NCERT Solutions for Class 12 Biology Chapter 2
Sexual Reproduction in Flowering Plants Class 12
Chapter 2 Sexual Reproduction in Flowering Plants Exercise Solutions
Exercise : Solutions of Questions on Page Number : 40
Name the parts of an angiosperm flower in which development of male and female gametophyte take place.
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.
(a)
Microsporogenesis |
Megasporogenesis |
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1. |
It is the process of the formation of microspore tetrads from a microspore mother cell through meiosis. |
It is the process of the formation of the four megaspores from a megaspore mother cell in the region of the nucellus through meiosis |
2. |
It occurs inside the pollen sac of the anther. |
It occurs inside the ovule. |
(b) Both events (microsporogenesis and megasporogenesis) involve the process of meiosis or reduction division which results in the formation of haploid gametes from the microspore and megaspore mother cells.
(c)
Arrange the following terms in the correct developmental sequence:
Answer :
The correct development sequence is as follows:
Sporogenous tissue - pollen mother cell - microspore tetrad - Pollen grain - male gamete
With a neat, labelled diagram, describe the parts of a typical angiosperm ovule.
The various parts of an ovule are -
(1) Funiculus - It is a stalk-like structure which represents the point of attachment of the ovule to the placenta of the ovary.
(2) Hilum - It is the point where the body of the ovule is attached to the funiculus.
(3) Integuments -They are the outer layers surrounding the ovule that provide protection to the developing embryo.
(4) Micropyle - It is a narrow pore formed by the projection of integuments. It marks the point where the pollen tube enters the ovule at the time of fertilization.
(5) Nucellus - It is a mass of the parenchymatous tissue surrounded by the integuments from the outside. The nucellus provides nutrition to the developing embryo. The embryo sac is located inside the nucellus.
(6)
What is meant by monosporic development of female gametophyte?
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.
What are chasmogamous flowers? Can cross-pollination occur in cleistogamous flowers? Give reasons for your answer.
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.
Mention two strategies evolved to prevent self-pollination in flowers.
Self-pollination involves the transfer of pollen from the stamen to the pistil of the same flower. Two strategies that have evolved to prevent self-pollination in flowers are as follows:
(1) In certain plants, the stigma of the flower hasthecapability to prevent the germination of pollen grains and hence, prevent the growth of the pollen tube.It is a genetic mechanism to prevent self-pollination called self- incompatibility. Incompatibility may be between individuals of the same species or between individuals of different species. Thus, incompatibility prevents breeding.
(2)
What is self-incompatibility? Why does self-pollination not lead to seed formation in self-incompatible species?
Self-incompatibility is a genetic mechanism in angiosperms that prevents self-pollination. It develops genetic incompatibility between individuals of the same species or between individuals of different species.
What is bagging technique? How is it useful in a plant breeding programme?
Various artificial hybridization techniques (under various crop improvement programmes) involve the removal of the anther from bisexual flowers without affecting the female reproductive part (pistil) through the process of emasculation. Then, these emasculated flowers are wrapped in bags to prevent pollination by unwanted pollen grains. This process is called bagging.
What is triple fusion? Where and how does it take place? Name the nuclei involved in triple fusion.
Triple fusion is the fusion of the male gamete with two polar nuclei inside the embryo sac of the angiosperm. This process of fusion takes place inside the embryo sac.
When pollen grains fall on the stigma, they germinate and give rise to the pollen tube that passes through the style and enters into the ovule. After this, the pollen tube enters one of synergids and releases two male gametes there. Out of the two male gametes, one gamete fuses with the nucleus of the egg cell and forms the zygote (syngamy). The other male gamete fuses with the two polar nuclei present in the central cell to form a triploid primary endosperm nucleus. Since this process involves the fusion of three haploid nuclei, it is known as triple fusion. It results in the formation of the endosperm.
Why do you think the zygote is dormant for sometime in a fertilized ovule?
Differentiate between:
(b) Coleoptile and coleorrhiza;
(d) Perisperm and pericarp.
Differentiate between
Hypocotyl |
Epicotyl |
|
1. |
The portion of the embryonal axis which lies below the cotyledon in a dicot embryo is known as the hypocotyl. |
The portion of the embryonal axis which lies above the cotyledon in a dicot embryo is known as the epicotyl. |
2. |
It terminates with the radicle. |
It terminates with the plumule. |
(b)
Coleoptile |
Coleorrhiza |
It is a conical protective sheath that encloses the plumule in a monocot seed. |
It is an undifferentiated sheath that encloses the radicle and the root cap in a monocot seed. |
Integument |
Testa |
It is the outermost covering of an ovule. It provides protection to it. |
It is the outermost covering of a seed. |
(d)
|
Perisperm |
Pericarp |
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It is the residual nucellus which persists. It is present in some seeds such as beet and black pepper. |
It is the ripened wall of a fruit, which develops from the wall of an ovary. |
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Why is apple called a false fruit? Which part(s) of the flower forms the fruit?
What is meant by emasculation? When and why does a plant breeder employ this technique?
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.
If one can induce parthenocarpy through the application of growth substances, which fruits would you select to induce parthenocarpy and why?
Explain the role of tapetum in the formation pollen-grain wall.
What is apomixis and what is its importance?
Apomixis is the mechanism of seed production without involving the process of meiosis and syngamy. It plays an important role 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. Apomixis prevents the loss of specific characters in the hybrid. Also, it is a cost-effective method for producing seeds.