The Chapter 2 of the NCERT Class 12 Biology textbook is named as Sexual Reproduction in Flowering Plants. As the name suggests, students in this chapter are taught about the various aspects and factors of the Sexual Reproduction in Flowering Plants. Some of the most important topics covered in this chapter are Flower Structure, Pollination, Fertilization, Seed Development, Fruit Formation, Seed Dispersal, etc.
The expert faculty members at Adda247 have crafted the NCERT Solutions for Class 12 Biology Chapter 2 Sexual Reproduction in Flowering Plants as listed below in the article so that all the students can complete their exam preparation easily.
Overview of CBSE Class 12 Biology Chapter 2
The CBSE Class 12 Biology Syllabus defines that the students in their Chapter 2 need to study and learn about the various fundamentals and basics of Sexual Reproduction in Flowering Plants:
- Flower Structure: Anatomy of flowers, including reproductive parts (stamens and pistil)
- Microsporogenesis and Megasporogenesis: Formation of male and female gametophytes
- Pollination: Types (self and cross), agents (wind, water, insects), and adaptations
- Pollen-Pistil Interaction: Compatibility and rejection mechanisms
- Double Fertilization: Fusion of male gametes with the egg and polar nuclei
- Post-Fertilization Events: Formation of embryo, endosperm, seed, and fruit
- Special Mechanisms: Apomixis, polyembryony, and artificial hybridization
- Significance: Role of sexual reproduction in genetic diversity and plant breeding
NCERT Solutions for Class 12 Biology Chapter 2 Sexual Reproduction in Flowering Plants
During the preparation phase for the final examinations, students will get stuck on numerous questions. Therefore, to make their preparation phase easier, we have provided the NCERT Solutions for Class 12 Biology Chapter 2 Sexual Reproduction in Flowering Plants in the article below:
Question 1: Name the parts of an angiosperm flower in which development of male and female gametophyte takes place.
Answer:
- Male Gametophyte: Develops in the anther (part of the stamen). The pollen grains, produced in the pollen sacs of the anther through microsporogenesis, represent the male gametophyte.
- Female Gametophyte: Develops in the ovule (inside the ovary of the pistil). The embryo sac, formed through megasporogenesis, represents the female gametophyte.
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.
Answer:
| Feature | Microsporogenesis |
Megasporogenesis
|
| Definition | Process of forming microspores (pollen grains) |
Process of forming megaspores
|
| Location | Occurs in anther (pollen sacs) |
Occurs in ovule (nucellus)
|
| Outcome | Produces haploid microspores (male gametophyte) |
Produces haploid megaspores (leads to female gametophyte)
|
| Structures Formed | Pollen grains (male gametophyte) |
Embryo sac (female gametophyte)
|
- Cell Division: Both processes involve meiosis (reduction division) to produce haploid cells from diploid mother cells.
- Structures Formed:
- Microsporogenesis: Pollen grains.
- Megasporogenesis: Embryo sac (from one functional megaspore).
Question 3: Arrange the following terms in the correct developmental sequence: Pollen grain, sporogenous tissue, microspore tetrad, pollen mother cell, male gametes.
Answer: The correct developmental sequence is:
Sporogenous tissue → Pollen mother cell → Microspore tetrad → Pollen grain → Male gametes
Explanation:
- Sporogenous tissue: Found in the anther, it differentiates into pollen mother cells.
- Pollen mother cell: Undergoes meiosis to form a microspore tetrad.
- Microspore tetrad: Four haploid microspores are formed.
- Pollen grain: Each microspore develops into a mature pollen grain (male gametophyte).
Male gametes: The pollen grain’s generative cell divides mitotically to form two male gametes.
Question 4: With a neat, labelled diagram, describe the parts of a typical angiosperm ovule.
Answer:
The ovule is the structure within the ovary of a flower that develops into a seed after fertilization.
Description of Parts:
- Funiculus: Stalk attaching the ovule to the placenta.
- Hilum: Point of attachment of the funiculus to the ovule.
- Integuments: Protective layers (outer and inner) surrounding the ovule, forming the seed coat post-fertilization.
- Micropyle: A small opening in the integuments for pollen tube entry.
- Nucellus: Nutritive tissue within the ovule, surrounding the embryo sac.
- Embryo Sac: The female gametophyte, containing the egg cell, synergids, antipodal cells, and polar nuclei.
Question 5: What is meant by monosporic development of female gametophyte?
Answer: Monosporic development refers to the formation of the female gametophyte (embryo sac) from a single functional megaspore. In most angiosperms (e.g., Polygonum type):
- A megaspore mother cell in the ovule undergoes meiosis to produce four haploid megaspores.
- Three megaspores degenerate, and only one remains functional.
- This single megaspore undergoes three mitotic divisions to form the mature embryo sac with eight nuclei (egg cell, two synergids, three antipodals, and two polar nuclei).
This is called monosporic because only one megaspore contributes to the embryo sac.
Question 6: With a neat diagram, explain the 7-celled, 8-nucleate nature of the female gametophyte.
Answer: The female gametophyte (embryo sac) in angiosperms (Polygonum type) is a 7-celled, 8-nucleate structure formed within the ovule.
Structure:
- Cells and Nuclei:
- Egg Apparatus (3 cells): One egg cell and two synergids, located at the micropylar end. Each has one nucleus (3 nuclei total).
- Central Cell: Contains two polar nuclei, which fuse to form a diploid secondary nucleus before fertilization (1 cell, 2 nuclei).
- Antipodal Cells (3 cells): Located at the chalazal end, each with one nucleus (3 nuclei total).
- Total: 7 cells (1 egg, 2 synergids, 1 central cell, 3 antipodals) and 8 nuclei (3 + 2 + 3).
Formation:
- A single megaspore undergoes three mitotic divisions, forming an 8-nucleate structure, which organizes into 7 cells.
Question 7: What are chasmogamous flowers—“can they cross-pollinate?
Answer:
- Chasmogamous Flowers: These are flowers with exposed anthers and stigma, typically open and showy, allowing pollination by external agents like insects, wind, or water.
- Cross-Pollination: Yes, chasmogamous flowers can cross-pollinate because their reproductive organs are accessible to pollinators, facilitating pollen transfer between flowers of different plants. They may also self-pollinate if pollen from the same flower reaches the stigma.
Example: Flowers of mustard or hibiscus.
Question 8: Mention two strategies adopted by flowers to prevent self-pollination.
Answer: To prevent self-pollination and promote cross-pollination, flowers adopt:
- Dichogamy: The anther and stigma mature at different times:
- Protandry: Anthers mature before the stigma (e.g., sunflower).
- Protogyny: Stigma matures before anthers (e.g., custard apple).
Self-Incompatibility: A genetic mechanism where pollen from the same flower or plant is rejected by the stigma, preventing fertilization (e.g., tobacco, petunia).
Question 9: What is self-incompatibility? Why does self-pollination not lead to seed formation in self-incompatible plants?
Answer:
- Self-Incompatibility: A genetic mechanism in some plants that prevents self-pollination by inhibiting pollen germination or pollen tube growth on the stigma of the same flower or plant.
- Reason for No Seed Formation: In self-incompatible plants, the pollen and stigma recognize each other as genetically similar, triggering a biochemical response that blocks pollen tube growth or fertilization. This ensures cross-pollination, promoting genetic diversity. Without successful fertilization, no zygote or seed is formed.
Example: Plants like Brassica (mustard) exhibit self-incompatibility.
Question 10: What is bagging technique? How is it useful in a plant breeding programme?
Answer:
- Bagging Technique: Involves covering the emasculated flowers (with anthers removed) with a bag (usually made of butter paper) to prevent unwanted pollination by external agents.
- Use in Plant Breeding:
- Ensures controlled pollination by allowing only desired pollen to reach the stigma.
- Prevents contamination by unwanted pollen, maintaining the purity of the cross.
- Used in artificial hybridization to produce plants with desired traits (e.g., disease resistance, higher yield).
Example: Bagging is used in breeding programs for crops like rice or wheat.
Question 11: What is triple fusion? Where and how does it take place? Name the nuclei involved in triple fusion.
Answer:
- Triple Fusion: A unique feature of angiosperm fertilization where one male gamete fuses with two polar nuclei (or their fused secondary nucleus) in the embryo sac to form a triploid (3n) primary endosperm nucleus.
- Where: Occurs in the central cell of the embryo sac within the ovule.
- How: During double fertilization:
- One male gamete fuses with the egg cell (syngamy) to form a diploid zygote.
- The second male gamete fuses with the two polar nuclei in the central cell to form the triploid primary endosperm nucleus.
Nuclei Involved: Two polar nuclei (or their fused secondary nucleus) and one male gamete.
Question 12: Why is the process of fertilization in angiosperms termed as double fertilization?
Answer: Fertilization in angiosperms is called double fertilization because two fertilization events occur simultaneously within the embryo sac:
- Syngamy: One male gamete fuses with the egg cell to form a diploid (2n) zygote, which develops into the embryo.
- Triple Fusion: The second male gamete fuses with two polar nuclei (or the secondary nucleus) to form a triploid (3n) primary endosperm nucleus, which develops into the endosperm (nutritive tissue for the embryo).
This dual process is unique to angiosperms and ensures the formation of both the embryo and endosperm.
Question 13: Explain the role of tapetum in the formation of pollen grain wall.
Answer:
- Tapetum: The innermost layer of the anther wall, surrounding the sporogenous tissue in the pollen sac.
- Role in Pollen Grain Wall Formation:
- Nutrient Supply: Tapetum provides nutrients (e.g., sugars, amino acids) to developing microspores.
- Enzyme Secretion: Secretes enzymes and substances needed for pollen wall formation.
- Exine Formation: Contributes sporopollenin, a tough, resistant material that forms the outer pollen wall (exine), protecting pollen grains from environmental stress.
Pollenkitt: In insect-pollinated plants, tapetum secretes pollenkitt, a sticky substance that aids in pollination.
Question 14: What is apomixis and what is its importance?
Answer:
- Apomixis: A form of asexual reproduction in plants where seeds are formed without fertilization. The embryo develops from an unfertilized egg or other diploid cells (e.g., nucellus or integuments), producing genetically identical offspring.
- Importance:
- Hybrid Seed Production: Apomixis allows the production of hybrid seeds with uniform traits, maintaining hybrid vigor without repeated crossing.
- Crop Improvement: Useful in agriculture to propagate desirable traits (e.g., high yield, disease resistance) consistently.
- Time and Resource Efficiency: Eliminates the need for pollination and fertilization, saving time and energy.
Example: Found in some grasses and citrus species.
Question 15: What is meant by emasculation? When and why does a plant breeder employ this technique?
Answer:
- Emasculation: The removal of anthers from a bisexual flower before they mature to prevent self-pollination.
- When Employed: Used during artificial hybridization in plant breeding programs, typically before anther dehiscence (when pollen is released).
- Why Employed:
- Prevents self-pollination, ensuring that only desired pollen from another plant is used for cross-pollination.
- Facilitates controlled breeding to produce hybrids with specific traits (e.g., higher yield, disease resistance).
- Maintains genetic purity of the cross.
Example: Emasculation is performed in crops like cotton or tomato during hybridization.
Question 16: If one pollen grain produces two male gametes, how many pollen grains are needed to fertilize 10 ovules?
Answer:
- Each pollen grain produces two male gametes: one for syngamy (to fertilize the egg) and one for triple fusion (to form the endosperm).
- For one ovule, one pollen grain is sufficient, as it provides both male gametes needed for double fertilization.
Therefore, to fertilize 10 ovules, 10 pollen grains are required.
Question 17: What is polyembryony and how can it be commercially exploited?
Answer:
- Polyembryony: The phenomenon where a single seed contains more than one embryo. It can occur naturally (e.g., in citrus) or be induced.
- Types: True polyembryony (multiple embryos from one embryo sac) or false polyembryony (multiple embryo sacs in one ovule).
- Commercial Exploitation:
- Uniform Seedlings: Produces genetically identical plants, useful for propagating elite varieties (e.g., citrus, mango).
- Nucellar Embryos: In citrus, nucellar embryos (formed from nucellus) are used to produce disease-free, uniform plants.
- Horticulture: Ensures consistent quality in fruit crops and ornamental plants.
- Clonal Propagation: Facilitates the propagation of desirable traits without hybridization.
Example: Citrus seeds often exhibit polyembryony, used in orchards for uniform fruit production.
Question 18: Why does the zygote begin to divide only after the division of the primary endosperm cell (PEC)?
Answer: The zygote begins to divide only after the division of the primary endosperm cell (PEC) because:
- Nutrient Dependency: The developing embryo (formed from the zygote) relies on the endosperm for nourishment.
- Endosperm Formation: The PEC, formed by triple fusion, divides to form the endosperm, which provides nutrients (e.g., starch, proteins) to the embryo.
- Synchronized Development: The endosperm must develop first to ensure a sufficient nutrient supply before the zygote starts dividing to form the embryo.
This ensures the embryo’s survival and proper development within the seed.
