NCERT solutions for Class 12 chapter 2 reproduction in flowering plants
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Important points to remember :
- Flowers are the seat of sexual reproduction in angiosperms. In the flower, androecium consisting of stamens represents the male reproductive organs and gynoecium consisting of pistils represents the female reproductive organs.
- A typical anther is bilobed, dithecous and tetrasporangiate. Pollen grains develop inside the microsporangia. Four wall layers, the epidermis, endothecium, middle layers and the tapetum surround the microsporangium.
- Pollen grains represents the male gametophytic generation. The pollen grains have a two-layered wall, the outer exine and inner intine. The exine is made up of sporopollenin and has germ pores.
- The pistil has three parts – the stigma, style and the ovary. Ovules are present in the ovary. The ovules have a stalk called funicle, protective integument(s), and an opening called micropyle. The mature embryo sac is 7-celled and 8-nucleate.
- 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.
- Pollination is the mechanism to transfer pollen grains from the anther to the stigma. Pollinating agents are either abiotic (wind and water) or biotic (animals).
- Pollen-pistil interaction involves all events from the landing of pollen grains on the stigma until the pollen tube enters the embryo sac (when the pollen is compatible) or pollen inhibition (when the pollen is incompatible).
- Angiosperms exhibit double fertilisation because two fusion events occur in each embryo sac, namely syngamy and triple fusion.
- The products of these fusions are the diploid zygote and the triploid primary endosperm nucleus (in the primary endosperm cell). Zygote develops into the embryo and the primary endosperm cell forms the endosperm tissue. Formation of endosperm always precedes development of the embryo.
- The developing embryo passes through different stages such as the proembryo, globular and heart-shaped stages before maturation. Mature dicotyledonous embryo has two cotyledons and an embryonal axis with epicotyl and hypocotyl. Embryos of monocotyledons have a single cotyledon.
1. Name the parts of an angiosperm flower in which development of male
and female gametophyte take place.
Answer : Pollen is the first cell of male gametophyte while ovule represents female gametophyte. Development of male and female gametophytes takes place in anther and ovary, respectively. The male gametophyte or the pollen grain develops inside the pollen chamber of the anther, whereas the female gametophyte (also known as the embryo sac) develops inside the nucellus of the ovule from the functional megaspore.
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.
1. Microsporogenesis is the process of the formation of microspore tetrads from a microspore mother cell through meiosis. While Megasporogenesis is the process of the formation of the four megaspores from a megaspore mother cell in the region of the nucellus through meiosis
2. Microsporogenesis occurs inside the pollen sac of the anther and Megasporogenesis occurs inside the ovule.
b) Both events (microsporogenesis and megasporogenesis) involve the process of meiosis or reduction division which results in the formation of haploid gametes from the microspore and megaspore mother cells.
c) Microsporogenesis results in the formation of haploid microspores from a diploid microspore mother cell. On the other hand, megasporogenesis results in the formation of haploid megaspores from a diploid megaspore mother cell.
3. Arrange the following terms in the correct developmental sequence:
Pollen grain, sporogenous tissue, microspore tetrad, pollen mother cell,
Answer : The correct development sequence is as follows:
Sporogenous tissue – pollen mother cell – microspore tetrad – Pollen grain – male gamete
Because During the development of microsporangium, each cell of the sporogenous tissue acts as a pollen mother cell and gives rise to a microspore tetrad, containing four haploid microspores by the process of meiosis (microsporogenesis). As the anther matures, these microspores dissociate and develop into pollen grains. The pollen grains mature and give rise to male gametes.
4. With a neat, labelled diagram, describe the parts of a typical angiosperm
Answer : The various parts of an ovule are –
1) Funiculus – The funiculus is the tissue, stalk like structure that attaches the developing seed with the mother plant (fruit) through the placenta. It is the conduit for food and water to the developing seed.
2) Hilum – It is the outer surface of the seed or the point where the body of the ovule is attached to the funiculus.
3) Integuments –They are the outer layers surrounding the ovule that provide protection to the developing embryo.
4) Micropyle – It is a narrow pore formed by the projection of integuments. It marks the point where the pollen tube enters the ovule at the time of fertilization.
5) Nucellus – It is a mass of the parenchymatous tissue surrounded by the integuments from the outside. The nucellus provides nutrition to the developing embryo. The embryo sac is located inside the nucellus.
6) Chalazal – It is the based swollen part of the nucellus from where the integuments originate.
5. What is meant by monosporic development of female gametophyte?
Answer : The formation of the female gametophyte from a single functional megaspore is called as monosporic development.
Or // In a majority of flowering plants, one of the megaspores is functional while the other three degenerate. Only the
functional megaspore develops into the female gametophyte (embryo sac). This method of embryo sac formation from a single megaspore is termed monosporic development.
6. With a neat diagram explain the 7-celled, 8-nucleate nature of the female
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. nuclear divisions are not followed immediately by cell wall formation. After the 8-nucleate stage, cell walls are laid down leading to the organisation of the typical female gametophyte or embryo sac.
Out of eight, six nuclei are surrounded by cell walls and organised into cells; the remaining two nuclei, called polar nuclei are situated below the egg apparatus in the large central cell.
Three cells are grouped together at the micropylar end and constitute the egg apparatus. The egg apparatus, in turn, consists of two synergids and one egg cell. The synergids have special cellular thickenings at the micropylar tip called filiform apparatus, which play an important role in guiding the pollen tubes into the synergid. Three cells are at the chalazal end and are called the antipodals. The large central cell, as mentioned earlier, has two polar nuclei. Thus, a typical angiosperm embryo sac, at maturity, though 8-nucleate is 7-celled.
7. What are chasmogamous flowers? Can cross-pollination occur in cleistogamous flowers? Give reasons for your answer.
Answer : Flower which remains open and exposes the anther and stigma are called chasmogamous flowers. They are large, colourful flowers with nectar/scent to attract pollinators. Cleistogamous flowers do not open at all. Because anthers and stigma are not exposed, cross pollination cannot take place in such cases. In this flowers the anther and the stigma lie close to each other Hence we can say only self-pollination is possible in these flowers.
8. Mention two strategies evolved to prevent self-pollination in flowers.
Answer : Various strategies adopted by flowering plants to prevent self-pollination are:
1. Dichogamy: It is the condition in which anthers and stigma mature at different times. Either anther matures before stigma (protandry) or stigma becomes receptive before anthers (protogyny). For example, in Aristolochia, protogyny occurs.
2. Herkogamy: It is the presence of a mechanical barrier between compatible pollen and stigma due to which self pollination become impossible.
3. Self-incompatibility: 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.
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.
10. What is bagging technique? How is it useful in a plant breeding
Answer : Emasculated flowers
have to be covered with a bag of suitable size, generally made up of butter
paper, to prevent contamination of its stigma with unwanted pollen. This
process is called bagging.
When the stigma of bagged flower attains
receptivity, mature pollen grains collected from anthers of the male parent
are dusted on the stigma, and the flowers are rebagged, and the fruits
allowed to develop. If the female parent produces unisexual flowers, there is no need for emasculation. The female flower buds are bagged before the flowers open.
When the stigma becomes receptive, pollination is carried out using the
desired pollen and the flower rebagged.
11. What is triple fusion? Where and how does it take place? Name the
nuclei involved in triple fusion.
Answer : The fusion of the male gamete with two polar nuclei inside the embryo sac of the angiosperm is called triple fusion. This process of fusion takes place inside the embryo sac of angiosperms. 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 and pollen tube release 2 male gamets into embryo sac where one male gamete fuse with egg results in zygote formation. While anoder male gamete fuse with the two polar nuclei and forms PEN (primary endosperm nucleus. This process involves the fusion of three haploid nuclei, hence it is known as triple fusion. It results in the formation of the endosperm.
12. Why do you think the zygote is dormant for sometime in a fertilised
Answer : The zygote is shaped by means of the fusion of the male gamete with the nucleus of the egg cell. The zygote stays dormant for a while and waits for the endosperm to form, which develops from the process of double fertilization or triple fusion. The result of double fedtilization is endosperm and this endosperm provides food for the developing embryo.
13. Differentiate between:
a) hypocotyl and epicotyl
- The portion of the embryonal axis which lies below the cotyledon in a dicot embryo is known as the hypocotyl. While The portion of the embryonal axis which lies above the cotyledon in a dicot embryo is known as the epicotyl.
- Hypocotyl terminates with the radicle. Epicotyl terminates with the plumule.
- Hypocotyl shows epigeal germination, hypocotyl elongates so that cotyledons come out of soil. While epicotyl shows hypogeal germination, epicotyl elongates so that cotyledons remain in the soil.
- Hypocotyl forms an important part of the embryonic root system while epicotyl forms an important part of the embryonic shoot system.
(b) coleoptile and coleorrhiza
- coleoptile is the epicotyl bearing shoot apex and leaf primordia is enclosed in a foliar structure called coleoptile. While coleorrhiza is the radicle and root cap are enclosed in a sheath called coleorhiza.
- coleoptile grows much beyond the grain. while coleorhiza grows After emergence from grain it stops growing.
- coleoptile protects the plumule during emergence from soil. coleorhiza does no protect the radicle during its passage into the soil.
- Coleoptile after emergence from soil during germination becomes green and does photosynthesis. While Coleorhiza does not come out of the soil. It remains non-green.
- Coleoptile breaks the seed coat and elongates while coleorhiza breaks the seed coat but stop further growth.
(c) integument and testa
- Integument is the covering of the ovule and test is the outer covering of seed.
- Integument is thin, one or two layered and testa is quite thick and one layered.
- The cells of integuments are living while the cells of testa are dead.
- In Integument Sclereids are absent while In testa Cells are rich in sclereids.
- Integument arises from chalazal end of ovule. While testa is derived from outer integument of ovule after fertilization.
- Integument is pre -fertilized structure and testa is a post fertilized structure.
(d) perisperm and pericarp.
- Perisperm is unused nucellus in the seed. Pericarp is the covering of fruit that develops from ovary wall.
- Perisperm is present in only a few seeds. Pericarp is found in all fruits.
- Perisperm is often nonfunctional for seed. Pericarp is protective covering and also helps in dispersal and nutrition.
- Perisperm is a part of the seed. Pericarp is a part of the fruit.
- Perisperm is usually dry. Pericarp is dry or fleshy.
14. Why is apple called a false fruit? Which part(s) of the flower forms the fruit?
Answer : The fruits which are derived from the ovary and other accessory floral parts are called false fruits. False fruit is also called pseudo fruit or pseudo carp. Examples of such fruits are strawberry, pineapple, mulberry, apples, pears etc.
Generally fruits develop from the ovaries of flowers. These fruits are known as true fruits. However, sometimes, the other parts of a flower like thalamus and calyx may also involved in the formation of fruits.
In apple the major part which is the fleshy part is developed from the thalamus and not from the ovary thats why apple is called as false fruit.
15. What is meant by emasculation? When and why does a plant breeder
employ this technique?
Answer : The process of removal of anthers from the flower is called as emasculation. In bisexual flowers, emasculation is essential to prevent of self-pollination. In monoecious plants, male flowers are removed. It is done before dehiscence to prevent contamination of stigma with any undesired pollen and to ensure cross-pollination by desired pollens.
The most famous application of emasculation is seen in Mendel’s experiment of garden pea plant. This technique is carried out to avoid self pollination thus favouring cross pollination.
Emasculation is performed by plant breeders in bisexual flowers to obtain the desired variety of a plant by crossing a particular plant with the desired pollen grain. To remove the anthers, the flowers are covered with a bag before they open. This ensures that the flower is pollinated by pollen grains obtained from desirable varieties only.
16. If one can induce parthenocarpy through the application of growth
substances, which fruits would you select to induce parthenocarpy
Answer : Parthenocarpy is the process of developing fruits without involving the process of fertilization or seed formation. Therefore, the seedless varieties of economically important fruits include watermelons, tomatoes, grapes (such as Termarina rossa), and bananas. Additionally, there are numerous seedless citrus fruits, such as oranges, lemons and lime are produced using this technique. Such fruits are called as Parthenocarpic fruits or seedless fruits.
This technique involves inducing fruit formation by the application of plant growth hormones such as auxins.
17. Explain the role of tapetum in the formation of pollen-grain wall.
- The tapetum is innermost a specialised layer of nutritive cells / microsporangiam found within the anther, of flowering plants, where it is located between the sporangenous tissue and the anther wall.
- Tapetum is important for the nutrition and development of pollen grains, as well as a source of precursors for the pollen coat.
- During microsporogenesis, the cells of tapetum produce various enzymes, hormones, amino acids, and other nutritious material required for the development of pollen grains.
- As we know The tapetum is the inner most layer of the microsporangial wall that secretes precursors of sporopollenin which in turn forms sporopollenin. And Pollen grain outer layer exine is then laid down by deposition of sporopollenin.
18. What is apomixis and what is its importance?
Answer : Apomixis is an asexual reproduction that occurs without fertilization and not involve in meiosis. One of the best example of apomixis is the apomictic parthenogenesis. It its one in which the egg cell is produced through mitosis. It then develops directly into an embryo without the prior fertilization. It plays an important role in hybrid seed production. The method of producing hybrid seeds by cultivation is very expensive for farmers.
Apomixis produces seed progeny that are exact replicas of the mother plant. The major advantage of apomixis over sexual reproduction is the possibility to select individuals with desirable gene combinations and to propagate them as clones.
Also Read : NCERT solutions for class 12 biology chapter 1 Reproduction in Organisms
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