Essay on Classification of Asexual Reproduction

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In the asexual reproduction method, certain body cells of the parent organism undergo repeated mitotic cell divisions to form two (or more) new organisms of the same kind. Asexual reproduction takes place by six different methods. These are:

1. Fission 2. Budding 3. Spore formation

5. Fragmentation 6. Vegetative propagation

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“We will now describe all these methods of asexual reproduction in detail, one by one. Let us start with fission.

1. Fission

Many single-celled organisms like protozoa and bacteria just split (or break) into two identical halves during cell division, leading to the creation of new organisms. This is called fission. In biology, fission is the process of reproduction in unicellular organisms such as protozoa (like Amoeba, Paramecium, Leishmania etc.) and many bacteria. In the process of fission, a unicellular organism splits (or divides) to form (or more) new organisms. Fission is of two types: binary fission and multiple fission, depending on whether the parent organism splits to form two new organisms or more than two organisms. The two types of fission are discussed below:

(i) Binary Fission

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Binary fission is an asexual method of reproduction of organisms. In binary fission, the parent organism splits (or divides) to form two new organisms. When this happens, the parent organism ceases to exist and two new organisms come into existence. The unicellular organisms like Amoeba, Paramecium, Leishmania, bacteria, etc., reproduce by binary fission. This is described below.

Amoeba reproduces by binary fission by dividing its body into two parts. This happens as follows: When the Amoeba cell has reached its maximum size of growth, then first the nucleus of Amoeba lengthens and divides into two parts. After that the cytoplasm of Amoeba divides into two parts, one part around each nucleus. In this way, one parent Amoeba divides to form two smaller Amoebae (called daughter Amoebae). And we say that one Amoeba produces two Amoebae.

The two daughter Amoebae produced here grow to their full size by eating food and then divide again to produce four Amoebae, and so on. In the unicellular organisms such as Amoeba, the splitting of the parent cell during fission (or cell division) can take place in any plane.

Paramecium is a unicellular animal having short thread-like structures called cilia over its surface (see Figure 6). Paramecium also reproduces by the method of binary fission. A fully grown Paramecium divides its body into two parts to form two smaller Paramecia. This happens by the division of nucleus followed by the division of cytoplasm.

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Leishmania is a unicellular animal (which is protozoan). It is a parasite which causes the disease known as kala-azar (or black fever). Kala-azar is also known as leishmaniasis. Leishmania has a greater degree of organisation in its body, having a whip-like structure called flagellum at its one end.

Leishmania reproduces by the process of binary fission. In Leishmania, the splitting of parent cell during fission (or cell division) takes place in a definite plane (longitudinally) with respect to flagellum at its end. In this respect Leishmania differs from Amoeba (in which fission can take place in any plane).

From the above discussion we conclude that the simple animals like Amoeba, Paramecium and Leishmania reproduce by binary fission. The micro-organisms like bacteria also reproduce by the method of binary fission. Please note that the word ‘binary’ means ‘two’ and the word ‘fission’ means ‘splitting’. So, the term ‘binary fission’ means ‘splitting into two’.

We can observe the binary fission of Amoeba or Paramecium under a microscope. This can be done as follows: Collect some water from a pond or any other stagnant water body (especially where weeds, hay and husk are dumped). Put a few drops of this pond water on a clean slide and observe first under low magnification and then under high magnification of microscope. We will see the Amoeba or Paramecium dividing (or reproducing) by the method of binary fission.

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The term ‘multiple’ means ‘many’ or ‘several’. So, multiple fission means ‘splitting into many’ or ‘splitting into several’. Let us discuss the multiple fission now.

(ii) Multiple Fission :

Multiple fission is also an asexual method of reproduction in organisms. In multiple fission, the parent organism splits (or divides) to form many new organisms at the same time. This happens as follows: Sometimes (particularly during unfavourable conditions), a cyst or protective wall is formed around the cell of a single-celled organism (like that of Plasmodium).

Inside the cyst, the nucleus of cell splits (or divides) several times to form many smaller nuclei called daughter nuclei. Little bits of cytoplasm collect around each daughter nuclei and thin membranes are formed around them. In this way, many new daughter cells are formed from a single parent cell within the cyst.

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In fact, as many daughter cells are formed as the number of daughter nuclei produced by the divisions of the parent nucleus. When the favourable conditions arrive, the cyst breaks open and the many daughter cells present in it are released, each forming a new organism.

In this way, Plasmodium is the malarial parasite which produces malaria disease in human beings. Malarial parasite Plasmodium is carried by female Anopheles mosquitoes from one person to another thereby spreading the malaria disease.

Before we discuss the next asexual method of reproduction called budding, we should know the mea of the term ‘bud’. The ‘bud’ here means a ‘small outgrowth’ from the body of a living organism. Let discuss the method of ‘budding’ now.

2. Budding:

Budding is an asexual method of reproduction. In budding, a small part of the body of the parent organism grows out as a ‘bud’ which then detaches and becomes a new organism. The asexual reproduction by budding is observed in Hydra and yeast. This is described below.

Hydra is a simple multicellular animal. Hydra reproduces by the process of budding (by using its regenerative cells). This happens as follows: In Hydra, first a small outgrowth called ‘bud’ is formed on the side of its body by the repeated mitotic divisions of its cells [see Figure 13(b)]. This bud then grows gradually to form a small Hydra by developing a mouth and tentacles. And finally the tiny new Hydra detaches itself from the body of parent Hydra and lives as a separate organism. In this way, the parent Hydra has produced (or created) a new Hydra. Thus, Hydra reproduces asexually by growing buds from its body. This is called budding. Please note that the bud formed in a Hydra is not a single cell. It is a group of cells.

We will now describe the reproduction in yeast plant by the process of budding. Please note that each single cell of yeast is a complete plant in itself.

Yeast is tiny, unicellular, non-green plant (which is a fungus). Yeast reproduces by budding. It shows a parent yeast cell (which is a complete plant). In yeast, first a bud appears on the outside of the cell wall. The nucleus of parent yeast cell then divides into two parts and one part of the nucleus moves into the bud.

Ultimately, the bud separates off from the parent yeast cell and forms a new yeast cell (or new yeast plant). The budding in yeast, however, often takes place so fast that the first buds start forming their own buds and all of them remain attached to the parent yeast cell forming a chain of yeast cells. After some time, all the yeast cells of the chain separate from one another and form individual yeast plants.

We can study the process of asexual reproduction in yeast by budding in the laboratory as follows: Take 100 mL of water in a conical flask and dissolve 10 grams of sugar in it. Then add 5 grams of yeast powder (or yeast granules) to this sugar solution and stir it well with a glass rod. Put a cotton plug in the neck of the conical flask. This conical flask containing sugar solution and yeast mixture is kept aside in a warm place for 3 to 5 days. When froth is observed in the flask, the yeast culture is ready for examination.

Take out a small quantity of the yeast culture solution from near the bottom of the conical flask with the help of a dropper and place a drop of this culture solution on a clean slide. Add a very little of iodine solution over the culture solution drop to stain it.

Place a cover slip over the slide. Keep the slide under the microscope and observe it first under low power and then under the high power of the microscope. Note the formation of buds on the yeast cells and how they separate from the parent cell.

In some organisms like sponges and corals, the buds remain attached to the parent organism permanently. These buds then grow and produce buds of their own. In this way, a colony of sponges or corals is formed.

Before we discuss the next asexual method of reproduction called ‘spore formation’, we should know something about ‘spores’. Spores are the microscopic ‘asexual reproductive bodies’ which are covered by a hard protective coat. This coat enables them to survive under unfavourable conditions like lack of food, lack of water and extreme temperatures. But when the conditions are favourable (food and water is available, and temperature is suitable), then the spores grow to produce new plants. Thus, spores are a kind of seeds of plants. These spores are very light and keep floating in air all around us. They are so small that we cannot see them with naked eyes. Keeping these points in mind, it will now be easier for us to understand the asexual reproduction by spore formation.

3. Spore Formation :

Spore formation is the asexual method of reproduction. The reproduction by spore formation takes place in plants. In spore formation, the parent plant produces hundreds of microscopic reproductive units called ‘spores’. When the spore case of the plant bursts, then the spores spread into air.

When these air-borne spores land on food (or soil) under favourable conditions (like damp and warm conditions), they germinate and produce new plants. Most of the fungi (like Rhizopus, Mucor, etc.), bacteria and non-flowering plants such as ferns and mosses reproduce by the method of spore formation. The common bread mould is a fungus plant whose scientific name is Rhizopus. The common bread mould (or Rhizopus fungus) reproduces by the method of spore formation. This is described below.

The tiny spores of ‘bread mould’ (a fungus plant) are almost always present in the air. If we keep a moist slice of bread aside for a few days, then the spores of bread mould plant present in air settle on the moist bread and germinate to form new fungus plants. The bread mould plants first look like a white cottony mass covering the bread slice which later on turns black. If we observe the surface of this slice of bread through a magnifying glass, then the bread mould plant growing on it will appear to be like.

The common bread mould plant consists of fine, thread­like projections called hyphae and thin stems having knob­like structures called sporangia. Each knob­like structure (or sporangium) contains hundreds of minute spores enclosed in a spore case. When the spore case bursts, the tiny spores are dispersed in air.

These spores are the asexual reproductive units which can produce more bread mould plants under suitable conditions. Actually, it was one such air-borne spore which grew on the moist slice of bread kept aside by us for a few days. If we remove one sporangium from the bread mould, keep it on a slide, put a cover slip over it and observe this slide through a microscope, we can see the spores.

The spore formation method of asexual reproduction is used by unicellular organisms as well as by multicellular organisms. For example, bacteria are the unicellular organisms which reproduce by spore formation whereas fungi such as Rhizopus (bread mould) and Mucor, and non-flowering plants such as ferns and mosses are multicellular organisms which reproduce by spore formation method.

4. Regeneration :

In some organisms (plants as well as animals) small cut parts of their body can grow (or regenerate) to form whole new organisms complete in all respects. The process of getting back a full organism from its body parts is called regeneration. The simple animals like Hydra and Planaria show regeneration.

This means that in these organisms, whole new organisms can be reproduced from their cut body parts. In other words, if Hydra or Planaria somehow get cut into a number of pieces, then each body piece can grow into a complete organism. This point will become clearer from the following example.

Planaria is a flatworm which is found in freshwater ponds and slow-moving streams. Planaria possesses great power of regeneration. If the body of Planaria somehow gets cut into a number of pieces, then each body piece can regenerate into a complete Planaria by growing all the missing parts.

It shows one Planaria worm. This Planaria worm somehow gets cut into three pieces. After a certain time, each cut piece of the body of Planaria worm grows into a complete Planaria worm. In this way, three Planaria worms are produced from just one Planaria worm. Similarly, if the body of a Hydra gets cut into a number of pieces, then each body piece of Hydra can grow into a complete Hydra. Please note that regeneration is not exactly the same as reproduction because most simple animals would not depend on being cut into pieces to be able to reproduce.

The regeneration of an organism from its cut body part occurs by the process of growth and development. This happens as follows: The cells of cut body part of the organism divide rapidly to make a ‘ball of cells’.

The cells present in the ‘ball of cells’ move to their proper places within the ball where they have to form various organs and body parts of the organism. The cells then change their shapes (or become specialised) to form different types of tissues. These different tissues form various organs and body parts of the organism. In this way a complete organism is regenerated.

The organisms like Planaria and Hydra are simple multicellular organisms which can be regenerated from their cut body parts to form complete organisms. We will now explain why the complex multicellular organisms (like mammals) cannot give rise to complete individuals from their cut body parts through the process of regeneration.

Regeneration can be used to reproduce only those organisms which organisation consisting of only a few specialised cells (or tissues). In complex multicellular organisms, specialised cells make up tissues; tissues make up organs; organs make up organ systems; and final organ systems make up organisms.

Since complex multicellular organisms have a very high degree of organisation in their body, they cannot be reproduced from their cut body parts by the process of regeneration. For example, a dog is a complex multicellular organism which cannot be regenerated from its cut body part say, a cut tail. This is because the cells present in the cut tail of a dog cannot produce dog’s organs like heart, brain, lungs, stomach, intestines and limbs, etc, needed for the making of a complete dog, The complex multicellular organisms need more complex ways of reproduction like sexual reproduction (which we will study after a while).

5. Fragmentation :

Some of the multicellular organisms having relatively simple body organisation can break up easily into smaller pieces (or fragments) on maturing. These pieces or fragments can then grow and form new organisms complete in all respects. This is another method of reproduction called ‘fragmentation’ which can be defined as follows:

The breaking up of the body of a simple multicellular organism into two (or more) pieces on maturing, each of which subsequently grows to form a complete new organism, is called fragmentation. The breaking up of the body of an organism in fragmentation to form new organisms occurs naturally (on its own) when the parent organism matures. Fragmentation is an asexual method of reproduction.

The reproduction by fragmentation method can occur in simple multicellular plants as well as animals. The organisms like Spirogyra and sea anemones can reproduce by the method of fragmentation. Please note that Spirogyra is a plant whereas sea anemones are marine animals. Let us discuss the reproduction in Spirogyra in a little more detail.

Spirogyra is a green, filamentous alga plant which is found in ponds, lakes and slow moving streams. Spirogyra filament simply breaks into two or more fragments on maturation, and each fragment then grows into a new Spirogyra.

This breakup of the filament of a mature Spirogyra on its own brings about asexual reproduction. Thus, Spirogyra reproduces by the asexual method of fragmentation. This is shown in Figure 25. In a mature Spirogyra filament is undergoing fragmentation to produce three new Spirogyra These three Spirogyra will mature in due course of time and break again to produce even more Spirogyra. And this process of reproduction goes on and on.

We can study Spirogyra in the laboratory as follows: Collect some water from a pond (or lake) which appears dark green and contains long filament-type (thread-type) structures. Take out the green coloured mass from the pond water sample and separate its threads or filaments by using two needles.

Place one filament on a clean slide, put a drop of glycerin over it and cover it with a cover slip. Keep this slide under the microscope and see it first under the low power and then under the high power of microscope. Observe the detailed structure of the green filament of Spirogyra and draw a diagram accordingly.

Please note that the main difference between fission and fragmentation is that in fission, a unicellular organism breaks up to form two (or more) daughter organisms, whereas in fragmentation, a multicellular organism breaks up to form two (or more) daughter organisms.

6. Vegetative Propagation :

Vegetative propagation is an asexual method of reproduction. The reproduction by vegetative propagation occurs only in plants. In vegetative propagation, new plants are obtained from the parts of old plants (like stems, roots and leaves), without the help of any reproductive organs.

Vegetative propagation usually involves the growth and development of one (or more) buds present on the old part of the plant to form a new plant. These buds are in the dormant state (inactive state) in the old part of the plant.

When provided suitable conditions (like moisture, warmth, etc.), these buds grow to form new plants. Please note that vegetative propagation is also called vegetative reproduction. Here is an example of the vegetative propagation (or vegetative reproduction) in grass.

It is a common observation that green grass plants spring up in dry fields after the rains. This happens due to vegetative propagation as follows: The fields have dry stems of the old grass plants all over them.

These dry stems have buds which are in the inactive state. By getting rain water, the buds present on dry grass stems get activated and grow to produce new grass plants. Thus, the green grass grows in the fields after rains from the dry, old stems of grass plants present in the fields, by the method of vegetative propagation.

Buds are present on the stems as well as the leaves of the Bryophyllum plant which can develop into new plants. So, Bryophyllum plants can be reproduced by vegetative propagation by using either a piece of its stem or its leaves.

For example, if we plant a broken piece of the stem of a Bryophyllum plant in the ground, we will get a new Bryophyllum plant growing from it in a week’s time. Even the leaves of a Bryophyllum plant can produce new plants. This happens as follows: The leaves of a Bryophyllum plant have special type of buds in their margins (or edges).

These buds may get detached from the leaves, fall to the ground and then grow to produce new Bryophyllum plants. The buds can also drop to the ground together with the leaf and then grow to produce new plants. Sometimes even before a leaf drops off from a Bryophyllum plant, we can see new plantlets already growing on it.

When such a mature leaf of the Bryophyllum plant falls on the ground, then each plantlet can grow into a new plant. Thus, the leaves of Bryophyllum plant can produce new plants. Another plant called Begonia also reproduces by vegetative propagation through its leaves.

Money plant can also be grown by vegetative propagation by using a piece of its stem as follows: Cut a piece of stem of money plant in such a way that it contains at least one leaf on it (The point on stem where a leaf is attached is called a node).

Dip one end of this stem in water. After a few days we will find that new roots appear at the point where leaf was attached. The piece of stem will gradually grow into a new money plant. Please note that if we cut the stem of money plant in between two leaves, then it will not grow into a new plant. This is because it does not have a growing point (here a node) in it.

We will now describe the vegetative propagation of plants by using tubers which are the modified, underground stems (or roots). A tuber is the thickened, underground stem (or root) of a plant which is swollen with stored food.

The tuber has a number of ‘buds’ (called ‘eyes’). Each bud (or eye) of the tuber grows into a new plant when the old tuber is planted in the soil in the next growing season. There are two types of tubers stem tubers and root tubers. Potato is a stem tuber whereas sweet potato is a root tuber. We will now describe how vegetative reproduction in potato takes place by using tubers.

Potato tuber is an underground stem of the potato plant. Potato tuber can be used for the vegetative reproduction of potato plants. Each potato tuber can produce more than one plant. This happens as follows:

A potato tuber has many buds (called eyes) on its body. These buds act as organs for vegetative reproduction. When a potato tuber is planted in the soil, then the various buds of the potato tuber start growing to form new potato plants. We have shown the new potato plants growing from only two buds of the potato tuber. Other buds can also do the same.

Please note that it is not necessary to plant the whole potato tuber in the ground to produce new potato plants. We can even plant ‘cut pieces’ of a potato tuber in the ground to obtain new potato plants.

But all these cut pieces of potato tuber should have a bud (or eye) on them. So, if we cut a potato tuber into a number of pieces in such a way that every piece has a bud (or eye) on it and plant them in the ground, then each cut piece of potato tuber will produce a new potato plant in due course of time.

Each potato plant produces more than one tuber and each tuber has more than one bud (which produces more than one new plant). Due to this, the vegetative propagation method of producing potato plants by tubers is much faster than the production of potato plants from seeds.

We can study the vegetative propagation of potatoes as follows : Take a potato and cut it into small pieces in such a way that some pieces contain a bud (or eye) in them. Place the potato pieces having buds on wet cotton kept in a tray. Keep the tray aside for a few days (but sprinkle water on the cotton daily to keep it wet).

We will see that green shoots and roots appear from the buds of the potato pieces. These are the new potato plants. If, however, we take potato pieces without buds in this experiment, then no new potato plants will grow from them.

The roots of a guava plant have buds which can develop into new guava plants. In fact, a large number of plants can be reproduced by the method of vegetative propagation. Some of the examples of the plants which can be reproduced by vegetative propagation are: Bryophyllum, Guava, Potato, Onion, Banana, Garlic, Water hyacinth, Tulip, Mint, Strawberry and Lily. We will now describe the artificial propagation of plants.

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