SEXUAL REPRODUCTION IN FLOWERING PLANTS
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.
The ovule is a small structure attached to the placenta by means of a stalk called funicle.
The junction between an ovule and a funicle is called hilum. Sometimes, the funicle extends beyond the hilum to form a ridge called raphae.
The ovule is surrounded by one or two protective envelopes called integuments.
Integument encircles the ovule entirely except at the tip, resulting in a small opening called micropyle.
The basal part of an ovule opposite to micropyle is called chalaza.
The cells with high or abundant reserve food material enclosed within integument is called nucellus.
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.
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.
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.
Unisexuality: Male and female flowers are present on different plants.
Dichogamy: The condition in which the stamens and stigma of a bisexual flower mature at different times.
Protandry: This is the condition where anthers mature earlier than the stigma and release pollens
Protogyny: This is the condition where the stigma matures earlier than the anther.
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