Sexual Reproduction in Flowering Plants

The flower is the site of sexual reproduction in angiosperms, and this chapter follows the events that occur before, during and after fertilisation. You will study the structure of the stamen and the four wall layers of the microsporangium (epidermis, endothecium, middle layers and the nourishing tapetum), microsporogenesis, and the structure of the pollen grain with its tough sporopollenin exine, germ pores and vegetative and generative cells. You will also study the pistil, the parts of an ovule, megasporogenesis, and the formation of the seven-celled, eight-nucleate embryo sac by monosporic development. The chapter then explains pollination, including autogamy, geitonogamy and xenogamy, cleistogamous flowers of Viola and Oxalis, and pollination by wind, water (Vallisneria, Zostera) and insects, with outbreeding devices and pollen-pistil interaction used in artificial hybridisation through emasculation and bagging. You will learn about double fertilisation (syngamy and triple fusion), the development of the triploid endosperm and the embryo, seeds and fruits, including parthenocarpy, and special mechanisms such as apomixis and polyembryony.

Detailed Worksheet: Sexual Reproduction in Flowering Plants Section A - Definitions (10 marks) 1. Name the four wall layers of a microsporangium and state the function of the tapetum. (2 marks) 2. What is sporopollenin? Why is it significant for pollen grains? (2 marks) 3. Distinguish between geitonogamy and xenogamy. Why is geitonogamy genetically similar to autogamy? (2 marks) 4. What is double fertilisation? Name the products formed. (2 marks) 5. Define apomixis and polyembryony with one example of each. (2 marks) Section B - Calculations and Applications (15 marks) 6. How many meiotic divisions are required to produce 100 viable seeds in an angiosperm? Show your reasoning for the number of microspore mother cells and megaspore mother cells involved. (3 marks) 7. The leaf cells of a plant have 24 chromosomes (2n). Calculate the chromosome number in the (i) microspore mother cell (ii) pollen grain (iii) egg cell (iv) synergid (v) endosperm (vi) embryo. (3 marks) 8. An anther has four microsporangia, each containing 50 microspore mother cells. Calculate the number of pollen grains produced by one anther and by a flower with 10 stamens. (3 marks) 9. Classify each seed or fruit: (i) wheat and castor seeds (ii) pea and groundnut seeds (iii) black pepper and beet seeds (iv) apple and strawberry (v) banana developed without fertilisation. (3 marks) 10. Pollen grains of rice and wheat lose viability within 30 minutes of release, while those of some members of Rosaceae, Leguminosae and Solanaceae remain viable for months. Explain how a plant breeder in Punjab can still use pollen of rice collected from another state for hybridisation. (3 marks) Section C - Diagrams (10 marks) 11. Draw a labelled diagram of a typical anatropous ovule showing the funicle, hilum, integuments, micropyle, chalaza, nucellus and embryo sac. (4 marks) 12. Draw a labelled diagram of a mature embryo sac showing the egg apparatus (egg and synergids with filiform apparatus), central cell with polar nuclei, and antipodals. (3 marks) 13. Draw a labelled diagram of the L.S. of a monocot embryo (grass) showing scutellum, coleoptile, shoot apex, epiblast, radicle, root cap and coleorhiza. (3 marks) Section D - Analysis and Higher-order Thinking (15 marks) 14. Explain the sequence of events from the landing of a compatible pollen grain on the stigma to fertilisation, including the role of the filiform apparatus. Analyse how pollen-pistil interaction helps a plant reject incompatible pollen. (5 marks) 15. Explain the outbreeding devices that flowering plants have evolved to discourage self-pollination, such as asynchrony, positioning of anther and stigma, self-incompatibility and unisexuality. Analyse why a breeder working with a bisexual flower must emasculate it and bag it, while unisexual flowers do not need emasculation. (5 marks) 16. Compare wind-pollinated and insect-pollinated flowers on five features. Analyse the special relationship between the Yucca plant and its moth, and the floral rewards and deceptions offered to insects. (5 marks) Instructions: Time allowed 2 hours. Attempt all sections. Draw all diagrams neatly in pencil and label every part. Show working for all calculations.