Sexual Reproduction in Flowering PlantsClass 12 Biology NCERT Solutions
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Q1EXERCISES
Name the parts of an angiosperm flower in which development of male and female gametophyte take place.
Solution
In an angiosperm flower, the development of gametophytes takes place in the following parts:
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Male Gametophyte: The development of the male gametophyte (pollen grain) occurs in the anther, which is a part of the stamen (the male reproductive organ). Specifically, it develops inside the microsporangium of the anther.
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Female Gametophyte: The development of the female gametophyte (embryo sac) occurs in the ovule (megasporangium), which is located inside the ovary of the pistil (the female reproductive organ).
Q2EXERCISES
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.
Solution
The differentiation between microsporogenesis and megasporogenesis is as follows:
| Feature | Microsporogenesis | Megasporogenesis |
|---|---|---|
| Definition | It is the process of formation of microspores from a pollen mother cell (PMC). | It is the process of formation of megaspores from a megaspore mother cell (MMC). |
| Location | It occurs inside the microsporangium (pollen sac) of the anther. | It occurs inside the megasporangium (ovule) of the ovary. |
| Starting Cell | Pollen Mother Cell (PMC) or microspore mother cell. | Megaspore Mother Cell (MMC). |
| End Product | It results in the formation of four functional microspores, arranged in a cluster called a microspore tetrad. | It results in the formation of four megaspores, typically in a linear tetrad, out of which only one remains functional and the other three degenerate. |
Type of Cell Division:
Both microsporogenesis and megasporogenesis involve meiotic division (meiosis), which reduces the chromosome number from diploid (2n) in the mother cells to haploid (n) in the spores.
Structures formed at the end:
- At the end of microsporogenesis, a microspore tetrad is formed.
- At the end of megasporogenesis, four megaspores are formed (of which one is functional).
Q3EXERCISES
Arrange the following terms in the correct developmental sequence: Pollen grain, sporogenous tissue, microspore tetrad, pollen mother cell, male gametes.
Solution
The correct developmental sequence is as follows:
- Sporogenous tissue: A mass of homogenous cells in a young anther.
- Pollen mother cell (PMC): Each cell of the sporogenous tissue differentiates into a PMC.
- Microspore tetrad: The PMC undergoes meiosis to form a cluster of four haploid microspores.
- Pollen grain: Each microspore matures and develops into a pollen grain (male gametophyte).
- Male gametes: The generative cell within the pollen grain divides by mitosis to form two male gametes.
Q4EXERCISES
With a neat, labelled diagram, describe the parts of a typical angiosperm ovule.
Solution
A typical anatropous ovule of an angiosperm consists of the following parts (as shown in Figure 1.7d of the textbook):
- Funicle: The stalk that attaches the ovule to the placenta.
- Hilum: The region where the body of the ovule fuses with the funicle. It represents the junction between the funicle and the ovule.
- Integuments: One or two protective outer layers that encircle the ovule. After fertilization, they harden to form the seed coat.
- Micropyle: A small opening at the tip of the integuments through which the pollen tube usually enters the ovule.
- Chalaza: The basal part of the ovule, located opposite the micropylar end.
- Nucellus: A mass of parenchymatous cells enclosed within the integuments. It provides nourishment to the developing embryo sac. Its cells have abundant reserve food materials.
- Embryo Sac (Female Gametophyte): A large, oval cell located within the nucellus that contains the female gamete (egg cell).
(A labelled diagram of a typical anatropous ovule should be drawn to accompany this description).
Q5EXERCISES
What is meant by monosporic development of female gametophyte?
Solution
Monosporic development of the female gametophyte refers to the method of embryo sac formation where the embryo sac develops from a single functional megaspore.
In most flowering plants, the megaspore mother cell (MMC) undergoes meiosis to produce four haploid megaspores. Out of these four megaspores, three degenerate and only one, usually the one at the chalazal end, remains functional. This single functional megaspore then undergoes mitotic divisions to form the 7-celled, 8-nucleate embryo sac (female gametophyte). Since the entire female gametophyte is derived from a single megaspore, this type of development is termed monosporic.
Q6EXERCISES
With a neat diagram explain the 7-celled, 8-nucleate nature of the female gametophyte.
Solution
The mature female gametophyte or embryo sac in most angiosperms is a 7-celled, 8-nucleate structure (as shown in Figure 1.8c of the textbook). The formation and organization are as follows:
The functional megaspore nucleus divides mitotically three times, resulting in eight nuclei within a single cytoplasm (free-nuclear division). After the 8-nucleate stage, cell walls are laid down, organizing the nuclei into cells.
The distribution of cells and nuclei is as follows:
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Egg Apparatus (at the micropylar end): This consists of three cells:
- One egg cell (female gamete).
- Two synergids, which have cellular thickenings called the filiform apparatus to guide the pollen tube. This group has a total of 3 cells and 3 nuclei.
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Antipodal Cells (at the chalazal end): There are three antipodal cells. Their function is largely nutritive. This group has a total of 3 cells and 3 nuclei.
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Central Cell: This is the large, single cell located in the center of the embryo sac. It contains two nuclei, called polar nuclei, which are not enclosed by a cell wall relative to each other before fertilization. This single cell contains 2 nuclei.
Summary:
- Total Cells: 3 (egg apparatus) + 3 (antipodals) + 1 (central cell) = 7 cells.
- Total Nuclei: 3 (in egg apparatus) + 3 (in antipodals) + 2 (polar nuclei in central cell) = 8 nuclei.
(A labelled diagram of a mature embryo sac should be drawn to accompany this description).
Q7EXERCISES
What are chasmogamous flowers? Can cross-pollination occur in cleistogamous flowers? Give reasons for your answer.
Solution
Chasmogamous Flowers:
Chasmogamous flowers are flowers that open at maturity, exposing their stamens (anthers) and pistil (stigma) to pollinating agents. They are similar to the flowers of most species and can undergo both self-pollination (autogamy) and cross-pollination (geitonogamy and xenogamy).
Cross-pollination in Cleistogamous Flowers:
No, cross-pollination cannot occur in cleistogamous flowers.
Reasons:
- Flowers do not open: Cleistogamous flowers are flowers that never open. Their reproductive organs (anthers and stigma) remain enclosed within the petals throughout their life.
- No exposure to external pollen: Since the flowers are sealed, there is no chance for pollen from another flower (cross-pollen) to land on the stigma.
- Assured self-pollination: The anthers and stigma lie very close to each other. When the anthers dehisce inside the closed flower, the pollen grains come in direct contact with the stigma, ensuring self-pollination (autogamy). Therefore, cleistogamous flowers are invariably autogamous.
Q8EXERCISES
Mention two strategies evolved to prevent self-pollination in flowers.
Solution
Flowering plants have evolved many strategies (outbreeding devices) to prevent self-pollination and encourage cross-pollination. Two such strategies are:
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Dichogamy (Non-synchronization): In some species, the pollen release and stigma receptivity are not synchronized. This means the anther matures and releases pollen before the stigma becomes receptive (protandry), or the stigma becomes receptive long before the pollen is released (protogyny). This temporal separation prevents self-pollination.
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Self-incompatibility: This is a genetic mechanism that prevents self-pollen (from the same flower or another flower on the same plant) from fertilizing the ovules. It works by inhibiting pollen germination on the stigma or by preventing the growth of the pollen tube through the style, thus ensuring that fertilization does not occur.
Q9EXERCISES
What is self-incompatibility? Why does self-pollination not lead to seed formation in self-incompatible species?
Solution
Self-incompatibility is a genetic mechanism that prevents self-pollination from being effective. It is a form of pre-fertilization barrier that inhibits the fusion of gametes from the same individual plant, thus promoting outbreeding.
Self-pollination does not lead to seed formation in self-incompatible species because the pistil can recognize and reject pollen from the same plant. This rejection is a result of a chemical dialogue between the pollen and the pistil. The incompatibility mechanism works in one of two ways:
- Inhibition of Pollen Germination: The pistil prevents the germination of self-pollen on the surface of the stigma.
- Inhibition of Pollen Tube Growth: Even if the pollen germinates, the growth of the pollen tube is arrested as it travels through the style, preventing it from reaching the ovule and delivering the male gametes for fertilization.
In both cases, fertilization is prevented, and consequently, no seed formation occurs.
Q10EXERCISES
What is bagging technique? How is it useful in a plant breeding programme?
Solution
Bagging Technique:
Bagging is a technique used in artificial hybridization where the emasculated flowers are covered with a bag of a suitable size, usually made of butter paper. This bag prevents the contamination of the stigma with unwanted pollen from other sources.
Usefulness in Plant Breeding Programme:
The bagging technique is crucial in plant breeding programmes for the following reasons:
- Prevents Contamination: It protects the stigma of the selected female parent from being pollinated by any random, unwanted pollen carried by wind, insects, or other agents.
- Ensures Desired Cross: After bagging, when the stigma becomes receptive, pollen from a selected male parent with desired traits is dusted onto it. The flower is then rebagged.
- Controlled Hybridization: This process ensures that fertilization occurs only between the selected male and female parents, allowing the plant breeder to create a hybrid variety with a combination of desired characters from both parents. It is a fundamental step in producing commercially superior varieties.
Q11EXERCISES
What is triple fusion? Where and how does it take place? Name the nuclei involved in triple fusion.
Solution
Triple Fusion:
Triple fusion is one of the two fertilization events that occur in the embryo sac of an angiosperm. It is the fusion of one male gamete with the two polar nuclei present in the central cell of the embryo sac.
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Where it takes place: Triple fusion occurs inside the central cell of the embryo sac, within the ovule.
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How it takes place: After entering the embryo sac, the pollen tube releases two male gametes. One male gamete fuses with the egg cell (syngamy). The second male gamete moves towards the center of the embryo sac and fuses with the two polar nuclei.
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Nuclei involved: The three nuclei involved in triple fusion are:
- One male gamete nucleus (haploid, n).
- Two polar nuclei (each haploid, n).
The product of this fusion is the triploid () Primary Endosperm Nucleus (PEN), which develops into the endosperm.
Q12EXERCISES
Why do you think the zygote is dormant for sometime in a fertilised ovule?
Solution
The zygote remains dormant for some time after fertilization because it waits for the endosperm to develop. This is an evolutionary adaptation to ensure that the developing embryo receives adequate nutrition.
The primary endosperm cell (PEC), formed after triple fusion, divides repeatedly to form the endosperm tissue, which is rich in reserve food materials. The development of the embryo (embryogeny) from the zygote begins only after a certain amount of endosperm has been formed. This delay ensures that there is an assured supply of nourishment available for the embryo's growth and development. If the embryo developed before the endosperm, it might starve due to a lack of food.
Q13EXERCISES
Differentiate between:
(a)
hypocotyl and epicotyl;
(b)
coleoptile and coleorrhiza;
(c)
integument and testa;
(d)
perisperm and pericarp.
Solution
(a) Hypocotyl and Epicotyl
| Hypocotyl | Epicotyl |
|---|---|
| It is the cylindrical portion of the embryonal axis that lies below the level of the cotyledons. | It is the portion of the embryonal axis that lies above the level of the cotyledons. |
| It terminates at its lower end in the radicle (root tip). | It terminates in the plumule (stem tip). |
(b) Coleoptile and Coleorrhiza
| Coleoptile | Coleorrhiza |
|---|---|
| It is a hollow, foliar protective sheath that encloses the shoot apex and a few leaf primordia in a monocot embryo. | It is an undifferentiated protective sheath that encloses the radicle and the root cap in a monocot embryo. |
| It is the upper part of the embryonal axis, above the scutellum. | It is the lower part of the embryonal axis. |
(c) Integument and Testa
| Integument | Testa |
|---|---|
| It is the protective envelope of the ovule. It is a living tissue. | It is the outer protective seed coat. It is a hard, non-living layer. |
| It is a pre-fertilization structure. | It is a post-fertilization structure, formed from the hardening of the outer integument(s). |
(d) Perisperm and Pericarp
| Perisperm | Pericarp |
|---|---|
| It is the residual, persistent nucellus found in some seeds like black pepper and beet. | It is the wall of the fruit, which develops from the wall of the ovary. |
| It is a part of the seed. | It is a part of the fruit that encloses the seed(s). |
Q14EXERCISES
Why is apple called a false fruit? Which part(s) of the flower forms the fruit?
Solution
Apple is called a false fruit (or pseudocarp) because the main edible, fleshy part of the fruit does not develop from the ovary. Instead, it develops from another floral part, the thalamus (receptacle), which grows and surrounds the ovary.
In a true fruit, the fruit develops exclusively from the ovary after fertilization. In an apple, the ovary develops into the central core part containing the seeds, while the fleshy part we eat is the modified thalamus. Therefore, since a part other than the ovary contributes significantly to the fruit formation, it is classified as a false fruit.
Q15EXERCISES
What is meant by emasculation? When and why does a plant breeder employ this technique?
Solution
Emasculation is the process of removing the anthers from a bisexual flower bud before they dehisce (release pollen). This is typically done using a pair of forceps.
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When it is employed: A plant breeder employs this technique during artificial hybridization experiments, specifically when the chosen female parent plant bears bisexual flowers.
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Why it is employed: The primary reason for emasculation is to prevent self-pollination. By removing the anthers, the breeder ensures that the stigma of the flower will not be pollinated by its own pollen. This allows the breeder to control the pollination process and dust the stigma with pollen from a desired male parent plant, thereby creating a specific cross-breed with desired characteristics.
Q16EXERCISES
If one can induce parthenocarpy through the application of growth substances, which fruits would you select to induce parthenocarpy and why?
Solution
If parthenocarpy (development of fruit without fertilization) can be induced, I would select fruits where the seeds are numerous or are considered an inconvenience to the consumer. The goal would be to enhance the fruit's economic value and edibility.
Examples of fruits to select:
- Grapes: Seedless grapes are highly popular.
- Watermelon: A seedless watermelon is much easier to eat.
- Pomegranate: A seedless version would make the arils (the edible part) entirely consumable without hard seeds.
- Oranges and Lemons: Seedless citrus fruits are preferred for direct consumption and for making juice.
- Guava: Guavas often contain many small, hard seeds that can be unpleasant to eat.
Why:
The primary reason for selecting these fruits is that parthenocarpy results in the production of seedless fruits. The absence of seeds makes the fruit more desirable, convenient for consumption, and commercially valuable. In fruits like pomegranate and watermelon, the presence of seeds significantly affects the eating experience, so producing seedless varieties would be a major improvement.
Q17EXERCISES
Explain the role of tapetum in the formation of pollen-grain wall.
Solution
The tapetum is the innermost nutritive layer of the microsporangium (anther wall). It plays a crucial role in the development and formation of pollen grains, especially their wall.
The roles of the tapetum in pollen-grain wall formation are:
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Providing Nutrition: The primary function of the tapetum is to provide nourishment to the developing microspore mother cells and the microspores (pollen grains). Its cells are dense with cytoplasm and often have more than one nucleus, indicating high metabolic activity.
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Formation of Exine: The tapetum synthesizes and secretes sporopollenin, which is one of the most resistant organic materials known. Sporopollenin is the main component of the exine, the hard outer layer of the pollen grain wall. This layer protects the pollen grain from physical damage, chemical degradation, and desiccation.
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Secretion of Enzymes and Hormones: The tapetum secretes enzymes like callase, which dissolves the callose wall enclosing the microspore tetrad, thus releasing the individual microspores. It also secretes hormones that are essential for pollen development.
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Formation of Pollenkitt: In insect-pollinated species, the tapetum produces an oily, sticky substance called pollenkitt, which forms a layer over the pollen grains. This substance helps the pollen to adhere to the bodies of insects.
Q18EXERCISES
What is apomixis and what is its importance?
Solution
Apomixis:
Apomixis is a form of asexual reproduction that mimics sexual reproduction by producing seeds without fertilization. In this process, an embryo develops directly from a diploid cell of the ovule (such as a diploid egg cell or a nucellar cell) without undergoing meiosis and syngamy. The resulting seeds are genetically identical to the parent plant.
Importance of Apomixis:
Apomixis is of great importance in agriculture and horticulture, particularly in the hybrid seed industry.
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Preservation of Hybrid Characters: Hybrid varieties often exhibit superior traits (hybrid vigour), but these traits segregate and are lost if seeds from hybrid plants are sown in the next generation. Apomixis prevents this segregation. If a hybrid plant is made apomictic, its desirable characteristics can be fixed, and the progeny will be genetically identical clones of the parent hybrid.
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Cost-Effective for Farmers: Production of hybrid seeds is a costly and laborious process. As a result, farmers have to buy expensive hybrid seeds every year. If hybrids can be made to reproduce through apomixis, farmers can save the seeds from their crop and use them for sowing in subsequent seasons without any loss of hybrid vigour. This makes agriculture more affordable and increases productivity.
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Rapid Multiplication: It allows for the rapid multiplication of genetically uniform individuals that are well-adapted to their environment.