NCERT Solutions for Class 12 Biology Chapter 1 Sexual Reproduction in Flowering Plants

NCERT
SOLUTIONS
CLASS - 12th

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Class : 12th
Subject : Biology
Chapter : 2
Chapter Name : Sexual Reproduction in Flowering Plants

Q1 Name the parts of an angiosperm ower 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
nucellus of the ovule from the functional megaspore.

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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.
(a)

(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) Microsporogenesis results in the formation of haploid microspores from a diploid microspore
mother cell. On the other hand, megasporogenesis results in the formation of haploid megaspores
from a diploid megaspore mother cell.

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Q3 Arrange the following terms in the correct developmental sequence: Pollen grain, sporogenous
tissue, microspore tetrad, pollen mother cell, male gametes

Answer. The correct development sequence is as follows:
Sporogenous tissue — pollen mother cell — microspore tetrad — Pollen grain — male gamete
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.

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

Answer. An ovule is a female megasporangium where the formation of megaspores takes place.

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) Chalazal — It is the based swollen part of the nucellus from where the integuments originate.

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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 owering 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.

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

Answer.

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
rst 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.

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

Answer. There are two types of owers present in plants namely Oxalis and Viola —
chasmogamous and cleistogamous owers. Chasmogamous owers have exposed anthers and
stigma similar to the owers of other species. Cross-pollination cannot occur in cleistogamous
owers. This is because cleistogamous owers never open at all. Also, the anther and the stigma
lie close to each other in these owers. Hence, only self-pollination is possible in these owers.

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

Answer. Self-pollination involves the transfer of pollen from the stamen to the pistil of the same
ower. Two strategies that have evolved to prevent self-pollination in owers are as follows:
(1) In certain plants, the stigma of the ower has the capability 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) In some plants, the gynoecium matures before the androecium or vice-versa. This
phenomenon is known as protogyny or protandry respectively. This prevents the pollen from
coming in contact with the stigma of the same ower.

Q9 What is self-incompatibility? Why does self-pollination not lead to seed formation in self-
incompatible species?

Answer. 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. The plants which exhibit this phenomenon have the
ability to prevent germination of pollen grains and thus, prevent the growth of the pollen tube on
the stigma of the ower. This prevents the fusion of the gametes along with the development of
the embryo. As a result, no seed formation takes place.

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

Answer. Various arti cial hybridization techniques (under various crop improvement programmes)
involve the removal of the anther from bisexual owers without affecting the female reproductive
part (pistil) through the process of emasculation. Then, these emasculated owers are wrapped in
bags to prevent pollination by unwanted pollen grains. This process is called bagging. This
technique is an important part of the plant breeding programme as it ensures that pollen grains of
only desirable plants are used for fertilization of the stigma to develop the desired plant variety.

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

Answer. 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. One
male gamete nucleus and two polar nuclei are involved in this process.

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

Answer. The zygote is formed by the fusion of the male gamete with the nucleus of the egg cell.
The zygote remains dormant for some time and waits for the endosperm to form, which develops
from the primary endosperm cell resulting from triple fusion. The endosperm provides food for
the growing embryo and after the formation of the endosperm, further development of the embryo
from the zygote starts.

Q13 Differentiate between:
(a) Hypocotyl and epicotyl;
(b) Coleoptile and coleorhiza;
(c) Integument and testa;
(d) Perisperm and pericarp.

Answer.
(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 ower forms the fruit?

Answer. Fruits derived from the ovary and other accessory oral parts are called false fruits. On
the contrary, true fruits are those fruits which develop from the ovary, but do not consist of the
thalamus or any other oral part. In an apple, the eshy receptacle forms the main edible part.
Hence, it is a false fruit.

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 owers without affecting
the female reproductive part (pistil), which is used in various plant hybridization techniques.
Emasculation is performed by plant breeders in bisexual owers to obtain the desired variety of a
plant by crossing a particular plant with the desired pollen grain. To remove the anthers, the
owers are covered with a bag before they open. This ensures that the ower 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 arti cial 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. Parthenocarpy is the process of developing fruits without involving the process of
fertilization or seed formation. Therefore, the seedless varieties of 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.

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

Answer. Tapetum is the innermost layer of the microsporangium. It provides nourishment to the
developing pollen grains. During microsporogenesis, the cells of tapetum produce various
enzymes, hormones, amino acids, and other nutritious material required for the development of
pollen grains. It also produces the exine layer of the pollen grains, which is composed of the
sporopollenin.

Q18 What is apomixis and what is its importance?

Answer. 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
dif cult to maintain hybrid characters as characters segregate during meiosis. Apomixis prevents
the loss of speci c characters in the hybrid. Also, it is a cost-effective method for producing seeds.