Chapter 1. Sexual Reproduction in Flowering Plants

Class 12 Biology Chapter 1 Sexual Reproduction in Flowering Plants Exercises :

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

Answer : In an angiosperm flower, the development of the male gametophyte (pollen) takes place in the anther, which is part of the stamen, while the development of the female gametophyte (embryo sac) occurs within the ovule, located inside 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.

Answer :  Microsporogenesis:

Location: Microsporogenesis occurs in the anther of the flower.

Purpose: It is the process of forming male reproductive cells (microspores), which will develop into pollen grains (male gametophytes).

Cell Division Type: Microsporogenesis involves meiosis. Meiosis is a type of cell division that reduces the chromosome number by half.

Resulting Structures: At the end of microsporogenesis, four microspores are produced within each microsporangium. Each microspore has the potential to develop into a pollen grain, which is the male gametophyte.

Megasporogenesis:

Location: Megasporogenesis occurs in the ovule of the flower.

Purpose: It is the process of forming female reproductive cells (megaspores), which will develop into the embryo sac (female gametophyte).

Cell Division Type: Megasporogenesis also involves meiosis. Similar to microsporogenesis, it is a type of cell division that reduces the chromosome number by half.

Resulting Structures: At the end of megsaporogenesis, only one megaspore survives, and this megaspore develops into the embryo sac, which is the female gametophyte.

Both microsporogenesis and megasporogenesis involve meiosis but occur in different parts of the flower and lead to the formation of structures with distinct roles in plant reproduction. Microsporogenesis results in the production of pollen grains (male gametophytes), while megsaporogenesis results in the formation of the embryo sac (female gametophyte)

3. Arrange the following terms in the correct developmental sequence:

Pollen grain, sporogenous tissue, microspore tetrad, pollen mother cell, male gametes.

Answer :  The correct developmental sequence is:

Sporogenous Tissue  Pollen Mother Cell  Microspore Tetrad  Pollen Grain  Male Gametes

Sporogenous tissue: This is the tissue that gives rise to the pollen mother cell.

Pollen mother cell: The sporogenous tissue undergoes division to form the pollen mother cell.

Microspore tetrad: The pollen mother cell undergoes meiosis to produce a group of four haploid microspores called a microspore tetrad.

Pollen grain: Each microspore within the tetrad matures into a single pollen grain, which is the male gametophyte.

Male gametes: Inside the mature pollen grain, there are two male gametes (sperm cells) that are released during pollination for fertilization.

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

Answer :  The diagram of a typical angiosperm ovule :

   

The parts of a typical angiosperm ovules are :

Hilum : The body of the ovule fuses with funicle in the region called hilum. The hilum represents the junction between ovule and funicle.

Funicle : The ovule is a small structure attached to the placenta by means of a stalk called funicle.

Micropyle : Integuments encircle the nucellus except at the tip where a small opening called the micropyle is organised. Opposite the micropylar end, is the chalaza, representing the basal part of the ovule.

Micropylar Pole: The micropylar pole is a specific region in the seed of flowering plants where the micropyle is located. The micropyle is a small opening in the seed coat that allows for water and gases to enter during germination.

Outer Integument: The outer integument is one of the two protective layers that make up the seed coat (integuments) of an ovule in a plant. It surrounds the inner integument and the embryo sac, providing protection to the developing embryo.

Inner Integument: The inner integument is the innermost protective layer of the seed coat (integuments) in a plant's ovule. Together with the outer integument, it forms a protective covering for the embryo sac and the developing embryo within the seed.

Nucellus : Enclosed within the integuments is a mass of cells called the nucellus .

Embryo Sac: The embryo sac is the female gametophyte found within the ovule of flowering plants. It contains the egg cell and is central to fertilization, where it fuses with a pollen grain's male gametes to form a zygote, initiating the development of a new plant embryo.

Chalazal Pole: The chalazal pole is one of the two poles of the embryo sac within the ovule, located opposite the micropylar pole. It anchors the embryo sac and plays a role in nutrient uptake, facilitating the growth of the embryo within the seed.

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

Answer :  Monosporic development of the female gametophyte means that only one spore, known as a megaspore, develops into the female gametophyte (embryo sac) in a flower. This is in contrast to polytrophic development, where multiple spores give rise to the female gametophyte, but in monosporic development, only a single spore survives and matures into the female gametophyte.

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

Answer :

   

Female Gametophyte Structure (Embryo Sac):

The female gametophyte, also known as the embryo sac, has a unique structure with seven cells and eight nuclei. Here's an explanation of this structure:

Central Cell (Large Central Cell): The central cell is a large, central cell within the embryo sac. It contains two nuclei known as polar nuclei.

Egg Cell (Ovum): At one end of the embryo sac, there is a single egg cell, which is responsible for fertilization when it fuses with a sperm cell from pollen.

Two Synergid Cells: Near the egg cell, there are two smaller cells called synergid cells. These cells play a role in guiding the pollen tube to the embryo sac during fertilization.

Three Antipodal Cells: At the opposite end of the embryo sac from the egg cell, there are three antipodal cells. These cells have less defined roles but may be involved in nutrient absorption or other functions.

The 7-celled, 8-nucleate structure of the female gametophyte is a unique adaptation in angiosperms (flowering plants) that facilitates the process of double fertilization, where one sperm cell fuses with the egg cell to form the embryo, and another sperm cell fuses with the polar nuclei to form the endosperm. This double fertilization is a key characteristic of angiosperm reproduction .

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

Answer :   Chasmogamous flowers are flowers that open up and expose their reproductive parts to the environment. These flowers allow for easy access by pollinators, such as insects or birds, to transfer pollen and facilitate cross-pollination.

In contrast, cleistogamous flowers are flowers that remain closed and do not open up. They typically self-pollinate, meaning that the pollen from the same flower fertilizes the ovules within that flower. Cross-pollination is less likely to occur in cleistogamous flowers because the closed structure prevents external pollinators from accessing the reproductive parts, reducing the chance of pollen transfer between different plants

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

Answer :  Two strategies that have evolved in flowers to prevent self-pollination are:

Dichogamy: This strategy involves a temporal separation of the maturation of male and female reproductive organs within the same flower. For example, in some flowers, the stamens (male parts) mature before the pistil (female part) or vice versa. This temporal separation reduces the chances of self-pollination within the same flower.

Herkogamy: Herkogamy refers to the physical separation of the male and female reproductive organs within a flower. It can involve structural features like the positioning of the anthers and stigma at different heights or in different locations within the flower. This spatial separation prevents self-pollination by ensuring that pollen from the same flower does not easily come into contact with the stigma of the same flower.

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

Answer : Self-incompatibility is a reproductive mechanism in plants that prevents self-pollination (the transfer of pollen from the same flower or plant) from leading to seed formation.

In self-incompatible species, self-pollination does not result in seed formation because the plant has evolved mechanisms to recognize and reject its own pollen. This recognition involves biochemical processes. When a flower receives its own pollen, these processes prevent the pollen from successfully fertilizing the ovules. As a result, self-pollination is blocked, and the plant promotes cross-pollination, where pollen from another plant is needed for successful fertilization and seed formation. This genetic diversity achieved through cross-pollination can lead to healthier and more adaptable offspring

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

Answer :  The bagging technique in plant breeding is like putting a bag or covering on a flower to keep it safe from unwanted pollen.

Useful :

No Surprise Babies: Bagging prevents plants from making babies with other plants nearby. This way, breeders can be sure which plants are the parents.

Pick the Best Matches: Breeders can choose which plants should be the parents to get the traits they want in the baby plants.

Protect from Strangers: It stops outside pollen, like from the wind or insects, from messing up the breeding plan.

Speed Things Up: Bagging makes breeding faster because breeders don't have to wait for random pollination to happen.

So, bagging helps plant breeders make better and faster progress in developing new plant varieties.

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

Answer : Triple fusion is a process in flowering plants where three nuclei from two different cells join together. It happens in the central cell of the embryo sac, which is inside the female reproductive part of the flower called the ovule.

The nuclei involved in triple fusion are:

The two polar nuclei from the central cell.

One of the two sperm nuclei from a pollen grain.

This triple fusion process is crucial for forming the endosperm, which provides nutrition to the developing embryo in the seed.

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

Answer : The zygote is dormant for some time in a fertilized ovule to give the embryo and the surrounding structures (endosperm) time to develop properly before the plant begins to grow. This dormancy allows the plant to establish a better chance of survival when it starts to grow.

13. Differentiate between:

(a) hypocotyl and epicotyl;

(b) coleoptile and coleorrhiza;

(c) integument and testa;

(d) perisperm and pericarp.

Answer : (a) Hypocotyl and Epicotyl:

Hypocotyl: This is the part of the embryo plant below the cotyledons. It usually develops into the lower part of the stem.

Epicotyl: This is the part of the embryo plant above the cotyledons. It typically becomes the upper part of the stem and carries the first leaves.

(b) Coleoptile and Coleorrhiza:

Coleoptile: This is a protective sheath covering the young shoot (epicotyl) of a germinating grass seed. It helps the shoot break through the soil surface.

Coleorrhiza: This is a protective sheath surrounding the young root (radicle) of a germinating grass seed. It aids in pushing through the soil.

(c) Integument and Testa:

Integument: This is the outermost layer that surrounds the ovule in a plant's ovary.

Testa: The testa is the seed coat, which is formed from the integument after fertilization. It protects the seed.

(d) Perisperm and Pericarp:

Perisperm: Perisperm is a tissue in some seeds that stores nutrients. It's derived from the nucellus and provides nourishment to the developing embryo.

Pericarp: Pericarp is the fruit's outermost layer. It surrounds the seed(s) and protects them. It's different from perisperm, as it's part of the fruit, not the seed.

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

Answer : An apple is called a false fruit because the part we commonly eat and think of as the fruit is not derived from the ovary of the flower. Instead, the edible part of the apple comes from the thickened receptacle (the base of the flower) that has grown and surrounded the true fruit, which is the seeds or pips inside the core.

In an apple, the part we eat (the fleshy part) is formed from the receptacle, while the seeds inside the core are the true fruit. So, the apple is a false fruit because what we consider the fruit is not the actual fruit of the apple tree.

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

Answer : Emasculation is a technique in plant breeding where a plant breeder removes or prevents the development of the male reproductive parts (anthers or pollen) from a flower before they release pollen. This is done to control pollination and ensure that only desired pollen is used for breeding.

Plant breeders employ emasculation when they want to make sure that specific plants or flowers are the parents. By removing the male parts, they can prevent unwanted pollination and make controlled crosses between different plants. This technique helps breeders develop new plant varieties with desired traits and characteristics.

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

Answer : One might choose to induce parthenocarpy in fruits like seedless grapes, bananas, or cucumbers. These fruits are typically consumed for their flesh, and having them without seeds makes them more convenient to eat. Inducing parthenocarpy in these fruits ensures that they develop without seeds, making them seedless and more appealing to consumers.

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

Answer : The tapetum is like a helper cell in the flower. It provides nutrients and materials to the developing pollen grains, helping them grow and form a strong protective wall. This wall is essential for the pollen grains to survive and function when they are released for pollination. So, the tapetum plays a crucial role in building the pollen-grain wall, making sure the pollen is healthy and ready for fertilization.

18. What is apomixis and what is its importance?

Answer : Apomixis is a way that some plants make seeds without the need for a male and female parent. It's like making copies of the parent plant.

Its importance is that it helps keep the same good traits in plants from one generation to the next. This can be useful in farming and for preserving certain types of plants. It saves time and resources because it skips the usual process of finding a mate and fertilizing the seeds. However, it's not as common as regular sexual reproduction in plants.