Reproduction

Sexual reproduction in plants.

We have learned in previous articles about asexual reproduction in plants. We shall learn about sexual reproduction in plants.

What do you know about sexual reproduction in plants?

Plants reproduce sexually for the same reasons as any other organism- to maintain genetic diversity. Sexual reproduction in plants is limited to flowering plants. Flowering plants are those plants that produce flowers. Have you ever wondered why flowers are so colourful or smell so good?

Plants invest a lot of energy in producing flowers. The flowers are bright to attract insects and bees. These insects and bees transport the pollen from one flower to another, thus, helping with the process of sexual reproduction in plants.

Flowers have only one role- sexual reproduction in plants. We have studied about the parts of a flower previously. Can you recollect the parts of a flower?

sexual reproduction in plants

Flowers are composed of sepals, petals, stamens and carpels. The sepals and petals protect the stamens and carpels. The stamens and carpels are the reproductive organs of the plant. The stamens are the male reproductive organs and the carpel is the female reproductive organ of a plant.

Plants are of two types- unisexual and bisexual.

Unisexual Plants– These plants have flowers that comprise only of stamens or carpels. Examples include papaya and watermelon. Therefore, these plants can only be fertilised by pollen from another plant.

Bisexual plants- Flowers of these plants contain both stamens and carpels. Thus, these plants can be fertilised by pollen from the same plant or another plant. Examples include Hibiscus and mustard.

sexual reproduction in plants

Stamens produce pollen. Pollen is the sticky, yellow powder that sticks to our hands if we touch it. The powder corresponds to the male gamete of the plant. When bees and other insects come in contact with the pollen, the pollen sticks to their feet and are transported to other flowers. This process is called pollination. Bees play a critical role in pollination. We will discuss the role of bees in agriculture in another article.

The carpel is the female reproductive organ of the plant. The carpel comprises of the ovary, the style and the stigma. The ovary is the bottom, swollen part of the carpel. The style is an elongated tube that functions to transport the pollen and the stigma is the sticky top part of the carpel. Pollen is deposited on the stigma and it travels through the style to reach the ovary. Here, it fuses with the female gamete to produce seeds. Once a flower is pollinated, the rest of the parts of the flower shrivel and fall off.

There are two terms one usually comes across about pollination:-

Self-pollination- Pollen from the same plant fertilise the ovule.

Cross-pollination– Pollen from another plant fertilises the ovule.

To summarise, sexual reproduction in plants is vital for maintaining genetic diversity and forms the basis of most agricultural innovation. Flowers are the reproductive organs of the plants and these flowers can either be fertilised by pollen from the same plant or another plant.

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Sexual Reproduction : Why and how!

What is the significance of sexual reproduction?

In the previous article on breeding we learned about the various methods of asexual reproduction. So, what are the limitations of asexual reproduction?

During the process of reproduction, the genetic code present in the DNA is copied. This process is quite efficient with a tiny margin of error. However, errors do happen, and these mistakes are the source of genetic variation in nature. Genetic variation is necessary for propagation and survival of the species. In asexual reproduction, as each progeny is an exact copy of the parent, the scope of genetic variation is quite limited. Therefore, to ensure genetic variation, organisms have developed the sexual form of reproduction.

Apart from the most primitive form of life like eukaryotes and prokaryotes, most plants and animals reproduce sexually. So, what does sexual reproduction mean?

Can a hen produce a chick by herself? No, the chicken needs the rooster to produce a chick. Similarly, a cow cannot produce a calf by herself. She needs a bull to produce progeny. Thus, in sexual reproduction, both males and females are required to produce an offspring.

The male and female of a species produce gametes. Gametes are specialised cells that combine to produce progeny. The male gamete is usually mobile while the female gamete is stationary and has stores of energy. The male gamete is also called sperm, and the female gamete is called ova or ovum.

In sexual reproduction, the male and female gamete combine to produce a zygote. However, this creates a unique problem. Each time the gametes fuse, the amount of DNA would double. If this process continues, then the amount of DNA will grow exponentially and over a period there will only be DNA on earth.

So, how does nature work around this problem?

Simple, nature keeps the amount of DNA constant across generations by halving the amount of DNA present in gametes. For example, all human cells contain 46 chromosomes. However, the gametes i.e. Sperm and Ova contain only 23 chromosomes.

When the sperm combines with the Ovum, the number of chromosomes is restored back to 46. Thus, the amount of DNA is preserved across generations.

Thus, the non-reproductive cells have twice the amount of DNA as compared with gametes. The non-reproductive cells are diploid, and the gametes are haploid.

As sexual reproduction requires the presence of male and female gametes, organisms have well developed male and female reproductive organs. The male and female reproductive organs are the cause of the differences in the bodies of the male and female organisms.

We shall learn more about sexual reproduction in plants and humans in another article.

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Modes of Asexual Reproduction

In asexual reproduction, the offspring inherits its genetic material from a single parent. Asexual reproduction is the most common method of reproduction. This method of reproduction is primarily seen in the less evolved organisms like bacteria, fungi and protozoa. However, many plants also reproduce by asexual means. In the previous article on modes of reproduction, we had learned about the different types of reproduction. We learned that sexual reproduction maintains genetic diversity, while asexual reproduction is useful for rapid growth in population. In this article, I shall discuss the various types of asexual reproduction.

Fission

We had learned about nuclear fission in the article on nuclear energy. Can you recollect what a nuclear fission is? Nuclear fission is the process by which an atom splits into two atoms. Asexual reproduction by fission is similar to a nuclear fission. However, unlike nuclear fission, where the daughter atoms are different to the parent molecule, the daughter cells are entirely analogous to the parent cell. Bacteria and amoeba are some of the organisms that reproduce through fission. Fission is of two types- binary and multiple. Organisms like Leishmania (the causative agent of Kala-Azar) split into two exact halves while the malarial parasite splits into multiple daughter cells. The former is binary fission and the latter is multiple fission.

Budding

Have you seen how a hydra reproduces? Hydra reproduces by budding. There is initially a small outgrowth from the body of the hydra that grows into a little hydra. When the daughter hydra is fully developed, it detaches from the parent and becomes an independent individual.

Vegetative Propagation

How many of you have a garden at home? If not a garden, many of us would have grown plants in pots. My favourite flower is the rose. Do you know how the rose is cultivated? You can just cut a twig from a rose plant, plant it in soil and it will grow into another rose plant. Can you think of any other plant that can be grown this way? Banana, jasmine and oranges can all be grown through this method. Vegetative propagation is the method by which the various parts of an individual plant- roots, shoots, and leaves, under proper conditions, can grow into an independent organism.

Spore formation

The Rhizopus is a species of fungi that reproduces by spore formation. The spores are blob-like structures present at the tip of the Rhizopus. These spores contain sporangia; cells that are capable of producing new rhizopus. These spores are shed in the atmosphere and winds carry these spores to distant areas where the sporangia can grow into new rhizopus.

So, how have we adapted asexual reproduction to fulfil our requirements? We have harnessed technologies like tissue culture; where cells from one plant can be grown in artificial media under disease-free conditions. It is now very easy to produce a large number of saplings with minimum effort using tissue culture. We can also improve the yield of crops by a technique of grafting.

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Modes of reproduction: Asexual Vs Sexual reproduction

How many modes of reproduction are known to you? Do all living beings reproduce in the same way? Obviously not. Plants produce seeds; birds and reptiles reproduce by producing eggs and mammals like humans give birth to live offspring. Each organism?s mode of reproduction is evolved to suit its ecological niche.

There are two modes of reproduction ? asexual and sexual.

Asexual reproduction

Asexual reproduction is that mode of reproduction where an offspring inherits all its genetic material from a single organism. Asexual reproduction is the most fundamental means of reproduction and single-celled organisms like bacteria and protozoa reproduce by asexual reproduction. Organisms can reproduce asexually in many different ways- fission, fragmentation, vegetative propagation, spore formation, etc.

During asexual reproduction, there is no exchange of genetic material. In the previous article on reproduction, we learned that there is a transfer of genetic material during the process of reproduction. However, the transfer of genetic material does not occur in asexual reproduction. The offspring inherits all its genetic material from a single organism only. Therefore, genetic diversity is not high. We also learned that genetic diversity is essential for evolution. So why do organisms reproduce asexually?

There are many reasons why organisms reproduce asexually. Firstly, asexual reproduction is one of the simplest modes of reproduction. Therefore, you will note that most less evolved organisms reproduce asexually. Secondly, asexual reproduction may help with rapid growth in population, especially in a stable environment. And thirdly, some organisms, due to developmental constraints have entirely relinquished the sexual mode of reproduction.

Sexual Reproduction

Sexual reproduction is a mode of reproduction that requires two individuals to mate to produce an offspring. There is an interchange of genetic material during sexual reproduction. Therefore, sexual reproduction helps with genetic diversity.

Sexual reproduction is the most complex of the modes of reproduction. Sexual reproduction requires specialised organs. Therefore, sexual reproduction is mostly seen in multicellular organisms. Gametes are the reproductive cells of an organism. The sperm is the male reproductive cell and the ova or eggs are the female reproductive cells.

The first process in sexual reproduction is meiosis. During meiosis, the number of chromosomes in the gametes are halved. When the two gametes fuse, the number of chromosomes is again restored to the usual number. For example, the human cell contains 46 chromosomes while the sperm and ova contain only 23 chromosomes. However, the fertilised ova, which forms the embryo has 46 chromosomes (23 from the mother and 23 from the father).

Apart from being complex, sexual reproduction is also slow and requires a much higher expenditure of energy.

To summarise, there are two modes of reproduction- asexual and sexual. Asexual reproduction is a simple and rapid method of reproduction but does not maintain genetic diversity. On the other hand, sexual reproduction is a the more complex of the modes of reproduction, requires specialised organ systems and it is useful for maintaining genetic diversity.

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Reproduction: Why do organisms reproduce?

What is a reproduction and why do organisms reproduce? It would be very interesting to listen to the answers to these questions. Reproduction is not an essential life process. Unlike other life processes like respiration or nutrition, if an organism does not reproduce, it is not going to die. Also, reproduction requires the expenditure of energy. If reproduction is a wasteful process, why do organisms reproduce?

There are many different reasons why organisms reproduce. One reason is to ensure the survival of a species. The species is the most fundamental unit in ecology. A species is any large group of organisms that can interbreed to produce offspring. Cows belong to one species and horses belong to another. Cows and oxen can breed to produce calves. However, cows cannot mate with other animals. Therefore, they are two different species.

Imagine the following scenario- What would happen if cows do not reproduce? Then one by one all the cows would die at the end of their lifespan and then the entire species would become extinct. Therefore, organisms reproduce to ensure the survival of their species.

But is the survival of the species, the only reason for reproduction?

Reproduction also has additional functions. Reproduction also helps with maintenance of genetic diversity. Do all members of a species look alike? No. Take humans for example. Does each one of us look alike? Even brothers look different. So how does reproduction affect the way we look?

Previously, we have learned that how we look is a function of our DNA. DNA is the genetic code of life. During reproduction, there is an exchange of DNA and this exchange of DNA leads to an offspring that is different from the parents in subtle ways. Meiosis is the process by which there is an exchange of genetic material during the process of reproduction.

Why is genetic diversity or variation necessary?

Genetic diversity is important for evolution. The environment is always changing. The Dinosaurs used to rule the earth at some point in time but are now extinct. Why did they become extinct? They could not adapt to the changing environment, therefore, they became extinct. However, the cockroach as a species are still thriving. For a species to survive, it needs to evolve constantly to the changing environment. Genetic diversity is important for this adaptive process.

Do you know how bacteria become resistant? Amongst the many millions of bacteria, a few bacteria are resistant to particular antibiotics. When we take antibiotics, the bacterium that is resistant survives while the other bacteria die. The resistant bacteria then reproduce, producing offsprings that are resistant to the antibiotic. Thus, the resistant bacteria thrive and hence the species continues.

Therefore, reproduction is vital for ensuring the continuity of a species and it is also necessary for maintaining genetic variation in the nature.

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