NCERT Textual Exercise (Solved) 53 Sexual Reproduction in Flowering Plants 1. Name the parts of an angiosperm flower in which development of male and female gametophytes take place. Ans. Anther and ovule are the parts of angiospermic flower where development of male and female gametophytes takes place respectively. 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. Ans. Microsporogenesis Megasporogenesis The process of formation and differentiation of pollen grains from microspore mother cells by meiosis is called microsporogenesis. The process of formation and differentiation of megaspores from megaspore mother cells by meiosis is known as megasporogenesis. Pollen grains are produced in the anther which is a broader knob like fertile part of the stamen. Ovules (which are the future seeds) are formed in the ovary. All the four pollen grains that are formed from microspore mother cell are functional. Only one out of the four megaspores is functional. Each microspore mother cell and megaspore mother cell contain two sets of chromoromes and are therefore diploid. The diploid megaspore mother cell and microspore mother cell enlarge and undergo meiosis to produce four haploid cells called megaspores and microspores respectively. The chromosome number is reduced by half and therefore megaspores and microspores are haploid. Microsporogenesis and megasporogenesis give rise to pollen grains and embryo sac respectively. Pollen grain is the male gametophyte and embryo sac represents the female gametophyte. 3. Arrange the following terms in the correct developmental sequence: pollen grain, sporogenous tissue, microspore tetrad, pollen mother cell, male gametes. Ans. Sporogenous tissue – pollen mother cell – microspore tetrad – pollen grains – male gametes. 4. With a neat, labelled diagram, describe the parts of a typical angiosperm ovule.
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NCERT Textual Exercise (Solved)
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1. Namethepartsofanangiospermflowerinwhichdevelopmentofmaleandfemale gametophytes take place.
Ans. Antherandovulearethepartsofangiospermicflowerwheredevelopmentof male and female gametophytes takes place respectively.
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.
Ans.
Microsporogenesis MegasporogenesisThe process of formation and differentiation of pollen grains from microspore mother cells by meiosis is called microsporogenesis.
The process of formation and differentiation of megaspores from megaspore mother cells by meiosis is known as megasporogenesis.
Pollen grains are produced in the anther which is a broader knob like fertile part of the stamen.
Ovules(whicharethefutureseeds)are formed in the ovary.
All the four pollen grains that are formed from microspore mother cell are functional.
Only one out of the four megaspores is functional.
Each microspore mother cell and megaspore mother cell contain two sets of chromoromes and are therefore diploid. The diploid megaspore mother cell and microspore mother cell enlarge and undergo meiosis to produce four haploid cells called megaspores and microspores respectively.
The chromosome number is reduced by half and therefore megaspores and microspores are haploid.
Microsporogenesis and megasporogenesis give rise to pollen grains and embryo sac respectively. Pollen grain is the male gametophyte and embryo sac represents the female gametophyte.
3. Arrange the following terms in the correct developmental sequence: pollen grain, sporogenous tissue, microspore tetrad, pollen mother cell, male gametes.
Ans. Sporogenoustissue–pollenmothercell–microsporetetrad–pollengrains– male gametes.
4. With a neat, labelled diagram, describe the parts of a typical angiosperm ovule.
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Ans. A typical angiospermic ovule is a small structure which is formed in the ovary (SeeFig.2.17).Ovulefirstdevelopsasaprojectionontheplacentaandiscomposed of multilayered cellular tissue called the nucellus. The hypodermal cell of the nucellus enlarges and is transformed into megaspore mother cell. This cell undergoes meiosis to produce four haploid cells only one of which developsandformsembryosac(femalegametophyte).Anovulemaybesurrounded by one or two protective layers called integuments, leaving a small opening at one end termed as micropyle which acts as a passage for the entry of the pollen tube into the ovule. Thus, a typical ovule consists of a fully developed embryo sac with the nucellus and integuments.
Fig. 2.17 Longitudinal section of a mature anatropous ovule 5. What is meant by monosporic development of female gametophyte? Ans. Inmanyfloweringplants,onlyoneoutofthefourmegasporesenlargeand
develop into female gametophyte or embryo sac. The other three megaspores degenerate. This type of embryo sac formation is called as monosporic type of development.
6. With a neat diagram explain the 7-celled, 8-nucleate nature of the female gametophyte.
Ans. Embryosac(orfemalegametophyte)isformedbythreesuccessivemitoticdivisions that take place in the nucleus of megaspore.
The nucleus of the functional megaspore divides mitotically to form two nuclei which move to the opposite poles, forming the 2-nucleate embryo sac. Two more sequential mitotic nuclear divisions result in the formation of the 4-nucleate and later the 8-nucleate stages of the embryo sac. After the 8-nucleate stage, cell walls are laid down leading to the organisation of the typical female gametophyte or embryo sac. Three cells out the eight cells are grouped together at micropylar end and form the egg apparatus. The three cells at the chalazal end are called antipodals. The large central cell consists of two polar nuclei.
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Fig. 2.18 A Diagrammatic representation of the mature embryo sac. Thus, a typical female gametophyte consists of 7 cells but it is 8 nucleated
(SeeFig.2.18). 7. What are chasmogamous flowers? Can cross-pollination occur in
cleistogamousflowers?Givereasonsforyouranswer. Ans. Chasmogamousflowersoropenflowersarethosewhichpollinatebytheir
own pollen. In them, the anthers and stigmas are brought together by growth, bending or folding for pollination.
Cross pollination cannot occur in cleistogamousflowers.Theseflowersremainclosedthuscausingonlyself-pollination.Incleistogamousflowers,anthersdehiscenceinsidetheclosedflowers.Sothepollengrainscomeincontact with stigma. Thus there is no chance of cross pollition, e.g. Oxalis, Viola.
8. Mentiontwostrategiesevolvedtopreventselfpollinationinflowers. Ans. Continued self pollination decreases the vigour and vitality of a particular
race.Italsodecreasestheimmunityoftheplants.Thus,floweringplantshave developed many devices to discourage self pollination and to encourage cross pollination.
Dichogamy and self sterility are two most common devices that ensure cross pollination.
Dichogamy – Maturation of anther and stigma at different times in a bisexual flowerpreventselfpollination.Selfsterility(orselfincompatibility)–Due
9. What is self incompatibility? Why does self pollination not lead to seed formation in self incompatible species?
Ans. When the pollen grains of an anther do not germinate on the stigma of the sameflower,thensuchafloweriscalledselfsterileorincompatibleandsuch condition is known as self incompatibility or self sterility.
The transference of pollen grains shed from the anther to the stigma of the pistil is called pollination. This transference initiates the process of seed formation.Selfpollinationisthetransferofpollengrainshedfromtheanthertostigmaofpistilinthesameflower.Butinsomeflowerthisdoesnothappenbecause of the presence of same sterile gene on pistil and the pollen grain is called self incompatibile. Thus seeds formation does not take place.
10. Whatisbaggingtechnique?Howisitusefulinaplantbreedingprogramme? Ans. Coveringtheemasculatedflowersbyabagofsuitablesizeorbutterpaperis
knownasbagging.Thistechniqueismainlyusedinartificialhybridisation.Plant breeders often use this technique to prevent the contamination of stigma oftheflowersfromunwantedpollengrains.
11. What is triple fusion? Where and how does it take place? Name the nuclei involved in triple fusion.
Ans. Fusion of the second male gamete with the two polar nuclei located in the centralcelltoformthetriploidprimaryendospermnucleus(PEN)iscalledtriple fusion or vegetative fertilization.
This process takes place in the embryo sac. After reaching the ovary, pollen tube enters into the embryo sac from the micropylar end. After penetration, the tip of the pollen ruptures releasing the two male gametes. The one male gamete fuses with the egg to form the diploid zygote. This process is called syngany and the other male gamete fuses with the two polar nuclei to form the triploid primary endosperm nucleus. This process is known as triple fusion. These two types of fertilization constitite the process of double fertilization. ItwasdiscoveredbyS.G.Nawaschin(1898)andGuignardinLilium and Frittillaria. One of the male gametes and the two polar nuclei in the central cell are the nuclei involved in triple fusion.
12. Why do you think the zygote is dormant for sometime in a fertilised ovule? Ans. The zygote after a period of rest develops into embryo. Most zygotes remain
dorment till certain amount of endosperm forms. They do so to provide assured nutrition to the developing embryo.
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13. Differentiate between: (i) hypocotylandepicotyl (ii) coleoptileandcoleorrhiza (iii) integumentandtesta (iv) perispermandpericarp Ans. (i) Theregionoftheembryonalaxisbetweentheradicleandthepoint
ofattachmentofthecotyledonsiscalledthehypocotyl(belowthecotyledons) whereas the portion between the plumule and cotyledons iscalledtheepicotyl(abovethecotyledons).
(ii) The plumule is surrounded by a leaf-like covering called thecoleoptile whereas the protective covering of radicle and root cap is called coleorrhiza.
(iii) Eachovule is surroundedbyoneor twoprotective layers calledinteguments, leaving only a small opening at one end called the micropyle whereas the outer seed coat is called the testa. The seed coats are formed from the two integuments.
(iv) Periplasmistheresidual,persistentnucelluswhereaspericarpisthewall of the fruit.
14. Whyisapplecalledafalsefruit?Whichpart(s)oftheflowerformsthefruit? Ans. Thefruitsinwhichthalamusandotherfloralpartsdevelopalongwiththe
ovary are called false fruits. For example, apple, strawberry, cashew, etc. In applethemainedibleportionofthefruitisthefleshyreceptacle.
Botanically any riped ovary is called a true fruit. 15. What is meant by emasculation? When and why does a plant breeder employ
this technique? Ans. Emasculationistheremovalofstamens,mainlytheanthersfromtheflower
buds before they dehiscence. This is mainly done to avoid self pollination. Emasculation isoneof themeasures in theartificialhybridisation.Plantbreeders employ this technques to obtain an overall superior variety.
16. If one can induce parthenocarpy through the application of growth substances, which fruits would you select to induce parthenocarpy and why?
Ans. Parthenocarpy is the production and development of seedless fruits. Orangesarethefruitswhichweselecttoinduceparthenocarpy.Seedsare
very irritating components when eating fruits so by inducing parthenocarpy we can obtain oranges without seeds and with more pulp in it.
17. Explain the role of tapetum in the formation of pollen-grain wall. Ans. Tapetum is the innermost layer of the microsporangium. The tapetal cells
are multinucleated and polyploid. They nourish the developing pollen grains. These cells contain Ubisch bodies that help in the ornamentation of
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the microspores or pollen grains walls. The outer layer of the pollen grain is called exine and is made up of the sporopollenin secreted by the Ubisch bodies of the tapetal cells. This compound provides spiny appearance to the exine of the pollen grains.
18. What is apomixis and what is its importance? Ans. Apomixis is a mode of asexual reproduction that produces seeds without
fertilization. For example, some species of Asteraceae and grasses. This method is important in hybrid seed industry.Hybrids are extensivelycultivated for increasing productivity. But the main drawback is that the hybrid seeds are to be produced every year because the seeds of the hybrid plants do not maintain hybrid characters for longer period due to segregation of the fruits. This can be avoided if apomixis can be introduced in hybrid seeds. For this reason scientsts are trying hard to identify genes for apomixis.
