Biology Class 12 NCERT Solutions: Chapter 2 Sexual Reproduction in Flowering Plants Part 3

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Q.9 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 flower.

  • This prevents the fusion of the gametes along with the development of the embryo. As a result, no seed formation takes place.

  • When inspite of viable pollens they are unable to germinate on the stigma of the same flower it is called self-incompatibility and this leads to cross pollination e.g., radish

Image of Self- Incompatibility

Image of Self- Incompatibility

Q.10 What is bagging technique? How is it useful in a plant breeding programme?

Answer:

  • 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.

  • 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.

  • During artificial hybridization, emasculated flowers are covered with a bag of suitable size, generally made up of butter paper. This step is called bagging

  • When stigma of bagged flower attains receptivity, mature pollen grains are collected from anther of male parent and are dusted on stigma.

  • Now the flowers are rebagged and fruits are allowed develop. Advantages of bagging technique: (i) It is useful for cross breeding. (ii) Commercially superior varieties are obtained

Q.11 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.

Q.12 Why do you think the zygote is dormant for some time 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.

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