Biology Class 12 NCERT Solutions: Chapter 2 Sexual Reproduction in Flowering Plants Part 2

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Q.5 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 flowering plants, a single megaspore mother cell present at the micropyle 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.

Q.6 With a neat diagram explains the 7-celled, 8-nucleate nature of the female gametophyte.

Answer:

Structure of female gametophyte

Structure of 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 edivide 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.

Q.7 What are chasmogamous flowers? Can cross-pollination occur in cleistogamous flowers? Give reasons for your answer.

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. 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.

Q.8 Mention two strategies evolved to prevent self-pollination in flowers.

Answer:

  • 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:

  • In certain plants, the stigma of the flower hasthecapability to prevent the germination of pollen grains and hence, prevents 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.

  • 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 flower.

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