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NCERT Solutions for Class 12 Biology
Page No. 40
Q1. Name the parts of an angiosperm flower in which development of male and female gametophyte take place.
Answer: 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 nucleus of the ovule from the functional megaspore.
Q2. 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: (i) Differences between microsporogenesis and megasporogenesis
|1) Formation of microspores and production of male gametophyte is termed micro-sporogenesis.|
2) It occurs inside the microsporangia or anther.
3) Many microspore mother cells are differentiated in a microsporangium.
4) All the four cells of the microspore tetrads are functional.
|1) Formation of megaspores , female gametophyte from megaspore mother cells is called|
2) It occurs in the nucellus of the ovule/megasporangium.
3) Only one megaspore mother cell is differentiated in a megasporangium.
4) Only one of the four cells in the megaspore tetrad continues to grow.
The type of cell division that occurs during these events is Meiosis or Reduction division which produces haploid gametes from diploid cells.
The structures formed at the end of the two events are
(a) Microspore or pollen grain at the end of Microsporogenesis.
(b) Megaspore which develops into female gametophyte or embryo sac at the end of megasporogenesis.
Q3. Arrange the following terms in the correct developmental sequence:
Pollen grain, sporogenous tissue, microspore tetrad, pollen mother cell, male gametes
Answer: Following is the correct developmental sequence:Sporogenous tissue → pollen mother cell → microspore tetrad → pollen grain → male gameteWhen the microsporangium is developing, every cell of the sporogenous tissue serves as a pollen mother cell, giving rise to a microspore tetrad possessing four haploid microspores through the meiosis process (microsporogenesis). When the anthers mature, these microspores dissociate and develop into pollen grains. The pollen grains mature and give rise to male gametes.
Q4. With a neat, labelled diagram, describe the parts of a typical angiosperm ovule.
Answer: An angiosperm ovule consists of the following parts:
- The ovule is attached to placenta by means of a stalk called funicle or funiculus.
- The point of attachment of funiculus to the body of ovule is called hilum.
- The main body of ovule is made of parenchymatous tissue called nucellus.
- Nucellus is covered on its outside by one or two coverings called integuments and hence ovule is rightly called as integument megasporangium.
- The integuments cover entire nucellus except for a small pore at upper end, which is called the micropyle. Micropyle is formed generally by inner integument or by both integuments.
- The place of junction of integuments and nucellus is called chalaza.
- In inverted ovules (most common type), the stalk or funiculus is attached to the main body of ovule for some distance to form a ridge like structure, called- raphe.
- In the nucellus of ovule, a large oval cell is present at micropylar end, which is known as embryo sac (female gametophyte), which develops from the megaspore.
Q5. What is meant by monosporic development of female gametophyte?
Answer: The female gametophyte or the embryo sac develops from a single functional megaspore. This is known as monosporic development of the female gametophyte. In most flowering plants, a single megaspore mother cell present at the micropylar pole of the nucellus region of the ovule 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 degenerate.
Q6. With a neat diagram explain the 7-celled, 8-nucleate nature of the female gametophyte.
The female gametophyte (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 differentiates 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, though it has 8 nucleate.
Q7. What are chasmogamous flowers? Can cross-pollination occur in cleistogamous flowers? Give reasons for your answer.
Answer: The anther and stigma of chasmogamous flowers are exposed for pollination. Cleistogamous flowers are incapable of cross-pollination. Therefore, only self-pollination is caused by these closed flowers. During the dehiscence process of cleistogamous flowers, anthers dehisce into closed flowers. As a result, pollen grains are exposed to stigma. Therefore, it is impossible for cross-pollination to occur, such as Oxalis and Viola.
Q8. Mention two strategies evolved to prevent self-pollination in flowers.
Answer: 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.
Dichogamy and self-sterility are.two most common devices that ensure cross-pollination. Dichogamy – Maturation of anther and stigma at different times in a bisexual flower prevent self-pollination.
Self-sterility (or self-incompatibility) – Due to the presence of self-sterile gene in some flowers, pollen grains do not germinate on the stigma of that flowers. e.g.,- tobacco, potato.
Q9. What is self-incompatibility? Why does self-pollination not lead to seed formation in self-incompatible species?
Answer: Self-incompatibility refers to a genetically controlled mechanism that prevents self-pollination and promote cross-pollination in flowers. Self-pollination cannot lead to the formation of seeds in a self-incompatible species. This happens because the pollens are unable to fertilize the ovules that would develop into an embryo and hence form seeds.
Q10. What is bagging technique? How is it useful in a plant breeding programme?
Answer: Bagging is a technique of covering the female reproductive parts of flowers to prevent stigma from the contamination with undesired pollen. It is useful in a plant breeding programme as only desired pollen grains for pollination and protection of the stigma from contamination of unwanted pollen.
Q11. What is triple fusion? Where and how does it take place? Name the nuclei involved in triple fusion.
Answer: Triple fusion. It is the fusion of two polar nuclei of embryo sac with one male gamete. It is named triple fusion because three nuclei i.e two polar nuclei and one male nuclei fuse together to form a 3n cell.
It occurs in embryo sac. Triple fusion occurs when during fertilization the other male nuclei moves towards the central two polar nuclei and fuse with them forming 3n primary endosperm nucleus.
Nuclei involved are two polar nuclei and one nucleus of male gamete.
Q12. Why do you think the zygote is dormant for sometime in a fertilised ovule?
Answer: The zygote remains inactive until the endosperm is formed as a result of triple fusion. The endosperm provides nutrition to the developing embryo and is formed from the primary endosperm cell that results from triple fusion.
Q13. Differentiate between
(a) Hypocotyl and epicotyl
(b) Coleoptile and coleorhiza
(c) Integument and testa
(d) Perisperm and pericarp
Q14. Why is apple called a false fruit? Which part(s) of the flower forms the fruit?
Answer: Botanically ripened ovary is called a true fruit. The fruits in which thalamus and other floral parts develop along with the ovary are called false fruits. For example – apple, strawberry, cashew etc. In apple, the main edible portion of the fruit is the fleshy thalamus. The ovary forms the fruit after fertilization or without fertilization in parthenocarpic fruits.
Q15. What is meant by emasculation? When and why does a plant breeder employ this technique?
Answer: Emasculation is the process of removing anthers from bisexual flowers without affecting the female reproductive part (pistil), which is used in various plant hybridization techniques.
Emasculation is performed by plant breeders 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.
Q16. If one can induce parthenocarpy through the application of growth substances, which fruits would you select to induce parthenocarpy and why?
Answer: The fruit of the parthenocarpy is seedless. The ovary does not fertilize them before they develop. A few seedless fruits are of great economic importance, such as banana, grape, orange, pineapple, guava, watermelon, and lemon. In fruits with edible seeds or seeds (e.g., pomegranates), parthenocarpy is not selected.
Q17. Explain the role of the tapetum in the formation of the pollen grain wall.
Answer: The tapetum is the innermost layer of the microsporangium. It provides nutrition to the developing pollen grains. It secretes enzymes, hormones and special proteins for the pollen grains to recognise compatibility, It produces Ubisch granules for the formation of exine of pollen grain. The tapetum secretes pollenkitt over the outer side of mature pollen.
- Q18. What is apomixis and what is its importance?
Answer: Apomixis is the process of seed formation without the occurrence of fertilization (process of meiosis and syngamy to be precise). It plays a vital role in the production of hybrid seeds. Apomixis inhibits the loss of desired characters in a hybrid. Also the process of production of hybrid seeds through cultivation is expensive. Therefore, apomixis is used for the production of hybrid seeds.
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