NCERT Solutions

The development of gametophytes in an angiosperm flower occurs in:

• Male Gametophyte (Pollen Grain): Develops inside the Anther (specifically within the microsporangium or pollen sac).
• Female Gametophyte (Embryo Sac): Develops inside the Ovary (specifically within the nucellus of the ovule).

• Type of Cell Division: Meiosis (Reductional Division).
• Structure formed (Microsporogenesis): Microspores (Pollen grains).
• Structure formed (Megasporogenesis): Megaspores (Functional megaspore develops into Embryo Sac).

The correct developmental sequence is:

Reasoning: Sporogenous tissue differentiates into PMC, which undergoes meiosis to form tetrads. Tetrads dissociate into pollen grains, which undergo mitosis to form male gametes.

• Funicle: The stalk that attaches the ovule to the placenta.
• Hilum: The junction where the body of the ovule fuses with the funicle.
• Integuments: Protective envelopes encircling the ovule (Outer and Inner).
• Micropyle: A small opening at the tip where integuments are absent (entry for pollen tube).
• Chalaza: Basal part of the ovule, opposite the micropyle.
• Nucellus: Mass of cells enclosed within integuments; has abundant food reserves.
• Embryo Sac: Female gametophyte located in the nucellus.

In the majority of flowering plants, one of the megaspores (usually the chalazal one) is functional while the other three degenerate. Only the single functional megaspore develops into the female gametophyte (embryo sac). This method of embryo sac formation from a single megaspore is termed monosporic development.

The functional megaspore undergoes three sequential mitotic divisions to form 8 nuclei.

• Micropylar End: 3 nuclei form the Egg Apparatus (1 Egg cell + 2 Synergids).
• Chalazal End: 3 nuclei form the Antipodal cells.
• Central Region: The remaining 2 nuclei (Polar Nuclei) move to the center and are situated within the large Central Cell.

These are flowers with exposed anthers and stigma (similar to flowers of other species). They open at maturity, allowing exposure to pollinating agents.

No, cross-pollination cannot occur in cleistogamous flowers.

Reason: Cleistogamous flowers never open at all. The anthers and stigma lie close to each other within the closed bud. When anthers dehisce, pollen grains come in contact with the stigma for pollination. Thus, they are invariably autogamous (self-pollinated).

1. Dichogamy (Synchronization inhibition): Pollen release and stigma receptivity are not synchronized. Either the pollen is released before the stigma becomes receptive (Protandry) or the stigma becomes receptive before pollen release (Protogyny).
2. Self-incompatibility: A genetic mechanism that prevents self-pollen (from the same flower or other flowers of the same plant) from fertilizing the ovules by inhibiting pollen germination or pollen tube growth in the pistil.

Self-incompatibility is a genetic mechanism in flowering plants that prevents self-fertilization.

Reason: It recognizes “self” pollen (from the same plant) and inhibits its growth. This inhibition occurs either by preventing pollen germination on the stigma or by retarding the pollen tube growth in the style. Consequently, the male gametes cannot reach the egg, and fertilization (and thus seed formation) does not occur.

• Bagging Technique: It involves covering the emasculated flowers (anthers removed) with a bag made of butter paper to prevent contamination of the stigma with unwanted pollen.
• Utility:
  1. It ensures that only the desired pollen grains are used for pollination.
  2. It protects the stigma from contamination by unwanted pollen, ensuring the purity of the hybrid seed produced.

Triple Fusion is the fusion of the second male gamete with the two polar nuclei located in the central cell to form the Primary Endosperm Nucleus (PEN).

It takes place in the central cell of the embryo sac. The pollen tube releases two male gametes into the synergid. One moves towards the egg, and the other moves towards the central cell, fusing with the two polar nuclei.

1. Male Gamete (Haploid, n).
2. Two Polar Nuclei (Haploid, n + n) or Secondary Nucleus (2n).

Result: Triploid (3n) Primary Endosperm Nucleus (PEN).

The zygote develops into an embryo, a process that requires a lot of nutrition. The endosperm is the nutritive tissue formed by triple fusion. The zygote remains dormant for some time to ensure that sufficient amount of endosperm is formed to provide nutrition to the developing embryo.

• False Fruit: A fruit is called false if floral parts other than the ovary contribute to fruit formation. In apple, the Thalamus contributes significantly to fruit formation along with the ovary.
• Parts involved: Thalamus (fleshy edible part) and Ovary (core).

• Emasculation: The removal of anthers from a bisexual flower bud before the anther dehisces using forceps.
• When: It is done during the bud stage of a bisexual flower intended to be the female parent.
• Why: To prevent self-pollination (contamination of stigma with its own pollen) in artificial hybridization experiments, ensuring that only desired pollen can be used for cross-pollination.

• Parthenocarpy: Formation of seedless fruits without fertilization.
• Selection: Fruits like Watermelon, Grapes, Oranges, Lemon, Banana.
• Reason: These fruits are valued for their fleshy pulp. Seeds often act as an irritant while eating. Inducing parthenocarpy makes them seedless, increasing their economic value and consumer preference.

The Tapetum is the innermost layer of the microsporangium wall.

• Nourishment: It provides nutrition to the developing pollen grains.
• Formation of Exine: Tapetal cells secrete Ubisch bodies which get coated with Sporopollenin. This sporopollenin is deposited on the pollen grain to form the hard outer layer called Exine.
• Pollenkitt: It secretes Pollenkitt (in insect-pollinated plants) to make pollen sticky.

Apomixis is a form of asexual reproduction that mimics sexual reproduction. It involves the production of seeds without fertilization (e.g., in Asteraceae, grasses).

1. Preservation of Hybrid Vigor: In hybrid seeds, characters segregate in the progeny. If hybrids are made into apomicts, there is no segregation of characters. The progeny are genetically identical to the parent.
2. Cost Reduction: Farmers do not need to buy expensive hybrid seeds every year. They can save seeds from apomictic crops and sow them year after year without loss of yield/traits.