Sexual reproduction in flowering plants ncert solutions: Class 12th biology chapter 1 ncert solutions
| Textbook | NCERT |
| Class | Class 12 |
| Subject | Biology |
| Chapter | Chapter 1 |
| Chapter Name | Sexual reproduction in flowering plants class 12 ncert solutions |
| Category | Ncert Solutions |
| Medium | English |
Are you looking for Ncert Solutions for Class 12 Biology Chapter 1 Sexual reproduction in flowering plants? Now you can download Ncert class 12 biology chapter 1 questions and answers pdf from here.
Question 1: Name the parts of an angiosperm flower in which development of male and female gametophyte take place.
Solution 1: The male gametophyte, or pollen grain, develops inside the pollen chambers of the anther. This process takes place in the microsporangia of the anther, where microspores develop into pollen grains, each containing the male gametophyte.
whereas The female gametophyte, also called the embryo sac, develops inside the nucellus of the ovule, which is located within the ovary. The embryo sac forms from a functional megaspore through the process of megagametogenesis.
Question 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.
Solution 2: Differences between Microsporogenesis and Megasporogenesis:
Definition:
- Microsporogenesis: The process by which microspores (male spores) are formed from microspore mother cells (microsporocytes) in the anther.
- Megasporogenesis: The process by which megaspores (female spores) are formed from megaspore mother cells (megasporocytes) in the ovule.
Location:
- Microsporogenesis: Occurs in the anther (in the pollen sacs or microsporangia).
- Megasporogenesis: Occurs in the ovule (within the nucellus in the ovary).
Purpose:
- Microsporogenesis: Leads to the development of pollen grains (male gametophytes).
- Megasporogenesis: Leads to the development of the embryo sac (female gametophyte).
Number of Spores Produced
- Microsporogenesis: Typically results in the formation of four microspores, all of which are functional.
- Megasporogenesis: Typically results in the formation of four megaspores, but usually only one is functional while the other three degenerate.
Cell Division Involved:
- Microsporogenesis: Undergoes meiosis to form microspores.
- Megasporogenesis: Undergoes meiosis to form megaspores.
Type of Cell Division:
- Both microsporogenesis and megasporogenesis involve meiosis (reductional division), which reduces the chromosome number from diploid (2n) to haploid (n).
Structures Formed at the End:
- Megasporogenesis: Results in the formation of megaspores, one of which develops into the embryo sac (the female gametophyte).
- Microsporogenesis: Results in the formation of microspores, which develop into pollen grains (the male gametophyte).
Question 3: Arrange the following terms in the correct developmental sequence: Pollen grain, sporogenous tissue, microspore tetrad, pollen mother cell, male gametes.
Solution 3: The following is the correct developmental sequence: Sporogenous tissue → pollen mother cell → microspore tetrad → pollen grain → male gamete.
Correct Developmental Sequence:
- Sporogenous tissue: The initial mass of diploid cells in the anther that will give rise to pollen.
- Pollen mother cell: Cells from the sporogenous tissue differentiate into pollen mother cells (microsporocytes), which undergo meiosis.
- Microspore tetrad: After meiosis, each pollen mother cell produces a tetrad of four haploid microspores.
- Pollen grain: The individual microspores dissociate from the tetrad and develop into mature pollen grains (male gametophytes).
- Male gametes: The pollen grain then matures and produces male gametes (sperm cells) within the pollen tube after pollination.
Question 4: With a neat, labelled diagram, describe the parts of a typical angiosperm ovule.
Solution 4: An ovule is a female megasporangium where the formation of megaspores takes place.
Parts of a Typical Angiosperm Ovule:
- Funiculus – It is a stalk-like structure which represents the point of attachment of the ovule to the placenta of the ovary.
- Hilum – It is the point where the body of the ovule is attached to the funiculus.
- Integuments –They are the outer layers surrounding the ovule that provide protection to the developing embryo.
- 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.
- 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.
- Chalazal – It is the based swollen part of the nucellus from where the integuments originate.
Question 5: What is meant by monosporic development of female gametophyte?
Solution 5: The female gametophyte or the embryo sac develops from a single functional megaspore. This is known as monosporic development of the female gametophyte. In most flowering plants, a single megaspore mother cell present at the micropylar pole of the nucellus region of the ovule undergoes meiosis to produce four haploid megaspores. Later, out of these four megaspores, only one functional megaspore develops into the female gametophyte, while the remaining three degenerate.
Question 6: With a neat diagram explain the 7-celled, 8-nucleate nature of the female gametophyte.
Solution 6:
The female gametophyte (embryo sac) develops from a single functional megaspore. This megaspore undergoes three successive mitotic divisions to form eight nucleate embryo sacs.
The first mitotic division in the megaspore forms two nuclei. One nucleus moves towards the micropylar end while the other nucleus moves towards the chalazal end. Then, these nuclei divide at their respective ends and re-divide to form eight nucleate stages. As a result, there are four nuclei each at both the ends i.e., at the micropylar and the chalazal end in the embryo sac.
At the micropylar end, out of the four nuclei only three differentiate into two synergids and one egg cell. Together they are known as the egg apparatus. Similarly, at the chalazal end, three out of four nuclei differentiates as antipodal cells. The remaining two cells (of the micropylar and the chalazal end) move towards the centre and are known as the polar nuclei, which are situated in a large central cell. Hence, at maturity, the female gametophyte appears as a 7-celled structure, though it has 8 nucleate.
Question 7: What are chasmogamous flowers? Can cross-pollination occur in cleistogamous flowers? Give reasons for your answer.
Solution 7: Chasmogamous Flowers: Chasmogamous flowers are flowers that open up fully at maturity, exposing their reproductive organs (anthers and stigma) to facilitate pollination. Since their petals open, these flowers typically promote cross-pollination through agents like wind, insects, or animals. In some cases, self-pollination can also occur.
Cleistogamous Flowers: Cleistogamous flowers, in contrast, never open, keeping their reproductive parts enclosed. As a result, self-pollination always occurs within these flowers, ensuring that the pollen from the anther directly fertilizes the ovule in the same flower.
Can Cross-Pollination Occur in Cleistogamous Flowers?
- No, cross-pollination cannot occur in cleistogamous flowers. The reasons are:
Closed Structure: Cleistogamous flowers remain closed, preventing external pollinators from accessing the anthers or stigma, which makes cross-pollination impossible.
Self-Pollination is Guaranteed: Because the reproductive organs are contained within the same flower and no external pollen can enter, the flower ensures self-pollination, which happens even in the absence of external pollinators.
Question 8: Mention two strategies evolved to prevent self-pollination in flowers.
Solution 8: To prevent self-pollination and promote genetic diversity, flowers have evolved several strategies. Here are two key strategies:
1. Dichogamy:
Definition: This strategy involves the maturation of male and female reproductive structures at different times within the same flower or between different flowers on the same plant.
Types:
- Protandry: The anthers mature and release pollen before the stigma is receptive, ensuring that the pollen from other flowers is more likely to fertilize the ovules.
- Protogyny: The stigma becomes receptive before the anthers release their pollen, again promoting cross-pollination from other plants.
2. Self-Incompatibility:
- Definition: This is a genetic mechanism that prevents self-pollination by rejecting pollen from the same plant.
- Mechanism: In self-incompatible plants, the stigma produces proteins that recognize and reject self-pollen, allowing only pollen from genetically different plants to fertilize the ovules. This ensures that cross-pollination occurs, promoting genetic diversity.
These strategies help ensure genetic variation and adaptability in plant populations.
Question 9: What is self-incompatibility? Why does self-pollination not lead to seed formation in self-incompatible species?
Solution 9: 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.
Question 10: What is bagging technique? How is it useful in a plant breeding programme?
Solution 10: Various artificial hybridization techniques (under various crop improvement programmes) involve the removal of the anther from bisexual flowers without affecting the female reproductive part (pistil) through the process of emasculation. Then, these emasculated flowers are wrapped in bags to prevent pollination by unwanted pollen grains. This process is called bagging.
This technique is an important part of the plant breeding programme as it ensures that pollen grains of only desirable plants are used for fertilization of the stigma to develop the desired plant variety.
Question 11: What is triple fusion? Where and how does it take place? Name the nuclei involved in triple fusion.
Solution 11: Triple fusion is a process that occurs during the fertilization stage of angiosperms (flowering plants). It involves the fusion of three nuclei to form a triploid (3n) cell, which ultimately develops into the endosperm of the seed. The endosperm provides nourishment to the developing embryo during seed development.
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. After this, the pollen tube enters one of synergids and releases two male gametes there. Out of the two male gametes, one gamete fuses with the nucleus of the egg cell and forms the zygote (syngamy). The other male gamete fuses with the two polar nuclei present in the central cell to form a triploid primary endosperm nucleus. Since this process involves the fusion of three haploid nuclei, it is known as triple fusion. It results in the formation of the endosperm.
Nuclei Involved in Triple Fusion: The nuclei involved in the process of triple fusion are:
- One male gamete nucleus (sperm cell) from the pollen grain
- Two polar nuclei (from the central cell of the embryo sac)
Question 12: Why do you think the zygote is dormant for sometime in a fertilised ovule?
Solution 12: The zygote is formed by the fusion of the male gamete with the nucleus of the egg cell. The zygote remains dormant for some time and waits for the endosperm to form, which develops from the primary endosperm cell resulting from triple fusion. The endosperm provides food for the growing embryo and after the formation of the endosperm, further development of the embryo from the zygote starts.
Question 13: Differentiate between:
Epicotyl and hypocotyl
Coleoptile and coleorrhiza
Integument and testa
Perisperm and pericarp
Solution 13:
1. Epicotyl vs. Hypocotyl:
| Epicotyl | Hypocotyl |
|---|---|
| The part of the embryonic axis above the cotyledons, which gives rise to the shoot system. | The part of the embryonic axis below the cotyledons, which gives rise to the root system. |
| The shoot (stem, leaves) of the plant. | The root of the plant. |
| Lies above the attachment point of the cotyledons (first leaves). | Lies below the attachment point of the cotyledons. |
| Helps in the development of leaves and shoot. | Helps in the development of the primary root. |
2. Coleoptile vs. Coleorrhiza:
| Coleoptile | Coleorrhiza |
|---|---|
| A sheath-like protective covering that encloses the shoot apex and first leaf in monocot seeds (grasses). | A sheath-like structure that covers and protects the root in monocot seeds (grasses). |
| Protects the shoot apex during germination. | Protects the radicle (root tip) during germination. |
| Ensures the safe emergence of the shoot through the soil. | Ensures the safe emergence of the root through the soil. |
| Present in monocots (e.g., wheat, maize). | Present in monocots (e.g., wheat, maize). |
3. Integument vs. Testa:
| Integument | Testa |
|---|---|
| The protective layer(s) surrounding the ovule before fertilization. | The outer seed coat that develops from the integuments after fertilization. |
| Protects the developing ovule. | Protects the seed from physical damage, pathogens, and water loss. |
| Present before fertilization, as part of the ovule structure. | Develops after fertilization from the integuments of the ovule. |
4. Perisperm vs. Pericarp:
| Perisperm | Pericarp |
|---|---|
| A nutritive tissue in some seeds that originates from the nucellus. | The wall of the fruit that develops from the ovary after fertilization. |
| Derived from the nucellus of the ovule. | Derived from the ovary wall after fertilization. |
| Provides nutrition to the developing embryo in some seeds (e.g., beet). | Protects the seed and aids in fruit dispersal (e.g., fleshy or dry pericarp). |
| Found in seeds of certain plants (e.g., black pepper, beet). | Present in all fruits (e.g., fleshy fruits like apples, dry fruits like nuts). |
These differences cover the important aspects of each pair, focusing on their structure, function, and origin.
Question 14: Why is apple called a false fruit? Which part(s) of the flower forms the fruit?
Solution 14: An apple is called a false fruit (also known as a pseudocarp) because it develops not only from the ovary but also from other parts of the flower, primarily the thalamus (receptacle). In true fruits, only the ovary develops into the fruit after fertilization, but in false fruits, other floral parts contribute to the formation of the fruit.
In the case of an apple, the thalamus (receptacle) enlarges and becomes fleshy, forming the edible part of the fruit. The ovary itself forms only the core of the apple, where the seeds are located. Therefore, the main edible portion of the apple is derived from the thalamus, not just the ovary, making it a false fruit.
Question 15: What is meant by emasculation? When and why does a plant breeder employ this technique?
Solution 15: Emasculation is the process of removing anthers from bisexual flowers without affecting the female reproductive part (pistil), which is used in various plant hybridization techniques.
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. Later, the mature, viable, and stored pollen grains are dusted on the bagged stigma by breeders to allow artificial pollination to take place and obtain the desired plant variety.
Question 16: If one can induce parthenocarpy through the application of growth substances, which fruits would you select to induce parthenocarpy and why?
Solution 16: Parthenocarpy is the process of developing fruits without involving the process of fertilization or seed formation. Therefore, the seedless varieties of economically important fruits such as orange, lemon, water melon etc. are produced using this technique. This technique involves inducing fruit formation by the application of plant growth hormones such as auxins.
Question 17: Explain the role of tapetum in the formation of pollen-grain wall.
Solution 17: The tapetum is the innermost layer of the microsporangium. It provides nutrition to the developing pollen grains. It secretes enzymes, hormones and special proteins for the pollen grains to recognise compatibility. It produces Ubisch granules for the formation of exine of pollen grain. The tapetum secretes pollenkitt over the outer side of mature pollen.
Question 18: What is apomixis and what is its importance?
Solution 18: Apomixis is a form of asexual reproduction that occurs in plants, where seeds are produced without the involvement of fertilization (syngamy). In apomixis, the embryo develops from an unfertilized egg or other parts of the ovule, bypassing the fusion of male and female gametes. This process leads to the formation of seeds that are genetically identical to the parent plant (clones).
There are two main types of apomixis:
- Diplospory: The embryo sac is formed from unreduced megaspore mother cells, meaning meiosis does not occur.
- Apospory: The embryo sac is formed from somatic (non-reproductive) cells in the ovule.
Importance of Apomixis:
- Clonal Propagation: Produces identical offspring, maintaining desirable traits across generations.
- Hybrid Vigor Preservation: Fixes hybrid traits without segregation, keeping high-performing plants stable.
- Cost-Effective: Farmers can reuse seeds without losing quality, reducing the need for buying hybrid seeds.
- Seed Formation in Absence of Pollinators: Ensures reproduction even without pollination or under unfavorable conditions.