NCERT Book Class 12 Biology Sexual Reproduction in Flowering Plants

SEXUAL REPRODUCTION IN FLOWERING PLANTS

Are we not lucky that plants reproduce sexually? The myriads of flowers that we enjoy gazing at, the scents and the perfumes that we swoon over, the rich colours that attract us, are all there as an aid to sexual reproduction. Flowers do not exist only for us to be used for our own selfishness. All flowering plants show sexual reproduction. A look at the diversity of structures of the inflorescences, flowers and floral parts, shows an amazing range of adaptations to ensure formation of the end products of sexual reproduction, the fruits and seeds. In this chapter, let us understand the morphology, structure and the processes of sexual reproduction in flowering plants (angiosperms).

2.1 FLOWER – A FASCINATING ORGAN OF ANGIOSPERMS

Human beings have had an intimate relationship with flowers since time immemorial. Flowers are objects of aesthetic, ornamental, social, religious and cultural value – they have always been used as symbols for conveying important human feelings such as love, affection, happiness, grief, mourning, etc. List at least five flowers of ornamental value that are commonly cultivated at homes and in gardens. Find out the names of five more flowers that areused in social and cultural celebrations in your family. Have you heard of floriculture – what does it refer to? 

To a biologist, flowers are morphological and embryological marvels and the sites of sexual reproduction. In class XI, you have read the various parts of a flower. Figure 2.1 will help you recall the parts of a typical flower. Can you name the two parts in a flower in which the two most important units of sexual reproduction develop?

2.2 PRE-FERTILISATION: STRUCTURES AND EVENTS

Much before the actual flower is seen on a plant, the decision that the plant is going to flower has taken place. Several hormonal and structural changes are initiated which lead to the differentiation and further development of the floral primordium. Inflorescences are formed which bear the floral buds and then the flowers. In the flower the male and female reproductive structures, the androecium and the gynoecium differentiate and develop. You would recollect that the androecium consists of a whorl of stamens representing the male reproductive organ and the gynoecium represents the female reproductive organ.

EXERCISES

1. Name the parts of an angiosperm flower in which development of male and female gametophyte take place.

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.

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

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

5. What is meant by monosporic development of female gametophyte?

6. With a neat diagram explain the 7-celled, 8-nucleate nature of the female gametophyte. the egg apparatus consisting of two synergids and an egg cell. At the chalazal end are three antipodals. At the centre is a large central cell with two polar nuclei.

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

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

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

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

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

12. Why do you think the zygote is dormant for sometime in a fertilised ovule?

13. Differentiate between:

 (a) hypocotyl and epicotyl;

 (b) coleoptile and coleorrhiza;

 (c) integument and testa;

 (d) perisperm and pericarp.

14. Why is apple called a false fruit? Which part(s) of the flower forms the fruit?

15. What is meant by emasculation? When and why does a plant breeder employ this technique?

16. If one can induce parthenocarpy through the application of growth substances, which fruits would you select to induce parthenocarpy and why?

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

18. What is apomixis and what is its importance?

Please refer to attached file for NCERT Class 12 Biology Sexual Reproduction in Flowering Plants