Sexual Reproduction in Flowering Plants PDF

# Chapter 2 Sexual Reproduction in Flowering Plants ## 2.1 Flower - A Fascinating Organ of Angiosperms - Humans have been fascinated with flowers for centuries, often using them for decoration, aesthetic, and cultural purposes. - They are a key part of sexual reproduction in flowering plants. - Th...

# Chapter 2 Sexual Reproduction in Flowering Plants ## 2.1 Flower - A Fascinating Organ of Angiosperms - Humans have been fascinated with flowers for centuries, often using them for decoration, aesthetic, and cultural purposes. - They are a key part of sexual reproduction in flowering plants. - The many different structures involved in flower reproduction are adaptations to ensure the formation of fruits and seeds. ## 2.2 Pre-fertilisation: Structures and Events - The development of flowers begins with hormonal and structural changes within the plant. - The floral primordium develops into inflorescences. - The flower differentiates specialized reproductive structures called the androecium and gynoecium. - The androecium, which is a whorl of stamens, represents the male reproductive organ. - The gynoecium, consisting of one or more carpels, represents the female reproductive organ. ### 2.2.1 Stamen, Microsporangium and Pollen Grain - A typical stamen consists of a filament and anther. - The filament attaches the anther to the thalamus or petal of the flower. - Anthers are bilobed and contain four pollen sacs, also known as microsporangia. - The microsporangia develop into pollen sacs, containing pollen grains. #### Structure of Microsporangium - Anther walls consist of four layers: epidermis, endothecium, middle layers, and tapetum. - The tapetum nourishes developing pollen grains. - The sporogenous tissue in the center of each microsporangium produces microspore tetrads. - Meiosis of the sporogenous tissue results in the formation of microspores. - Microspores develop into mature pollen grains. ##### Pollen Grain - Pollen grains represent the male gametophyte. - Pollen grain walls are two-layered: exine and intine. - Exine is made of sporopollenin, a resistant organic material that helps preserve pollen grains in the fossil record. - Exine contains apertures called germ pores. - Intine is a thin, continuous layer made of cellulose and pectin. - Pollen grains have a variety of designs and patterns on their surface. - Mature pollen grains contain two cells: the vegetative cell and the generative cell. - Most angiosperms shed pollen grains at a two-celled stage. - In some angiosperms, the generative cell divides mitotically before pollen grain release, forming two male gametes. - Pollen grains can cause allergies in some people. ## 2.3 Double Fertilisation - Double fertilization occurs when two male gametes from the pollen tube enter the ovule. - One male gamete fuses with the egg cell, forming a diploid zygote. - The other male gamete fuses with the two polar nuclei in the central cell, forming a triploid primary endosperm nucleus (PEN). - The PEN develops into endosperm, a nutritive tissue that supports the developing embryo. - Double fertilization is unique to flowering plants. ## 2.4 Post-fertilisation: Structures and Events - The events that occur after fertilization are called post-fertilization events. - Endosperm development precedes embryo development. - The primary endosperm cell divides repeatedly to form endosperm tissue, which provides nutrients for the developing embryo. ### 2.4.1 Endosperm - Endosperm may be either cellular or nuclear. - In the nuclear type, the primary endosperm nucleus undergoes several rounds of nuclear division before cell walls form. - In the cellular type, cell walls form immediately after each nuclear division. - The endosperm may be consumed by the developing embryo before seed maturation (e.g, pea, groundnut, beans) or persist in the mature seed (e.g., castor, coconut). ### 2.4.2 Embryo - The zygote divides mitotically to form the embryo. - The embryo develops at the micropylar end of the embryo sac. - Early stages of embryo development are similar in both monocots and dicots. - The mature dicot embryo consists of an embryonal axis, two cotyledons, a plumule, an epicotyl, a hypocotyl, and a radicle. - The mature monocot embryo has one cotyledon called the scutellum, which is situated towards the lateral side of the embryonal axis. ### 2.4.3 Seed - A seed is a fertilized ovule. - A seed typically consists of a seed coat, cotyledons, and an embryo axis. - Mature seeds may be albuminous or non-albuminous, depending on whether endosperm is present or consumed by the embryo. - Seeds are generally dry and dormant but become active when favorable conditions are available. - As the ovule matures into seed, the ovary develops into a fruit. - Fruits may be fleshy or dry, depending on the pericarp. - Fruits play a role in seed dispersal. - False fruits develop from the thalamus, as in apples and strawberries, while true fruits develop only from the ovary. ## 2.5 Apomixis and Polyembryony - Apomixis is a form of asexual reproduction that mimics sexual reproduction. - Apomixis results in the formation of seeds without fertilization. - Polyembryony is the occurrence of more than one embryo in a seed. - Polyembryony can occur through adventive embryony or nucellar polyembryony. ## Exercises 1. What are chasmogamous flowers? Can cross-pollination occur in cleistogamous flowers? Give reasons for your answer. 2. Mention two strategies evolved to prevent self-pollination in flowers. 3. What is self-incompatibility? Why does self-pollination not lead to seed formation in self-incompatible species? 4. What is bagging technique? How is it useful in a plant breeding programme? 5. What is triple fusion? Where and how does it take place? Name the nuclei involved in triple fusion. 6. Why do you think the zygote is dormant for sometime in a fertilized ovule? 7. Differentiate between: - hypocotyl and epicotyl - coleoptile and coleorrhiza - integument and testa - perisperm and pericarp 8. Why is apple called a false fruit? Which part(s) of the flower forms the fruit? 9. What is meant by emasculation? When and why does a plant breeder employ this technique? 10. If one can induce parthenocarpy through the application of growth substances, which fruits would you select to induce parthenocarpy and why? 11. Explain the role of tapetum in the formation of pollen grain wall. 12. What is apomixis and what is its importance?