NCERT Solutions for Class 12 Biology Chapter 1 Sexual Reproduction in Flowering Plants

Question 1. Name the parts of an angiosperm flower in which the development of male and female gametophytes takes place.

Answer:

In angiosperm flowers, the gametophyte development takes place at certain sites.

• Male gametophyte (pollen) forms within the anther's pollen sac (microsporangium).
• Female gametophyte (embryo sac) forms inside the ovule, in the nucellus.

Question 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 and megasporogenesis are important processes in plant reproduction that have key differences. Both involve meiosis, a type of cell division.

During microsporogenesis and megasporogenesis, meiosis occurs and results in the formation of four microspores and megaspores, respectively. Later, mitosis occurs to give rise to the microgametophyte and megagametophyte.

At the end of microsporogenesis, four functional haploid microspores are formed, whereas at the end of megasporogenesis, four megaspores are formed, out of which three degenerate and only one functional megaspore remains.

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

Answer:

The correct developmental sequence is as follows:

Sporogenous tissue → Pollen mother cell → Microspore tetrad → Pollen grain → male gametes

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

Answer:

The diagram of a typical angiosperm ovule is as follows:

An ovule consists of a funicle, micropyle, integuments, nucellus, embryo sac, hilum, chalaza, etc.

• Funicle - It is the stalk of the ovule which connects the ovule to the placenta. Funicle is short and multicellular.
• Hilum - It is the point where the funicle connects the main body of the ovule.
• Integuments - These are the layers that surround the ovule. There can be an outer integument and an inner integument. The main function of integuments is to protect the inner tissues.
• Micropyle - The opening which is present at the top of the integuments is called the micropyle. The main function of the micropyle is to allow the entry of the pollen tube into the ovule.
• Nucellus - Integuments surround parenchymatous tissue, which constitutes the main body of the ovule and is called the nucellus. The primary function of the nucellus is to provide nutrition to the developing embryo.
• Chalaza - The swollen part of the ovule present opposite to the micropyle is called the chalaza.
• Embryo sac - It is present in the nucellus, and it contains a female gamete called an egg.

Question 5. What is meant by the monosporic development of female gametophytes?

Answer:

Monosporic development is a particular process in the development of the female gametophyte.

• It signifies that the female gametophyte (embryo sac) develops from one functional megaspore.
• The megaspore develops into the embryo sac through mitotic divisions.

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

Answer:

The diagram of 7-celled 8-nucleate embryo sacs of plants is as follows:

The female gametophyte of plants is formed from a single functional megaspore with the help of four unequal mitotic divisions. These mitotic divisions give rise to 8 nuclei. Later, these nuclei assemble into 7 cells. To both chalazal and micropylar ends, three cells each move. The three cells at the chalaza are called antipodal cells, while those at the micropylar end constitute the egg apparatus. The egg apparatus includes two larger cells called synergid, while the egg is the smallest cell that acts as a female gamete.

Two nuclei migrate to the centre, and together they form a single cell called the central cell. This central cell consists of two nuclei called polar nuclei. Thus, the mature embryo sac possesses 7 cells ( 3 antipodal cells, 2 synergid cells, 1 egg cell, and 1 central cell) and 8 nuclei ( 3 antipodal cells, 2 synergid cells, 1 egg cell, and 2 central cells).

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

Answer:

Flowers can be of two types, i.e., chasmogamous flowers or cleistogamous flowers.

• Chasmogamous flowers are open flowers that have their anthers and stigma exposed.
• These flowers facilitate cross-pollination.
• On the other hand, cleistogamous flowers are closed flowers whose anthers and stigmas are not exposed.
• Since these flowers do not open at all, only self-pollination occurs in these, and cross-pollination is never seen.

Thus, Cross-pollination cannot occur in cleistogamous flowers.

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

Answer:

Self-pollination refers to the transfer of pollen grains from the anther of a flower to the stigma of the same flower. Continuous self-pollination can reduce the variations in the progeny and cause a reduction in vigour and vitality. Thus, plants have developed certain mechanisms to avoid self-pollination and ensure cross-pollination. The two mechanisms to prevent self-pollination are as follows:

1. Self-sterility or self-incompatibility - In this mechanism, the pollen grains of a flower do not germinate on the stigma of the same flower. This occurs due to the presence of some self-sterile genes. This mechanism is genetic.
2. Dichogamy- In this method, the timing of maturation of anthers and stigmas of a bisexual flower is different to prevent self-pollination.

Protandry- Anthers mature earlier than the stigma of the same flower. So, even if the pollens fall on these stigmas, they are unable to germinate. Example, sunflower, Salvia.

Protogyny- Stigmas mature earlier than anthers and get pollinated by mature pollen grains of other flowers. E.g. Mirabilis jalapa.

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

Answer:

Self-incompatibility refers to the genetic inability of fully functional pollen grains and ovules to produce viable seeds.

• This is mainly due to the presence of self-genes.
• If these genes are present in pollen as well as ovules, they will not be able to produce viable seeds.
• Self-pollination does not lead to the formation of seeds in self-incompatible species due to the presence of certain chemical substances that block the germination of pollen grains on the stigma and also obstruct their passage to the ovary.

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

Answer:

The bagging technique is a tool used in plant breeding. Bagging refers to covering emasculated flowers with the help of butter paper or plastic bags to avoid contamination of the stigma of these flowers with undesired pollens.

• This technique is used during artificial hybridisation experiments.
• This technique is useful in breeding programmes because it prevents contamination of stigma with unwanted pollens.
• So, through this technique, we can allow the breeding of species according to our needs and obtain superior species.

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

Answer:

A pollen grain produces two male gametes. Out of these, one male gamete fuses with the egg cell and produces a zygote. The second male gamete fuses with the two polar nuclei of the central cell to produce the primary endosperm nucleus. Since this fusion includes three haploid nuclei, it is called triple fusion.

• The process of triple fusion occurs in the central cell of the embryo sac.
• The pollen tube delivers one nucleus to the egg cell while the other one migrates to the centre and fuses polar nuclei.

In triple fusion, two polar nuclei and one male gamete are involved.

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

Answer:

The zygote starts developing into an embryo only when a certain amount of endosperm has developed. This is because endosperm provides nourishment to the developing embryo. Thus, a zygote remains dormant in a fertilised ovule till the endosperm has developed.

Question 13. Differentiate between (a) hypocotyl and epicotyl

Answer:

The differences between hypocotyl and epicotyl are as follows:

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

Answer:

The fruits are formed from the ovary of a flower after fertilisation.

• Such fruits that develop from the ovary are called true fruits.
• On the other hand, fruits formed from any part of the flower other than the ovary are called false fruits.
• Apple is called a false fruit because it develops from the thalamus and not from the ovary.
• The parts of flowers that take part in fruit formation are the ovary, thalamus, etc.

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

Answer:

The process of removal of anthers from the flower with the help of forceps without affecting the female reproductive parts is called emasculation. This technique is used by plant breeders to allow suggestive breeding between plants to produce plants with desired seeds. Hence, it can be used to prevent self-pollination or cross-pollination according to the needs of the breeder.

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

Answer:

The fruits formed as a result of parthenocarpy are generally seedless. Hence, for parthenocarpy, we can select fruits that have a lot of seeds in them. Such fruits include watermelon, orange, muskmelon, lemon, etc. So we can select watermelon to induce parthenocarpy because by this method, we can have seedless watermelons.

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

Answer:

The tapetum is the nutritive innermost layer of the microsporangium that provides nutrition to the developing microspores. The role of the tapetum in the formation of the pollen grain wall is as follows:

• Tapetum secretes ubisch granules required for the formation of sporopollenin in the exine (outer wall) of the pollen grain
• Tapetum also secretes pollen kit substances required for the formation of the pollen grain wall