Class 12 Biology Chapter 2 Sexual reproduction in Flowering plants, 2nd Year

Class 12 Biology

 Chapter: 2 

  Sexual reproduction in Flowering plants  

Q1. Name the parts of an angiosperm flower in which development of male and female gametophyte take place.

Ans: The male gametophyte develops inside the pollen chamber present in the anther, whereas the female gametophyte develops inside the nucellus present in the ovule of the ovary.

Q2. Differentiate between microsporangenesis and megasporangenesis. Which type of cell division occurs during these events? Name the structures formed at the end of these two events.

Ans: Microsporangenesis

• Microsporangenesis is the process of development of microspore from the pollen mother cell by meiosis.

• It generally takes place in the pollen chamber of anther.

Megasporangenesis

• Megasporangenesis is the process of development of megaspores from a megaspore mother cell by meiosis.

• It generally takes place in ovule of stigma.

Meiosis is the process of cell division occurs during these events which results in the development of haploid gametes from microspore and megaspore mother cells.

Haploid microspore is develops from a diploid microspore mother cell by the process of microsporangenesis or PMC. On the other hand, haploid megaspore is develops from a diploid megaspore mother cell by the process of megasporangenesis.

Q3. Arrange the following terms in the correct developmental sequence:

Pollen grains, sporangenous tissue, microspore tetrad, pollen mother cell, male gametes.

Ans: The correct developmental sequence is as follows:

Sporangenous tissue – pollen mother cell – microspore tetrad - pollen grain – male gamete

Q4. With a neat, labeled diagram, describe the parts of a typical angiosperm ovule.

Ans: 

• Funiculus is the structure through which ovule are attached to the placenta of the ovary.

• Helium is the point in which the body of the ovule is connected to the Funiculus.

• An integument is the outer covering of the ovule which protects the growing embryo.

• Micropyle is the narrow opening formed by the projection of integuments.

• Nucellus- Nucellus is the parenchymatous tissues covered by the integuments from the outside. It provides nutrients to the growing embryo.                The embryo sac is found inside the nucellus.

• Chalazal is the swollen part of the nucellus which is the origin of integuments.

Q5. What is meant by monosporic development of female gametophyte?

Ans: The female gametophyte forms from a single functional megaspore. This process is known as development of female gametophyte. A single megaspore mother cell is located at the micropylar pole of the nucellus present in the ovule, which is then undergoes meiosis to form four haploid megaspores. From this four megaspore, one is the functional megaspore grows into the female gametophyte, while the other three degenerates.

Q6. With a neat diagram explain the 7-celled, 8-nucleate nature of the female gametophyte.

Ans: 

The female gametophyte form from a single functional megaspore, which is then, undergoes three successive mitotic divisions to generate 8 nucleate embryo sacs. In the first mitotic division, two nuclei are formed from a megaspore. One nucleus moves to the micropylar end while the other moves to the Chalazal end. These nuclei divide and re-divide to form 8 nucleate stages. Each ends have 4 nuclei i.e. at the micropylar and Chalazal end of the embryo sac. Out of four nuclei, three nuclei are differentiated into two synergids and one egg cell present in the micropylar end. Similarly, three out of four nuclei present in the Chalazal end differentiates as antipodal cells. The cell remaining at the micropylar and Chalazal end move towards the centre. These remaining cells are called polar nuclei. At maturity, 7-celled, 8-nucleate of female gametophyte is formed.

Q7. What are chasmogamous flowers? Can cross-pollination occur in cleistogamous flowers? Give reasons for your answer.

Ans: The flowers in which anther and stigma are exposed similar to the flowers of other species are called Chasmogamous flowers. Example- Oxalis

No, cross-pollination cannot occur in cleistogamous. This is because the stigma and the anther are close to each other. Thus, self-pollination can occur in these flowers.

Q8. Mention two strategies evolved to prevent self-pollination in flowers.

Ans: Self pollination is the migration of pollen grains from the stamen to the pistil if same flower. 

There are two strategies evolved to prevent self-pollination in flowers-

• Incompatibility prevents self pollination

There are some plants in which stigma have the ability to prevent the germination of pollen grains which prevents the growth of pollen tube. This mechanism is said to be self-incompatibility.

• Protogyny prevents self-pollination.

In some flowering plants, the androecium matures before the gynoecium or vice-versa. This mechanism is said to be Protogyny. 

Q9. What is self-compatibility? Why does self-pollination not lead to seed formation in self-compatible species?

Ans: Self-compatibility has the ability of the stigma to prevent the germination of pollen grains which prevents the growth of pollen tube.

There are some plants in which stigma have the ability to prevent the germination of pollen grains which prevents the growth of pollen tube in the stigma of the flower. Thus, prevents the fusion of gametes. As a result no seed formation occurs.

Q10. What is bagging technique? How is it useful in a plant breeding programme?

Ans. In some hybridization technique, the removal of the anther is required from the bisexual flowers without affecting the female reproductive part by the process of emasculation. Then, these emasculated flowers are wrapped in the bag to prevent pollination by undesirable pollen grains. This technique is known as bagging.

This technique allows only the pollen grains of desirable plants are used for fertilization to yield desirable plant.

Q11. What is triple fusion? Where and how does it take place? Name the nuclei involved in triple fusion.

Ans: The fusion of male gamete within the angiosperm embryo sac with two polar nuclei is called triple fusion.

The fusion takes place in the embryo sac.

After germination of pollen grains on the stigma, it gives rise to a pollen tube which travels through the style and enters the ovule. One of the synergids and two male gametes are released in the ovary through pollen tubes. One male gametes fuse with the nucleus of the egg cell while the other one fuses with the two polar nuclei located in the central cell to form a triploid endosperm nucleus. In this process, three haploid nuclei get fused. So, it is known as triple fusion.

The nuclei involved in triple fusion are one male gamete nucleus and two polar nuclei.

Q12. Why do you think the zygote is dormant for sometime in a fertilized ovule?

Ans: The zygote is developed from the fusion of male gamete with the nucleus of the egg cell. The zygote remains dormant until the endosperm is formed. The endosperm develops from the primary endosperm cell formed from triple fusion. The endosperm nourishes the developing embryo and the growing of embryo from the zygote begins.

Q13. Differentiate between:

a) Hypocotyl and epicotyl

Ans: Hypocotyl

• The part of the embryonal axis lies under the cotyledon in a dicot embryo is called hypocotyl.

• It terminates with the radicle.

Epicotyl

• The part of the embryonal axis lies above the cotyledon in a dicot embryo is called epicotyls.

• It terminates with the plumule.

b) Coleoptile and coleorrhiza

Ans: Coleoptile

• Coleoptile is the defensive conical sheath that surrounds the plumule into a monocot seed.

Coleorrhiza

• Coleorrhiza is an undifferentiated sheath which surrounds the radicle and the root cap in a monocot seed.

c) Integument and Testa

Ans: Integument

• It is the outermost protective layer of an ovule.

Testa

• It is the outermost protective layer of seed.

d) Perisperm and Pericarp

Ans: Perisperm

• Perisperm is an important residual nucellus which is found in some seeds.

Pericarp

• Pericarp is the ripened wall of the fruit, which forms from the wall of an ovary.

Q14. Why apple is called a false fruit? Which parts of the flower forms the fruit?

Ans: The apple is formed from the floral parts other than the ovary is known as false fruit. True fruits are developed from the ovary. 

The receptacle parts of the flower forms the fruit.

Q15. What is meant by emasculation? When and why does a plant breeder employ this technique?

Ans: Emasculation is the hybridization technique of cutting out anther from bisexual flowers without affecting the female reproductive part.

Emasculation is used by plant breeder to get the desired variety of plants by crossing the specific plant with the desired pollen grain. The emasculated flowers are wrapped in the bag to prevent pollination by undesirable pollen grains. This technique is known as bagging. This technique allows only the pollen grains of desirable plants are used for fertilization to yield desirable plant.

Q16. If one can induce parthenocarpy through the application of growth substances, which fruit would you select to induce parthenocarpy and why?

Ans: Parthenocarpy is the technique in which fruits develop without the formation of seed.

Fruits such as lemon and orange can be used to induce parthenocarpy as the fruit formation is induced by plant growth hormones called auxin.

Q17. Explain the role of tapetum in the formation of pollen grain wall.

Ans: Tapetum provides nutrients to the growing pollen grains. During the process of microsporangenesis, the tapetum secretes various hormones, enzymes and other essential material needed for the development of pollen grains.

Q18.  What is apomixes and what is its importance?

Ans: The formation of seed without the requirement of meiosis process and syngamy is called apomixes. 

The importance of apomixes are-

• Helps to produce hybrid seed.

• Prevent the loss of desired characters in the hybrid.

• Cost-effective method for producing seeds.

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