NCERT Solution Class 12 Biology Chp-2 Reproduction in Flowering Plants :

 

1. Name the parts of an angiosperm flower in which development of male and female gametophyte take place.

An angiosperm flower consists of several parts, including the male and female reproductive structures where the development of the male and female gametophytes take place.

The male reproductive structure of a flower is called the stamen. It consists of two parts: the filament and the anther. The anther is where the development of the male gametophyte (pollen grains) takes place. Inside the anther, there are microsporangia that contain microsporocytes. These microsporocytes undergo meiosis to form microspores, which then develop into pollen grains.

The female reproductive structure of a flower is called the pistil or carpel. It consists of three parts: the stigma, style, and ovary. The stigma is the uppermost part of the pistil, which receives the pollen grains during pollination. The style is a long, slender stalk that connects the stigma to the ovary. The ovary is the swollen base of the pistil that contains one or more ovules. The ovules are where the development of the female gametophyte (embryo sac) takes place. Each ovule contains a megasporangium, which contains a megasporocyte. The megasporocyte undergoes meiosis to form four megaspores, but only one of these survives and develops into the female gametophyte.

So, in summary, the development of male gametophytes takes place in the anthers of the stamen, while the development of female gametophytes takes place in the ovules of the ovary.

2. Differentiate between microsporogenesis and megasporogenesis. Which type of cell division occurs during these events? Name the structures formed at the end of these two events.

Microsporogenesis and megasporogenesis are the two types of sporogenesis that occur in the male and female reproductive structures of angiosperms. They differ in various aspects, including the type of cell division that takes place, the structures formed at the end of these events, and the number of spores produced.

Microsporogenesis is the process of formation of male gametophytes or pollen grains. It occurs in the anthers of the flower. The process starts with the division of a diploid microspore mother cell (MMC) into four haploid microspores through meiosis. The haploid microspores then develop into pollen grains through mitotic divisions. Pollen grains are made up of two cells – a generative cell and a tube cell.

On the other hand, megasporogenesis is the process of formation of female gametophytes or embryo sacs. It occurs in the ovules of the flower. The process starts with the division of a diploid megaspore mother cell (MMMC) into four haploid megaspores, but only one of them develops into the female gametophyte. The haploid megaspore undergoes three rounds of mitotic divisions, resulting in the formation of eight haploid nuclei. These eight nuclei then arrange themselves into different cells to form an embryo sac. An embryo sac consists of seven cells, including three antipodal cells, two synergids, one egg cell, and one central cell with two polar nuclei.

During microsporogenesis and megasporogenesis, meiosis is the type of cell division that occurs. Meiosis is a special type of cell division that produces four genetically diverse haploid cells from one diploid cell.

At the end of microsporogenesis, four haploid microspores are formed from one diploid microspore mother cell (MMC) through meiosis. These microspores then undergo mitotic divisions to form the mature pollen grain, which contains two cells – a generative cell and a tube cell.

At the end of megasporogenesis, one haploid megaspore is formed from one diploid megaspore mother cell (MMMC) through meiosis. The haploid megaspore then undergoes mitotic divisions to form the female gametophyte or embryo sac. The mature embryo sac consists of seven cells, including three antipodal cells, two synergids, one egg cell, and one central cell with two polar nuclei.

3. Arrange the following terms in the correct developmental sequence: Pollen grain, sporogenous tissue, microspore tetrad, pollen mother cell, male gametes.

The correct developmental sequence of the terms is as follows:

  1. Sporogenous tissue
  2. Pollen mother cell
  3. Microspore tetrad
  4. Pollen grain
  5. Male gametes

4. With a neat, labelled diagram, describe the parts of a typical angiosperm ovule.

Diagram from NCERT book

The various parts of a typical angiosperm ovule are:

  1. Integument: It is the protective outer layer of the ovule. The integument has two layers, an outer layer called the outer integument, and an inner layer called the inner integument. The integuments enclose the nucellus and are fused at the micropyle end.
  2. Micropyle: It is a small opening at the tip of the ovule through which the pollen tube enters during fertilization.
  3. Hilum: It is the point of attachment of the ovule to the placenta of the ovary.
  4. Chalaza: It is the base of the ovule opposite the micropyle. The chalaza is the point where the funicle and integuments are attached to the nucellus.
  5. Funicle: It is the stalk-like structure that attaches the ovule to the placenta.
  6. Nucellus: It is the central part of the ovule that contains the female gametophyte, which develops from the megaspore.
  7. Female Gametophyte: It is the multicellular structure that contains the egg cell and other cells involved in fertilization. The female gametophyte develops from a haploid megaspore through several rounds of mitotic divisions.
  8. Embryo: It is the young plant that develops from the fertilized egg cell and eventually grows into the mature seed.

5. What is meant by monosporic development of female gametophyte?

In plants, the female gametophyte is the structure that produces the egg cell, which is involved in fertilization. The female gametophyte develops from a single haploid cell, known as the megaspore, which is produced by the meiosis of a diploid cell called the megasporocyte.

Monosporic development of the female gametophyte refers to the development of the female gametophyte from a single megaspore. In this type of development, the megaspore undergoes mitotic divisions to produce a multicellular female gametophyte.

In flowering plants, which are angiosperms, monosporic development of the female gametophyte occurs through a process called megagametogenesis. During megagametogenesis, the single haploid megaspore undergoes three rounds of mitotic division, resulting in eight haploid nuclei that are distributed among seven cells. These seven cells then differentiate to form the mature female gametophyte, which consists of three antipodal cells, two synergids, one egg cell, and one central cell with two polar nuclei.

6. With a neat diagram explain the 7-celled, 8-nucleate nature of the female gametophyte.

Draw  diagram from NCERT book

 

7. What are chasmogamous flowers? Can cross-pollination occur in cleistogamous flowers? Give reasons for your answer.

Chasmogamous flowers are the flowers that have opened and exposed reproductive structures such as stamens and pistils. These flowers are typically larger and more showy than cleistogamous flowers. They rely on pollinators like insects, birds, or bats to transfer pollen from the anthers to the stigma.

Cleistogamous flowers, on the other hand, are flowers that remain closed and do not open to expose their reproductive structures. They are usually small and inconspicuous and often grow close to the ground. They self-pollinate, which means that they do not rely on pollinators for fertilization.

In general, cross-pollination cannot occur in cleistogamous flowers because their reproductive structures are enclosed within the unopened flower. The pollen produced by the anthers must fall directly on the stigma, which is located within the flower, in order for fertilization to occur. Therefore, cleistogamous flowers tend to be self-fertilizing.

However, some cleistogamous flowers are capable of cross-pollination under certain conditions. For example, if the flowers are not able to self-fertilize due to genetic factors, they may produce pollen that is viable and able to fertilize the pistil of another flower. In some cases, pollinators may also be able to transfer pollen from one cleistogamous flower to another, which could result in cross-pollination.

8. Mention two strategies evolved to prevent self-pollination in flowers.

Preventing self-pollination in flowers is an important strategy to ensure genetic diversity and promote outcrossing, which can lead to healthier offspring. There are several ways in which flowers have evolved to prevent self-pollination, including the following two strategies:

  1. Dioecy: This strategy involves separating the male and female reproductive structures into separate flowers on different plants. Dioecious plants have either male flowers that produce only pollen or female flowers that have only pistils. Because the male and female flowers are located on separate plants, they are unable to self-pollinate, which promotes cross-pollination and genetic diversity.
  2. Self-incompatibility: This strategy involves a genetic mechanism that prevents a flower from being fertilized by its own pollen. Self-incompatibility systems are controlled by genes that determine whether a pollen grain is capable of fertilizing a particular flower. These genes can be either gametophytic or sporophytic. In gametophytic self-incompatibility, the pollen grain carries a gene that determines whether it can fertilize a particular flower. In sporophytic self-incompatibility, the plant’s own genes determine whether the flower can be fertilized by its own pollen. Self-incompatibility promotes cross-pollination by preventing a flower from being fertilized by its own pollen, which encourages the plant to outcross and increases genetic diversity.

9. What is self-incompatibility? Why does self-pollination not lead to seed formation in self-incompatible species?

Self-incompatibility is a genetic mechanism in plants that prevents self-fertilization or self-pollination, thereby promoting cross-fertilization or outcrossing. In self-incompatible plants, the stigma of a flower is capable of recognizing and rejecting its own pollen. This is achieved through a complex recognition system that involves proteins and other molecules that are expressed in both the pollen and the stigma.

In self-incompatible species, self-pollination does not lead to seed formation because the self-pollen is recognized and rejected by the stigma of the flower. This mechanism prevents self-fertilization, which can result in offspring with reduced genetic diversity and increased susceptibility to diseases and other stresses. In some cases, self-fertilization can even lead to inbreeding depression, where deleterious recessive alleles are expressed in offspring. By preventing self-fertilization, self-incompatibility promotes cross-fertilization, which can increase genetic diversity and improve the overall health and fitness of the plant population.

Self-incompatibility systems can be either gametophytic or sporophytic. In gametophytic self-incompatibility, the recognition system is controlled by genes expressed in the haploid pollen grains. In sporophytic self-incompatibility, the recognition system is controlled by genes expressed in the diploid sporophyte tissues of the stigma. Both types of self-incompatibility involve complex interactions between pollen and stigma proteins, which can vary among species and can be influenced by environmental factors such as temperature and humidity. Self-incompatibility is found in many important crop species, such as apples, cherries, and strawberries, and understanding the genetic and molecular basis of self-incompatibility is important for breeding programs aimed at improving these crops.

10. What is bagging technique? How is it useful in a plant breeding programme?

The bagging technique is a method used in plant breeding to prevent pollen contamination and to control pollination in order to produce hybrid seeds. In this technique, a plant or a group of plants is covered with a bag or a net to prevent unwanted pollen from reaching the stigmas of the flowers. The bag or net is usually made of a breathable material that allows air to circulate while preventing insects and other pollinators from accessing the flowers.

The bagging technique is useful in a plant breeding program for several reasons:

  1. It can be used to produce hybrid seeds with a high degree of purity, as it prevents unwanted pollen from other plants or from the same plant from fertilizing the flowers.
  2. It allows for controlled pollination, which can be important for studying the inheritance of specific traits or for developing new varieties with desired characteristics.
  3. It can prevent cross-contamination of genetically modified plants with non-GM plants or with wild relatives, which is important for maintaining the integrity of the gene pool and for ensuring the safety of the environment and the consumers.
  4. It can be used to protect plants from insect pests, such as fruit flies, that can damage or destroy the fruits.
  5. It can extend the flowering period of some plants, allowing breeders to obtain more seeds or to make crosses between different accessions.

The bagging technique is a valuable tool in plant breeding programs for ensuring the purity and controlled pollination of plants, which can help to improve crop yields, enhance genetic diversity, and develop new varieties with desirable traits.

11. What is triple fusion? Where and how does it take place? Name the nuclei involved in triple fusion.

Triple fusion is a process that occurs during double fertilization in angiosperms, where one of the sperm cells fuses with two polar nuclei in the embryo sac, resulting in the formation of a triploid nucleus. This process is essential for the development of the endosperm, a tissue that provides nutrients for the developing embryo.

Triple fusion takes place in the central cell of the female gametophyte, which contains two polar nuclei. After the pollen tube enters the ovule and reaches the embryo sac, the two sperm cells are released. One sperm cell fuses with the egg cell, resulting in the formation of a diploid zygote. The other sperm cell fuses with the two polar nuclei, resulting in the formation of a triploid endosperm nucleus.

The nuclei involved in triple fusion are the two polar nuclei and one of the sperm cells. The polar nuclei are haploid, while the sperm cell is also haploid. After the fusion, the resulting nucleus is triploid, with three sets of chromosomes. This triploid nucleus will undergo cell division to form the endosperm, which is a nutritive tissue that provides nourishment to the developing embryo.

 

12. Why do you think the zygote is dormant for sometime in a fertilised ovule?

After fertilization, the zygote in a fertilized ovule may remain dormant for some time before it begins to divide and develop into an embryo. There are several reasons why this dormancy occurs:

  1. The zygote needs time to undergo physiological changes before it can divide and develop into an embryo. During this period of quiescence, the zygote undergoes biochemical changes, such as the synthesis of new proteins and the reorganization of organelles, which prepare it for the developmental changes ahead.
  2. The endosperm, which is formed through triple fusion, needs time to develop and provide nutrients for the developing embryo. The endosperm is the primary source of nutrients for the embryo during its early stages of development, and it must be fully functional before the embryo can begin to grow.
  3. The ovule and the developing seed coat need time to mature and develop. The ovule must undergo changes to form the seed, and the seed coat must develop to protect the embryo and the endosperm. These developmental changes take time and must be completed before the embryo can begin to grow.
  4. Dormancy also helps to synchronize seed germination with favorable environmental conditions. Seeds that germinate at the wrong time or in unfavorable conditions may not survive, so it is advantageous for the embryo to remain dormant until conditions are favorable for germination and growth.

The dormancy of the zygote after fertilization allows for the necessary developmental changes to occur and helps to ensure the successful development of the seed into a mature plant.

13. Differentiate between:
(a) hypocotyl and epicotyl;
(b) coleoptile and coleorrhiza;
(c) integument and testa;
(d) perisperm and pericarp.

Answer :

(a) Hypocotyl and epicotyl are both parts of the embryo in a seed. Hypocotyl is the lower part of the embryo, located between the radicle and the cotyledons. It develops into the stem of the young plant. Epicotyl, on the other hand, is the upper part of the embryo, located between the cotyledons and the first true leaves. It develops into the shoot of the young plant.

(b) Coleoptile and coleorrhiza are both protective sheaths found in grasses and other monocots. Coleoptile is a sheath that surrounds and protects the emerging shoot of the seedling. It is usually found above the ground. Coleorrhiza, on the other hand, is a sheath that surrounds and protects the emerging root of the seedling. It is usually found below the ground.

(c) Integument and testa are both outer coverings of the seed. Integument is the outermost layer of the ovule, which develops into the seed coat after fertilization. Testa, on the other hand, is the outermost layer of the seed coat itself. The integument and the testa are both protective layers that help to protect the developing embryo and endosperm.

(d) Perisperm and pericarp are both structures found in the mature seed of angiosperms. Perisperm is a nutritive tissue that is derived from the nucellus and surrounds the embryo in some seeds. It provides nourishment to the developing embryo. Pericarp, on the other hand, is the outer layer of the fruit. It develops from the ovary wall and protects the seed. The pericarp may be fleshy or dry, depending on the type of fruits.

14. Why is apple called a false fruit? Which part(s) of the flower forms the fruit?

Apple is called a false fruit because the edible part of the apple, which is the fleshy part we eat, is not derived from the ovary of the flower. Instead, the fleshy part of the apple is derived from the thalamus, which is the receptacle of the flower. The ovary of the apple flower is located at the base of the receptacle, and its wall becomes papery and thin after fertilization, forming a core that holds the seeds.

The fruit is formed from the ovary of the flower. After fertilization, the ovules inside the ovary develop into seeds, and the ovary wall undergoes changes to become the fruit. The fruit can be fleshy or dry, and it can be either a simple fruit or a complex fruit, depending on the number of carpels in the ovary and the way they are fused.

15. What is meant by emasculation? When and why does a plant breeder employ this technique?

Emasculation is the process of removing the male reproductive organs, i.e. anthers, from a flower before they have matured and released pollen. This is done in order to prevent self-pollination and promote cross-pollination with a desired pollen source.

Plant breeders employ emasculation to control the genetic makeup of the offspring produced by a plant. By preventing self-pollination, the breeder can ensure that the pollen used for fertilization comes from a different plant with desired traits, such as disease resistance, high yield, or specific aesthetic features. This technique can also be used to produce hybrids by crossing two different varieties of a plant species.

Emasculation is typically performed before the anthers of the flower have matured and released pollen, to ensure that the desired pollen source is used for fertilization. The timing and technique used for emasculation may vary depending on the plant species and the goals of the breeding program.

16. If one can induce parthenocarpy through the application of growth substances, which fruits would you select to induce parthenocarpy and why?

Parthenocarpy is the development of fruits without fertilization, and it can be induced through the application of growth substances such as auxins or gibberellins. The selection of fruits to induce parthenocarpy would depend on various factors such as market demand, ease of cultivation, and yield.

Some fruits that could be selected to induce parthenocarpy include:

  1. Bananas – Bananas are one of the most popular fruits worldwide, and inducing parthenocarpy could lead to seedless fruit production, which is preferred by consumers.
  2. Cucumbers – Parthenocarpic cucumbers are easier to grow and have higher yields as they don’t require pollination.
  3. Tomatoes – Parthenocarpic tomatoes can be grown in areas where pollination by insects may not be possible, leading to more consistent fruit production.
  4. Oranges – Parthenocarpy can result in seedless oranges, which are more desirable for juicing.
  5. Grapes – Parthenocarpic grapes can result in seedless fruit production, which is preferred by consumers for its ease of consumption.

The selection of fruits to induce parthenocarpy would depend on various factors, but those listed above are some common options that could benefit from this technique.

17. Explain the role of tapetum in the formation of pollen-grain wall.

The tapetum is a specialized tissue layer found within the anther of a flower, which plays a crucial role in the formation of the pollen-grain wall.

During the development of pollen, the tapetum provides nourishment and support to the developing microspores (the precursor cells to pollen). The tapetum is responsible for producing and secreting various enzymes, proteins, and lipids that are essential for the development and differentiation of the microspores into mature pollen grains.

The tapetum also produces the sporopollenin, which is a complex polymer that makes up the outer layer of the pollen-grain wall. Sporopollenin is an extremely tough and durable material that protects the pollen grain from desiccation, mechanical damage, and environmental stresses.

In addition to producing sporopollenin, the tapetum also plays a role in transporting the sporopollenin to the developing pollen grains. Once the sporopollenin is produced, it is deposited onto the surface of the developing microspores, forming the outer layer of the pollen-grain wall.

The tapetum plays an essential role in the formation of the pollen-grain wall by providing nourishment, producing and secreting essential substances, and transporting sporopollenin to the developing pollen grains. Without the tapetum, the microspores would not be able to differentiate into mature pollen grains, and the pollen grains would not be able to protect themselves from environmental stresses.

18. What is apomixis and what is its importance?

Apomixis is a type of asexual reproduction in plants that involves the formation of seeds without the need for fertilization. In apomixis, the embryo develops from an unfertilized egg cell, which is produced by the maternal plant through meiosis.

Apomixis is important because it allows for the production of genetically identical offspring without the need for sexual reproduction. This can be beneficial for plant breeders as it can lead to the production of plants with desirable traits that are genetically stable and uniform. Additionally, apomixis can help to maintain hybrid vigor in certain crops, which can lead to increased yield and productivity.

Apomixis is also important for the preservation of rare or endangered plant species, as it allows for the production of viable seeds without the need for a viable pollen donor. This can help to maintain genetic diversity and prevent the loss of valuable genetic resources.

However, apomixis also has some limitations, as it can lead to the loss of genetic diversity over time and restrict the potential for evolutionary adaptation. Therefore, it is important to balance the benefits and drawbacks of apomixis when considering its use in plant breeding and conservation efforts.

NCERT SOLUTIONS

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NCERT SOLUTION Chapter 1 Reproduction in Organism

Click Here for NCERT solution Chapter 2 Sexual Reproduction in Flowering Plants

NCERT SOLUTION Chapter 3 Human Reproduction

NCERT SOLUTION CHAPTER 4 REPRODUCTIVE HEALTH

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