that the hyphal cells become multinucleate.
The fungal protoplast consists of a granular or reticulated cytoplasm, which, in the older region, leaves a vacuole in the middle of the cell. The nucleus may show a delicate nuclear reticulum with one or more nucleoli, its contents contracted into a dense chromatin body surrounded by a hyaline area. The division of the nucleus is mitotic as well as meiotic.
In some cases, the cell wall is made up of pure cellulose but in others little cellulose is present, the greater part of the wall being a complex fatty acid with a chitin base.
Owing to the absence of the photosynthetic pigments, fungi accumulate carbohydrate reserves, such as sugars and glycogen instead of starch. Oils and proteins are also found as reserve materials. The fungal protoplast also secretes various types of enzymes during its activity, and these not only help the fungus to deal with reserve materials but also assist the parasitic fungi to enter or break down the tissues of the host.
In many cases the hyphae grow together, intertwine, adhere and form a thick tissue, called the plectenchyma.
It is of two types:
(a) Prosoplectenchyma or prosenchyma when in a tissue the singl hyphal elements are recognizable, and
(b) Paraplectenchyma or pseudoparenchyma, in which the hyphal individually is entirely lost, so that it appears more or less isodiametric in section, resembling a parenchyma.
When the plectenchyma become tuberiform in appearance with a pseudoparenchymatic outer rind and a prosenchymatic central core, it is called a sclerotium. A sclerotium is a resting body, with abundant food reserves, which carries the fungus over unfavourable conditions of growth, and on the return of normal conditions either directly develops into a mycelium or gives rise to a fructification.
Sometimes plectenchyma is formed chiefly from parallel hyphae forming root-like strands, called rhizomorphs, which under suitable conditions give rise to the mycelium.
Term Paper # 2. Origin of Fungi:
As regards the origin of the true fungi, there are two opposite schools of view. Workers like DeBary (1881), von Tavel (1892) Bessey (1942), and Gaumann (1949) belonging to one school believe that they have come from the algae, while Dangeard (1886, 1903) Fischer (1892), Atkinson (1909), Cavers (1915), Scherffel (1925), and Cook (1928) favour their derivation from the protozoa.
The proponents of the algal origin of the fungi suggest that some algae, which somehow had lost their chlorophyll and thereby had to change their mode of nutrition from an autotrophic to a heterotrophic one, had given rise to the true fungi, and this had taken place in diverse lines. According to them, the Phycomycetes have come from one class of algae, and the Ascomycetes from another, while the Basidiomycetes have been evolved from the Ascomycetes.
A good deal of controversy centres around the hypothetical algal ancestor of the Phycomycetes. Some suggest that the probable ancestor was a siphonaceous green algae, while others regard it to be an oogamous Xanthophyceae, or one of its unicellular coccoid members.
The question which still remains unsolved in whether the Zygomycetes came first or the Oomycetes. Depending on the similarities in the structure of reproductive organs as well as in the gonimoblast filaments and ascogenous hyphae in the Rhodophyceae and the Ascomycetes, the latter are also thought by some, to have been derived from the former.
Those who favour a protozoan origin of fungi suggest that at first the chytrids appeared, and they, in turn, gave rise later on to the other Phycomycetes with an extensive hyphal system, and this origin had been polyphyletic. Believers in this idea are of opinion that the Oomycetes have originated from the Zygomycetes.
Term Paper # 3.
Classification of Fungi:
The fungi are more or less universally divided into four classes as follows:
1. Phycomycetes (e.g Synchytrium Saprolegnia, Pythium. Phytophthora, Peronospora. Albugo, Mucor, Rhizopus, Pilobolus, etc.).
The vegetative body is usually a coenocytic mycelium which may sometimes be absent or rudimentary.
A well-developed septate mycelium is always present:
2. Ascomycetes (e.g. Saccharomyces, Eurotium, Penicillium, Peziza, Ascobolus, Erysiphe, Claviceps, Pyronema, Humaria, Morchella, Sphaerotheca, etc.).
Characteristic asexual spores, the ascospores are formed endogenously within specialized spore mother cells, called asci.
3. Basidiomycetes (e.g. Ustilago, Tilletia, Puccinia, Polyporus, Agaricus, Phallus, Lycoperdon, Cyathus, etc).
Characteristic asexual spores, the basidiospores, are formed exogenously on specialized spore mother cells, known as basidia.
4. Deuteromycetes or Fungi imperfecti (e.g. Fusarium, Cercospora, Curvularia, Alternaria, Helminthosporium. Colletotrichum. etc).
Both ascospores and basidiospores are not produced but the fungi reproduce mostly by means of conidia.
Term Paper # 4.
Evolution among Fungi:
Though unquestionable fossil records of the Phycomycetes have been found even as far back as the Devonian, they are not of much help in clarifying the problem of evolution among the true fungi.
Whatever, might have been the ancestral form of the fungi, the non-mycelial chytrids are almost unanimously regarded as the most primitive among the Phycomycetes. From these chytrids there appeared two distinct series, one with uniflagellate swarmers and the other with biflagellate ones. In both these directions evolution has taken place from isogamy to oogamy, and most probably the Zygomycetes are reduced forms evolved from the Oomycetes.
If the phycomycetous origin of the Ascomycetes is accepted, then those Ascomycetes, which possess a single ascus produced by the fusion of two cells (Plectomycetes) are to be considered as the most primitive, from which the other types have arisen.
Most probably some higher Ascomycetes, in which the dikaryocic hypha produced the ascus, underwent modifications, and the ascus, and the basidium of the Basidiomycetes came into existence.
Based on this hypothesis, the Basidiomycetes possessing an aseptate basidium are the most primitive, from which the bi-directional evolution has taken place. The basidium, in one line of development is typically vertically septate, while in the other, it is always transversely septate.
Term Paper # 5.
Reproduction in Fungi:
Most fungi reproduce by vegetative, asexual and sexual methods.
The fungi may reproduce vegetatively by the fragmentation of the whole thallus or of considerable portions of it. Vegetative cells of some yeasts and also the ascospores and the basidiospores in some higher fungi may give rise to lateral outgrowths, which cut off buds and this process of cell multiplication is known as budding.
Asexual reproduction takes place by the production of one or more types of asexual reproductive bodies, called spores. Some of these types are formed by the sporophyte at a specific time in the lite cycles of higher fungi (Ascomycetes and Basidiomycetes) immediately following reduction division of the spore mother cells.
When formed within (endogenous) the mother cells, they are called ascospores, and the mother cell, an ascus; when the mother cell produces spores externally (exogenous), the spores are termed basidiospores, and the mother cell, a basidium.
These two types of spores actually correspond to the spores of vascular plants. Other types of spores are also formed for accessory methods of multiplication, but do not possess any relation to a sexual process and do not take part in the alternation of generations. They are purely vegetative in nature and produce mycelia, identical with those producing them.
These are device for rapid increase and dissemination of the organisms, and are either produced by the sporophyte (e.g. rusts and a few other Basidiomycetes) or by the gametophyte (e.g. most of the Phycomycetes and Ascomycetes).
Some of the principal types of these accessory spores are:
(1) Chlamydospores, formed by the direct transformation of hyphal cells, or segments of mycelium which become enlarged, thick walled, filled with food, or portions of the contents of hyphae contract, lose water and become surrounded by thick walls, and act as resting spores;
(2) Sporangiospores, formed within enlarged unicellular sacs, called sporangia, due to the successive cleavage of their protoplasts; these spores may be either naked and amoeboid, or are provided with flagella forming free-swimming cells, and are known as zoospores, and the sporangia, zoosporangia. When the spores, thus formed, become distinctly walled and non-motile, they are termed aplanospores or simply spores;
(3) Conidia, when cut off externally from the tip of a hypha, fall off and are disseminated by the wind; and
(4) Oidia, which are another kind of vegetatively formed cells arising directly from the breaking up of a hypha.
Sexual reproduction in fungi is extremely diverse when considered in detail, but when taken as a whole, it shows some evolutionary sequence. Among lower Phycomycetes isogamy is represented by the union of two morphologically identical cells, called gametes, which fuse in pairs to form a zygote.
A somewhat advanced condition is also noticed among Phycomycetes where there is a union of two flagellate gametes of unequal size (anisogamy) of which the larger one is regarded as the female cell. The next higher step in the sexual process, when considered in evolutionary sequence, is oogamy.
In this case, the female gamete (oosphere or ovum) is a large, non-flagellate, non-motile and passive cell and the male gamete (spermatozoid or antherozoid) is a small, flagellate, motile and active cell, as in a few Phycomycetes. Besides these normal processes, there are cases where variations occur.
For instance, in some Ascomycetes the male gamete is set free as a naked or very thin-walled, non-motile sperm which becomes attached to the wall of the oogonium or to a tube-like extension of the oogonium, called the trichogyne. Among higher Phycomycetes distinct sperm cells are not produced but only male nuclei are introduced into the oogonium.
Sometimes fusion takes place between two gametangia, distinguishable or indistinguishable whereby the multinucleate protoplasts of the two eventually unite. Among Basidiomycetes, with the exception of rust fungi, sex organs are entirely absent, but, the mycelium may exist in two or more sexually differentiated strains. In such cases, any cell of a plant of one sexual phase may unite with any cell of the plant of the opposite sexual phase, as a result of which a sporophytic or diploid (dikaryon) mycelium is produced.
The product of union of gametes is a zygote (zygospore or oospore), depending on the nature of the uniting gametes. Such spores are known as sexual spores, which are diploid, and they also take part in the alternation of generations.
Term Paper # 6.
Evolution and Degeneration of Sexuality in Fungi:
It is the usual practice with the botanists to consider plants having definite sex organs and sexual reproductive units (gametes) as primitive ones and placed low in the scale of evolution than those which do not possess any apparent sex organs or gametes.
The mode of sexual reproduction in fungi, in this respect, affords a very good example of gradual reduction and final obliteration of any trace of the sex organs or gametes. If one gradually advances from the Phycomycetes to the Basidiomycetes through the Ascomycetes, one can never fail to notice it, though occasional types may be found, which shows a tendency towards the primitive forms.
The different forms of sexuality found among fungi may be broadly summarized as follows:
i. Isogamy:
This is the simplest and most primitive form of sexuality, found among, lower Phycomycetes (e.g. Synchytrium endobioticum, Olpidium viciae, etc.), in which sexual union takes place between two flagellated sexual cells, that are not closely related to each other, giving rise at first to a flagellate zygote. Since the uniting gametes are morphologically identical with each other, they are called isogametes. These gametes always arise as daughter cells of the gametangia.
ii. Anisogamy:
This type of sexual union is exhibited by Allomyces javanicus in which the two uniting flagellated gametes differ only in size. They are also produced as daughter cells of the gametangia and the product of their union is always a zygote.
Some mycologists hold that the larger gamete may be regarded as the female and the smaller one the male. The gametangium that gives rise to the female gamete is usually larger than the gametangium producing the male gamete.
iii. Oogamy:
In this type of sexuality the gametes not only arise as daughter cells of the gametangia but they are sexually differentiated into male and female gametes. Typical oogamous reproduction is found in Monoblepharis where sexual union takes place between a flagellated small, motile male gamete or spermatozoid and a large, non-motile, non-flagellated female gamete or oosphere or ovum.
The product of the union is always an oospore (zygote). The sex organs are also differentiated into antheridium (male gametangium) and oogonium (female gametangium). In other cases as in Saprolegnia, Phytophthora, Pythium, Albugo, etc. though the antheridia and oogonia become more marked and well-developed the antheridium never gives rise to motile spermatozoids, which are represented by the male gamete nuclei.
In such cases the male nuclei from the antheridium pass into the oogonium through the fertilization-tube and are discharged near the oosphere. Only one male nucleus then unites with the female nucleus of the ovum, and an oospore is formed. In Saprolegnia there are several oospheres within the oogonium.
In this case, several antheridia are formed and from these different fertilization-tubes penetrate the wall of the oogonium and discharge the sperm nuclei to the individual oospheres and fertilization takes place. As a result of union several oospores are produced within the single oogonium. The type of sexual reproduction in which oospores are produced is known as oomycetous, and the group of fungi producing it is known as Oomycetes.
iv. Gametangial Copulation:
This type of sexuality represents the first step towards the gradual reduction of sex in fungi. In this case the gamete formation is suppressed and union takes place between gametangia instead of gametes. The role of gametes is taken over by the gametangia, and as a result of gametangial fusion their multinucleate contents fuse to form a strong coenocytic zygote, known as the zygospore.
Sexual reproduction resulting in zygospore formation is known as the zygomycetous type and the fungi forming zygospores are known as Zygomycetes. This type of sexuality is commonly found in Mucor, Rhizopus, Phycomyces, Pilobolus, etc. The species may be homothallic or heterothallic. In heterothallic forms the thalli as well as the gametangia produced by them are designated as ( + ) or female, and (—) or male.
v. Self-Fertilization:
This type of sexuality represents the next higher step towards reduction of sex in fungi and is found among the members of Ascomycetes. In this case only a functional oogonium is present, and an antheridium is either abortive or entirely absent. As a result, union cannot take place between two sex organs. There are two ways by which the sexual act is performed.
In Ascobolus citrinus the female sexual organ is multicellular and union takes place between two cells of the female sexual organs (parthenogamy). In Humaria granulata, on the other hand, the oogonium is multinucleate and fusion takes place between the sexual nuclei in pairs within the oogonium and not by cell fusion (autogamy).
vi. Pseudo-Fertilization:
In this case the degeneration is sexuality has gone a step further. The sex organs have completely disappeared and sexual, union takes place between two vegetative cells. In many heterothallic Basidiomycetes (e.g. Peniophora sambauci, Polyporus ostreiformis, Polystictus sanguineus, Polystictus hirsutus, Stereum fuscum, Lentinus subnudus, etc.) fusion between the vegetative cells of mycelia bearing opposite sexual strains takes place and the nuclei of complementary strains; become associated together.
In this case plasmogamy occurs and karyogamy is delayed till the formation of the basidium in which the two nuclei fuse to form a zygote nucleus. In some yeast (e.g. Saccharomyces, Zygosaccharomyces, etc.) union takes place between two vegetative cells, the nuclei fuse and a zygote is formed. The uniting cells may be entirely unrelated or may be mother and daughter cells.
vii. Parthenogenesis and Apogamy:
These processes represent the last step in the reduction of sexuality in which reproduction takes place vegetatively without cell- or nuclear-fusion. In parthenogenesis new individuals are produced from haploid sexual cells, while in apogamy the individuals arise from diploid sexual cells without meiosis.
Term Paper # 7.
Economic Importance of Fungi:
The fungi are of great economic importance on account of their both harmful as well as beneficial effects. A large number of fungi cause destructive havoc to our valuable crop and timber plants, various kinds of food and food products. They also attack the live-stock as well as human, beings. Some serious-diseases of the skin, eye, ear, nose and throat as well as intestinal and bronchial disorders are also caused by the various groups of fungi.
But, all of them are not harmful to the mankind, as most of the species bring about decomposition of dead bodies of plants and animals as well as of animal dung; thus the organic debris is removed from the surface of the earth and the fertility of the soil is enriched. Further, some species are edible and quite a good number of them produce various kinds of vitamins, organic acids, drugs, enzymes, alcohols and other medicinally and industrially important substances.
The Phycomycetes usually cause damage to different crop plants. For example, Pythium de Baryanum causes the ‘damping off’ disease resulting in the death of young seedlings. Phytophthora infestans is responsible for the serious ‘late blight’ and ‘rot’ of potato. Albugo Candida brings about much deformation of the inflorescences and fruits of cruciferous plants throughout the world.
Peronospora parasitica is the pathogen causing the ‘downy mildew’ of crucifers. Saprolegnia parasitica, causes serious damage to the fishing industry. The various species of Mucor and Rhizopus spoil the preserved fruits, pickles, jams and jellies, bread, butter, cheese, leather and various other consumable articles. A few members of the Order Mucorales, however, can produce some organic acids.
Of the different ascomycetous fungi, the three medicinally most important ones are the yeasts, penicillia and ergot fungi. Various species of Saccharomyces are very good sources of vitamin B. Penicillium notatum and P. chrysogenum yield the wonder drug ‘penicillin’. Claviceps purpurea produces the ‘ergot’, which is extensively used for stopping haemorrhage after child birth.
The different types of yeasts are employed for the preparation of various kinds of alcoholic drinks as well as in baking breads. Some species of Aspergillus are used in the preparation of diastase and organic acids, while some costly cheeses are prepared with the help of a few species of Penicillium. Morchella esculenta, the common morel, is an edible fungus.
The polypores among the Basidiomycetes are the chief agents in causing serious damages to the timber trees, lumbers, railway sleepers, telegraph and fencing posts, and wooden buildings and furniture. It is interesting to note that some of them, however, are taken as food by man, such as Daedalea flavida.
Besides various mushrooms, puff balls also form table delicacies. Some of them are, however, deadly poisonous to the human system, and may even cause death. The members of the orders Uredinales and Ustilaginales are highly destructive to the various important crop plants, like wheat, maize, oat, barley and others. Researches are going on for the discovery of antibiotics from some polypores. Auricularia auricula is locally used in some parts of India for curing ear sores.
Many of the Deuteromycetes not only cause diseases of some higher plants but also are responsible for the athlete’s foot, ringworm and other skin diseases in man.
No comments yet.