NCERT Solutions for class 12th Biology Chapter 2 Sexual Reproduction in Flowering Plants

Exercise

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

Sol. The process of the development of male gametophyte and female gametophyte is called microgametogenesis and megagametogenesis respectively.The male gametophyte or the pollen grain develops inside the pollen chamber of the anther, whereas the female gametophyte (also known as the embryo sac) develops inside the nucellus of the ovule from the functional megaspore.

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.

Sol. Differences between microsporogenesis and megasporogenesis

  • Meiosis occurs in both the events of microsporogenesis and megasporogenesis. Process of meiosis or reduction division results in the formation of haploid gametes from the microspore and megaspore mother cells.
  • Each microspore mother cell and megaspore mother cell contain two sets of chromosomes and are therefore diploid. The diploid megaspore mother cell and microspore mother cell enlarges and undergo meiosis toproduce, four haploid cells called megaspores and microspores respectively. The chromosome number is reduced by half and therefore megaspores and microspores are haploid.
  • At the end of these two events, microsporogenesis and megasporogenesis give rise to pollen grains and embryo sac respectively. Pollen grain is the male gametophyte and embryo sac represents the female gametophyte.

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

Sol. The correct developmental sequence is as follows:
Sporogenous tissue – Pollen mother cell – Microspore tetrad- Pollen grains­ Male gametes.
During the development of microsporangium, each cell of the sporogenous tissue acts as a pollen mother cell and gives rise to a microspore tetrad, containing four haploid microspores by the process of meiosis (microsporogenesis). As the anther matures, these microspores dissociate and develop into pollen grains. The pollen grains mature and give rise to male gametes.

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

Sol. A typical angiospermic ovule is a female megasporangium where the formation of mega spores takes place. It is formed in the ovary.

The various parts of an ovule are –

(i) Hilum – It is the point where the body of the ovule is attached to the funiculus.
(ii) Funiculus- It is a stalk-like structure which represents the point of attachment of the ovule to the placenta of the ovary.
(iii) Micropyle – It is small opening which is left by the integument in the ovule for the passage of pollen tube into the ovule.
(iv) Integuments -They are the outer layers surrounding the ovule that provide protection to the developing embryo. It forms seed coats i.e. testa and tegmen.
(v) Chalaza- It is the basal region of ovule from where the integuments arise.
(vi) Nucellus – It is a mass of the parenchymatous tissue surrounded by the integuments from the outside. The embryo sac is located inside the nucellus. It provides nourishment in the development of embryo sac.
(vii) Embryo sac – It is the female gametophyte which contains the egg apparatus, antipodal and polar nuclei.
(viii) Raphe – It is the longitudinal ridge formed by lengthwise fusion of funiculus with the body of ovule in a typical anatropous ovule.

Note: Angiosperm ovules are diverse in their position in the ovary, nucellus thickness, number and thickness of integuments, degree and direction of curvature, and histological differentiations.

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

Sol. In many flowering plants, monosporic development of the female gametophyte is a process in which only one out of the four megaspores enlarges and develops into female gametophyte or embryo sac.

A single megaspore mother cell which is present at the micropylar pole of the nucellus region of the ovule in most of flowering plants undergoes meiosis to produce four haploid megaspores. Later, out of these four megaspores, only one functional megaspore develops into the female gametophyte, while the remaining three degenerates.

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

Sol. A female gametophyte, commonly known as an embryo sac, develops from a single functional megaspore. This megaspore undergoes three successive mitotic divisions to form eight nucleate embryo sacs.

The first mitotic division in the megaspore forms two nuclei. One nucleus moves towards the micropylar end while the other nucleus moves towards the chalazal end. Then, these nuclei divide at their respective ends and re­ divide to form eight nucleate stages. As a result, there are four nuclei each at both the ends i.e., at the micropylar and the chalazal end in the embryo sac. At the micropylar end, out of the four nuclei, only three differentiate into two synergids and one egg cell. Together they are known as the egg apparatus. Similarly, at the chalazal end, three out of four nuclei differentiate as antipodal cells. The remaining two cells (of the micropylar and the chalazal end) move towards the centre and are known as the polar nuclei, which are situated in a large central cell.

Hence, at maturity, the female gametophyte appears as a 7-celled structure (an egg, two synergids, three antipodals and a central cell), though it has 8 nucleate.

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

Sol. Chasmogamous flowers (or open flowers) are bisexual flowers in which anthers and stigmata are exposed for pollination.
Cross-pollination (the process of transfer of pollen from the flower of one plant to the flower of a plant having a different genetic constitution) cannot occur in cleistogamous flowers, because these flowers never open at all and also, the anther and the stigma lie close to each other in these flowers. Hence, only self-pollination is possible in these flowers. Cleistogamous flowers remain in bud form throughout their life and form seeds via self-pollination. In such flowers, anthers dehisce inside the closed flowers. So the pollen grains come in contact with stigma. Thus, there is no chance of cross-pollination, e.g., Oxalis, Viola.

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

Sol. Self-pollination is the transfer of pollen from the anthers to the stigma of the same flower or of another. It always occurs in bisexual flowers. Continued self-pollination decreases the vigour and vitality of a particular race. Thus, flowering plants have developed many devices to discourage self-pollination and to encourage cross-pollination.

• Two strategies that have evolved to prevent self-pollination in flowers are dichogamy and self-incompatibility.
(i) Dichogamy is the maturation of anther and stigma at different times in a bisexual flower which prevent self-pollination.
(ii) Self-incompatibility (also called self sterility) is a genetic mechanism and prevent self-pollen (from the same flower or other flowers of the same plant) from fertilising the ovules by inhibiting pollen germination or pollen tube growth in the pistil.

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

Sol. •When the pollen grains of an anther do not germinate on the stigma of the same flower, then such a flower is called self-sterile or incompatible and such condition is known as self-incompatibility or self-sterility. It develops genetic incompatibility between individuals of the same species or between individuals of different species. Examples are tobacco and potato.
• Self-pollination does not lead to seed formation in self-incompatible species because in self-incompatibility, the plants which exhibit this phenomenon have the ability to prevent pollen germination, pollen tube growth, ovule fertilisation or embryo development at one of its stages. This prevents the fusion of the gametes along with the development of the embryo. As a result, no seed formation takes place.

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

Sol.• Bagging is the process of covering flowers on male as well as female parents in separate bags (made up of polyethene, muslin or paper) after emasculation to prevent contamination of its stigmas with unwanted pollens. When the stigmas of emasculated flowers mature the bags are removed, stigmas are dusted with pollen grains of desired male plants by means of a presterilised brush and flowers are rebagged till fruit develop. This technique is mainly used in artificial hybridisation.

• Bagging technique is an important part of plant breeding programme
because,

  1. It ensures that pollen grains of only desirable plants are used for fertilisation of the stigma to develop the desired plant variety.
  2. It helps to avoid inbreeding depression caused by self-pollination.
  3. Commercially superior hybrid varieties can be obtained by cross-pollination of different useful species.

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

Sol. • Triple fusion is the fusion of the second male gamete with the two polar nuclei located in the central cell to form the triploid primary endosperm nucleus (PEN). It is also called vegetative fertilisation.

• This process takes place in the embryo sac of angiosperm.
After reaching the ovary, the pollen tube enters into the embryo sac from the micropylar end. After penetration, the tip of the pollen tube ruptures releasing the two male gametes. One male gamete fuses with the egg to form the diploid zygote (called syngamy) and the other male gamete fuses with the two polar nuclei to form the triploid (3n) primary endosperm (called triple fusion). These two events of fertilisation constitute the process of double fertilisation.
It results in the formation of the triploid primary endosperm cell.

• Nuclei involved in triple fusion are two polar nuclei and the nucleus of the male gamete.

Note: Syngamy is a type of generative fertilisation whereas triple fusion is a type of vegetative fertilisation. Syngamy occurs in animals and fungi as well. But, triple fusion only occurs in angiosperms.

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

Sol. A zygote is formed by the fusion of the male gamete with the nucleus of the egg cell. It remains dormant for some time in a fertilised ovule because the outer conditions like high temperature, humidity, improper light, etc., are not favourable for germination. So therefore, it waits for the endosperm to form as it is the source of nutrition for the developing embryo. Endosperm develops from the primary endosperm cell resulting from triple fusion and provides food for the growing embryo and after the formation of the endosperm, further development of the embryo from the zygote starts.

Question 13. Differentiate between:
(a) Hypocotyl and epicotyl
(b) Coleoptile and coleorrhiza
(c) Integument and testa
(d) Perisperm and pericarp

Sol. (a) Differences between hypocotyl and epicotyl

Note: Similarities between coleoptile and coleorhiza: These are structures of the monocot seed. Both act as a protective sheath. Also, both undergo rapid growth in its early phase.
Difference between coleoptile and coleorhiza: Coleoptile is the sheath that protects the emerging shoot while coleorhiza is the sheath which protects the emerging root

(c) Differences between integument and testa

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

Sol. Fruit is the seed-bearing structure in flowering plants formed from the ovary after flowering.
• Apple is called a false fruit because in an apple, the fleshy receptacle forms the main edible part. False fruits are those fruits which arise from other floral parts (like thalamus, receptacle, or calyx) except the ovary.
• Ovary forms the fruit after fertilisation or without fertilisation in parthenocarpic fruits.

Note: False fruits are also called as parthenocarpic fruits. Parthenocarpy is the phenomenon in which fruits are formed without fertilisation. Parthenocarpic fruits are seedless fruits. E.g. apple, pineapple, banana.

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

Sol.

  • Emasculation is the removal of stamens (mainly the anthers) from the flower buds before their dehiscence. It does not affect the female reproductive part (pistil), which is used in various plant hybridization techniques.
  • Emasculation can be done in bisexual flowers to obtain the desired variety of a plant by crossing a particular plant with the desired pollen grain. To remove the anthers, the flowers are covered with a bag before they open. This ensures that the flower is pollinated by pollen grains obtained from desirable varieties only. Later, the mature, viable, and stored pollen grains are dusted on the bagged stigma by breeders to allow artificial pollination to take place and obtain the desired plant variety.
  • Plant breeders employed this technique in artificial hybridisation,
    • to prevent the pollination within same flower (self pollination).
    • to pollinate stigmas with pollens of desired variety to get disease­ resistant plants and to avoid unwanted pollen rejection.

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

Sol. Parthenocarpic fruits are seedless fruits. They develop from ovary without fertilisation or seed formation. Such fruits can be induced by the application of plant growth hormones (such as auxin) to produce seedless fruit.
Therefore, the seedless varieties of economically important fruits such as oranges, pineapple, guava, watermelon, lemon, banana, grapes are selected to induce parthenocarpy.

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

Sol. Tapetum is the innermost layer of the microsporangium. The tapetal cells are multinucleated and polyploid. It is located between sporangenous tissue and the anther wall. Tapetum is important for the nutrition and development of pollen grain, as well as a source of precursors for the pollen coat.
• Role of tapetum in the formation of pollen grain wall
During microsporogenesis, the cells of tapetum produce various enzymes, hormones, amino acids, and other nutritious material which are required for the development of pollen grains. Tapetal cells contain ubisch bodies that help in the ornamentation of the microspores or pollen grains walls (exine). The wall of the pollen grain consists of two layers: the inner intine wall and the outer exine wall. The inner layer, laid by the cells themselves consists, at least in part, of cellulose or hemicellulose. The outer layer of the pollen grain is called exine and is made up of sporopollenin (secreted by the ubisch bodies of the tapetal cells). This compound provides a spiny appearance to the exine of the pollen grains. The outer layer is the most durable layer and resistant to disintegration therefore treatment with intense heat, strong acids, or strong bases has very little effect upon it.

Note: Pollen grains are very common constituents of geologic sediments, both recent and ancient because of their high resistance to decay, their widespread dispersal by wind and water, and their abundant production by plants, Because of these features, pollen grains have provided much information on the origin and geologic history of terrestrial plant life.

Question 18. What is apomixis and what is its importance?

Sol. Apomixis is a form of asexual reproduction that produces seeds without fertilisation, e.g., some species of Asteraceae and grasses.
Importance of apomixis

  1. It helps in hybrid seed production. The method of producing hybrid seeds by cultivation is very expensive for farmers. Also, by sowing hybrid seeds, it is difficult to maintain hybrid characters as characters segregate during meiosis. This can be avoided if apomixis can be introduced in hybrid seeds.
  2. It prevents the loss of specific characters in the hybrid.
  3. It helps in producing hybrid varieties with increased yield.
  4. It is a cost-effective and time-efficient method for producing seeds.

Note: Apomixis is a genetically controlled reproductive process whose major advantage over sexual reproduction is the possibility to select individuals with desirable gene combinations and to propagate them as clones. The ability to generate maternal clones and therefore rapidly fix desirable genotypes in crop species could accelerate agricultural breeding strategies.

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