SEXUAL REPRODUCTION IN FLOWERING PLANTS
EXERCISE SOLUTIONS
Question 1: Name the parts of an angiosperm flower in which development of male and female gametophyte take place.
Ans: Development of male and female gametophytes takes place in anther and ovary, respectively.
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.
Meiosis occurs during micro and megasporogenesis.
Microspores (Pollen grains) are formed at the end of microsporogenesis and female gametophytes (embryo sacs) are formed at the end of megasporogenesis.
Question 3: Arrange the following terms in the correct developmental sequence:
Pollen grain, sporogenous tissue, microspore tetrad, pollen mother cell, male gametes
Ans: 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.
Question 4: With a neat, labelled diagram, describe the parts of a typical angiosperm ovule.
Ans:
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The ovule is a small structure attached to the placenta by means of a stalk called funicle.
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The junction between an ovule and a funicle is called hilum. Sometimes, the funicle extends beyond the hilum to form a ridge called raphae.
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The ovule is surrounded by one or two protective envelopes called integuments.
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Integument encircles the ovule entirely except at the tip, resulting in a small opening called micropyle.
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The basal part of an ovule opposite to micropyle is called chalaza.
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The cells with high or abundant reserve food material enclosed within integument is called nucellus.
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The female gametophyte located within the nucellus is called an embryo sac.
Question 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 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 degenerates.
Question 6: With a neat diagram explain the 7-celled, 8-nucleate nature of the female gametophyte.
Ans:
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The female gametophyte (embryo sac) develops from a single functional megaspore.
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This megaspore undergoes three successive mitotic divisions to form eight nucleate embryo sacs.
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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.
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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.
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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.
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Similarly, at the chalazal end, three out of four nuclei differentiates as antipodal cells.
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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.
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Hence, at maturity, the female gametophyte appears as a 7-celled structure, though it has 8 nucleate.
Question 7: What are chasmogamous flowers? Can cross-pollination occur in cleistogamous flowers? Give reasons for your answer.
Ans: Chasmogamous flowers are open flowers with exposed stamens and stigma which facilitate cross pollination. E.g., Oxalis and Viola
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.
Question 8: Mention two strategies evolved to prevent self-pollination in flowers.
Ans:
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Unisexuality: Male and female flowers are present on different plants.
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Dichogamy: The condition in which the stamens and stigma of a bisexual flower mature at different times.
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Protandry: This is the condition where anthers mature earlier than the stigma and release pollens
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Protogyny: This is the condition where the stigma matures earlier than the anther.
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Self-sterility or self-incompatibility: It is a genetic mechanism that prevents self-pollination.
Question 9: What is self-incompatibility? Why does self-pollination not lead to seed formation in self-incompatible species?
Ans: 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 flower. 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?
Ans: The process of removal of the anther from bisexual flowers without affecting the female reproductive part (pistil) is called emasculation.
Then, these emasculated flowers 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.
Question 11: What is triple fusion? Where and how does it take place? Name the nuclei involved in triple fusion.
Ans: 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.
Question 12: Why do you think the zygote is dormant for sometime in a fertilized ovule?
Ans: 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.
Question 13: Differentiate between:
(a) Hypocotyl and epicotyl;
(b) Coleoptile and coleorrhiza;
(c) Integument and testa;
(d) Perisperm and pericarp.
Question 14: Why is apple called a false fruit? Which part(s) of the flower forms the fruit?
Ans: Apple fruit is derived from the other accessory floral parts.
True fruits are those fruits which develop from the ovary, but do not consist of the thalamus or any other floral part. In an apple, the fleshy receptacle forms the main edible part. Hence, it is a false fruit.
Question 15: What is meant by emasculation? When and why does a plant breeder employ this technique?
Ans: 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.
Question 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 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.
Question 17: Explain the role of tapetum in the formation pollen-grain wall.
Ans: 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.
Question 18: What is apomixis and what is its importance?
Ans: Apomixis is a type of asexual reproduction seeds are formed without fertilisation.
In apomitic seeds, parental characters are maintained in the progeny as there is no meiosis or segregation of characters.
If desired hybrid seeds are made apomitics the farmers can keep on using the hybrid seeds to raise new crops year after year.