NCERT Solutions For Class 12 Biology Sexual Reproduction in Flowering Plants
Topics and Subtopics in NCERT Solutions for Class 12 Biology Chapter 2 Sexual Reproduction in Flowering Plants:
Topics in Chapter 2
2 Sexual Reproduction in Flowering Plants
2.1 Flower – A Fascinating Organ of Angiosperms
2.2 Pre-fertilisation : Structures and Events
2.3 Double Fertilisation
2.4 Post-fertilisation: Structures and Events
2.5 Apomixis and Polyembryony
2.6 Summary
Questions from the Textbook Solved
1. Name the parts of anangiosperm flower in which development of male and female gametophytes take place.
Ans: Development of male gametophyte (micro- gametogenesis) occurs in pollen sac of anther up to two – celled stage. The female gametophyte develops (megagametogenesis) in the nucellus of ovule.
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?
Ans: Differences between microsporogenesis and megasporogenesis:
Character | Microsporogenesis | Megasporogenesis |
Process | It is the formation of haploid microspores from the diploid microspore mother cell. | It is the formation of haploid megaspores from the diploid megaspore mother cell. |
Number of spore mother cells involved | A large number of microspore mother cells are involved. | Only a single megaspore mother cell is involved. |
Arrangement of spores | The arrangement of microspores in a tetrad is generally tetrahedral. | The arrangement of megaspores in a tetrad is commonly linear. |
Fate of spores | The microspore leads to the formation of the male gametophyte. | The megaspore leads to the formation of the female gametophyte. |
Functionality of spores | All the four microspores of a spore tetrad are functional. | Only one megaspore of a spore tetrad is functional. |
Site of occurrence | It occurs inside the microsporangium. | It occurs inside the megasporangium. |
Type of cell division: Meiosis or meiotic cell division occurs during microsporogenesis and megasporogenesis.
Microsporogenesis: Each diploid microspore mother cell divides meiotically (sporic meiosis) to produce four haploid pollen grains.
Megasporogenesis: Each diploid megaspore mother cell divides meiotically (sporic meiosis) to produce four haploid megaspores.
3. Arrange the following terms in the correct development sequence: Pollen grain, sporogenous tissue, microspore tetrad, pollen mother cell, male gametes.
Ans: Following is the correct developmental sequence: Sporogenous tissue → pollen mother cell → microspore tetrad → pollen grain → male gamete. When 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.
4. With a neat, labelled diagram, describe the parts of a typical angiosperm ovule.
Ans:
The parts of a typical angiosperm ovule are:
• Funicle is the stalk that attaches the ovule to placenta.
• Hilum is the points, where funicle is attached to the ovule body.
• Integuments are one or two layers surrounding the ovule that provide protection to the developing embryo.
• Micropyle is a pore like structure on one side of the ovule, where integuments are absent.
• Nucellus is mass of cells surrounded by the integuments from the outside. Cells of nucellus have abundant reserves of food material.
• Female gametophyte covered by a thin membrane is called embryo sac. It is located in the nucellus.
5. What is meant by monosporic development of female gametophyte?
Ans: 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 nucleus 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 threee degenerate.
6. With a neat diagram explain the 7-celled, 8- nucleate nature of the female gametophyte.
Ans:
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 end three cells each move. The three cells at chalaza are called antipodal cells while those at micropylar end constitute 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 of antipodal cell, 2 of synergid cells, 1 of egg cell and 2 of central cell).
7. What are chasmogamous flowers? Can cross-pollination occur in deistogamous flowers? Give reasons for your answer
Ans: There are two types of flowers present in plants namely Oxalis and Viola − chasmogamous and cleistogamous flowers. Chasmogamous flowers have exposed anthers and stigmata similar to the flowers of other species.
Cross-pollination cannot occur in cleistogamous flowers. This is because cleistogamous flowers never open at all. Also, the anther and the stigma lie close to each other in these flowers. Hence, only self-pollination is possible in these flowers.
8. Mention two strategies evolved to prevent self-pollination in flowers.
Ans: Self-pollination is the transfer of pollen from stamen to the pistil of the same flower. The two strategies that have evolved to prevent self-pollination in flowers are as follows:
(i) Self-incompatibility: In some plants, the pollen grains of a flower is not capable of completing growth on the stigma of the same flower. The stigma of the flower can prevent the germination of pollen grains and hence, prevent the growth of the pollen tube. Self- incompatibility is a genetic mechanism to prevent self-pollination. Incompatibility could be b/w individuals of the same species or between individuals of different species.Thus, incompatibility prevents breeding.
(ii) Dichogamy: The maturation of anthers and stigma at a different time in a bi-sexual flower which prevents self-pollination.
(iii) Protogyny: In a few plants, the gynoecium matures before the androecium or vice-versa. This phenomenon is called protogyny or protandry, respectively. This avoids the pollen from coming in contact with the stigma of the same flower.
9. What is self-incompatiblility? Why does self-pollination not lead to seed formation in self-incompatible species?
Ans: Self incompatibility is pre-fertilization barriers, it is the incompatibility with in spaces & functional male gametes fails to affect fertilization with female gametes. Extensive selfing of plants leads to highly homozygous individual which have low survival value. The nature has imposed self incompatibility to check it.
10. What is bagging technique? How is it useful in a plant breeding programme?
Ans:
Bagging technique - Bagging refers to covering of emasculated flowers with the help of butter paper or plastic bags to avoid contamination of stigma of these flowers to 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 need and obtain superior species.
11. What iss triple fusion? Where and how does it take place? Name the nuclei involved in triple fusion.
Ans:
A) Triple fusion:- It is fusion of second male gamete with the secondary nucleus to formed primary endosperm nucleus is called as triple fusion.
B) Triple fusion occurs in the centre of embryo sac where two Pollen nuclei are present.
C) The nuclei involved in triple fusion are two polar nuclei and one second male gamets.
12.Why do you think the zygote is dormant for sometime in a fertiUsed ovule?
Ans: The zygote after the period of rest develops into embryo. Most zygote remain dormant till certain amount of endosperm forms. They do so to provide assured Nutrition to the embryo.
13. Differentiate b/w:
(a)hy pocotyl and epi cotyl;
(b)coleoptile and coleorrhiza;
(c)integument and testa;
(d)perisperm and pericarp.
Ans: a.
Hypocotyl | Epicotyl |
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b.
Coleoptile | Coleorhiza |
• It is a conical protective sheath over the epicotyl bearing the shoot apex and leaf primordia. | • It is a protective sheath over the radicle and root cap. |
• It has a terminal pore for the emergence of the first leaf. | • It is a solid structure. |
• It comes out of the soil. | • It remains inside the soil. |
• It protects the plumule during its emergence from the soil. | • It does not protect the radicle during its passage into the soil. |
• It grows much beyond the grain. | • It stops growing after its emergence from the grain. |
• It emerges from the soil, turns green in colour and carries out photosynthesis. | • It remains inside the soil and is non-green. |
c.
Integument | Testa |
• It is the protective covering of the ovule. | • It is the protective covering of the seed. |
• It is a thin covering. | • It is a thick covering. |
• The cells are living. | • The cells are dead. |
• Sclereids are absent in the cells. | • Sclereids are present in abundance in the cells. |
• It arises from the chalazal end of the ovule. | • It is derived from the outer integument of the ovule. |
• It is a pre-fertilisation structure. | • It is a post-fertilisation structure. |
d.
Perisperm | Pericarp |
• It represents residual persistent nucellus. | • It represents the fruit wall derived from the ovary wall. |
• It is part of the seed. | • It is part of the fruit. |
• It is generally dry. | • It can be dry or fleshy. |
• It is often non-functional for the seed. | • It acts like a protective covering, helps in dispersal and providing nutrition. |
• It is present only in few seeds. | • It is found in all fruits. |
14. Why is apple called a false fruit? Which Part(s) of the flower forms the fruit?
Ans: 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. Ovary forms the fruit after fertilization or without fertilization in parthenocarpic fruits.
15. What is meant by emasculation? When and why does a plant breeder employ this technique?
Ans: 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 in order to produce plants with desired seeds. Hence, it can be used to prevent self-pollination or prevent cross-pollination according to the need of the breeder.
16. If one can induce parthenocarpy through the application of growth substances, which fruits would you select to induce parthenocarpy and why?
Ans: Parthenocarpy is the process of developing fruits without involving the process of fertilization or seed formation. Therefore, the seedless varietiesof economically important fruits such as orange, lemon, water melon etc. are produced using this technique. This technique involves inducing fruit formation by the application of plant growth hormones such as auxins.
17. Explain the role of tapetum in the formation of pollen grain wall.
Ans: Innermost wall of microsporangium is called tapetum. Tapetal cells may have uni, bi or multi nucleate and have dense cytoplasm.
Role played by tapetum are as following:-
a) It secretes hormones and various enzymes.
b) During meiosis in spore mother cells, tapetum supports nutrition to anther locule.
c) It produces ubisch bodies which are coated with sporopollenin to cause thickening of exine.
d) It secretes protein for pollen to be compatible during recognition.
18. What is apomixis and what is its importance?
Ans: Apomixis is the mechanism of seed production without involving the process of meiosis and syngamy (or seed production without fertilization). It’s a kind of asexual reproduction method which mimics sexual reproduction. It produces clonal progeny with maternal genotype. It occurs in some plant families like grasses, Asteraceae.
The main advantage of apomixis over sexual reproduction is the possibility to select individuals with desirable gene combinations and propagate them as clonesin lesser-time.
Apomixis prevents the loss of specific characters in the hybrid.
Hybrid varieties of seeds with better yield and quality can be produced.
Disease-free plants can also be produced with desirable characteristics.
By sowing hybrid seeds, it is difficult to maintain the hybrid characters as characters segregate during meiosis. However, apomixis prevents the loss of specific characters in the hybrid. Also, it is a cost-effective method for producing seeds.
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