Answer:
The embryo sac is an oval, multicellular structure covered by a thin membrane derived from the parent megaspore wall. The typical or Polygonum type of embryo sac contains 8 nuclei but 7 cells — 3 micropylar, 3 chalazal, and 1 central.

1. Micropylar end:
   The three cells at the micropylar end form the egg apparatus. It consists of one egg (oosphere) and two synergids (helper cells).

• The egg cell is larger, with a central or micropylar vacuole and a nucleus toward the chalazal end.
• Each synergid has a filiform apparatus, lateral hook, chalazal vacuole, and central nucleus.
• The synergids assist in the nutrition of the embryo sac, guidance of the pollen tube, and absorption of impact during fertilization.

1. Chalazal end:
   The three cells at the chalazal end are called antipodal cells. These are vegetative cells that may degenerate early or help absorb nourishment from surrounding nucellar cells. They are connected to the central cell through plasmodesmata.
2. Central cell:
   The central cell is the largest cell of the embryo sac. It has a vacuolated cytoplasm rich in reserve food and Golgi bodies. It contains two polar nuclei, which often fuse to form a diploid secondary nucleus.
   Thus, all the cells of the embryo sac are haploid, except the central cell which becomes diploid after the fusion of polar nuclei.

Question 7.
What are chasmogamous flowers? Can cross-pollination occur in cleistogamous flowers? Give reasons for your answer.

Answer:
Chasmogamous flowers are the open flowers in which anthers and stigmas are exposed for pollination. Such flowers are usually brightly coloured, fragrant, and produce nectar to attract pollinators. Since the reproductive parts are exposed, both self-pollination and cross-pollination can occur in chasmogamous flowers.

On the other hand, cleistogamous flowers are closed flowers in which the anthers and stigmas remain enclosed and never open. In such flowers, pollen grains fall directly on the stigma of the same flower, ensuring autogamy (self-pollination).

Hence, cross-pollination cannot occur in cleistogamous flowers, because the flowers never open, and there is no chance for pollen from another flower to reach the stigma.

Examples: Viola, Oxalis, Commelina.

Question 8.
Mention two strategies evolved to prevent self-pollination in flowers.
Answer:
To promote cross-pollination and prevent self-pollination, flowering plants have developed several strategies. Two important ones are:

1. Dichogamy:
   In this condition, the anthers and stigma mature at different times in a flower, preventing self-pollination.
   • If the anthers mature first, it is called protandry (e.g., Sunflower).
   • If the stigma matures first, it is called protogyny (e.g., Datura).
2. Self-incompatibility (Self-sterility):
   It is a genetic mechanism in which pollen from the same flower or plant is unable to fertilize the ovule of that flower. This ensures cross-pollination between genetically different individuals of the same species.

Other strategies (for reference):

• Herkogamy: Physical barrier between anthers and stigma.
• Unisexuality: Male and female flowers occur on different plants (e.g., Papaya).

Question 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 flowering plants that prevents self-pollen (from the same flower or plant) from fertilizing the ovule of that flower. It promotes cross-pollination and maintains genetic diversity in the population.

In self-incompatible species, when self-pollen lands on the stigma, it fails to germinate or the pollen tube growth is inhibited before reaching the ovule. As a result, fertilization does not occur, and therefore, seed formation does not take place.

Question 10.
What is bagging technique? How is it useful in a plant breeding programme?
Answer:

• If the female parent bears bisexual flowers, the removal of anthers from the flower bud before the anther dehisces using a pair of forceps is necessary. This step is called emasculation.
• The emasculated flowers are then covered with a bag of suitable size, usually made of butter paper, to prevent contamination of the stigma with unwanted pollen grains. This process is known as bagging.
• When the stigma of the bagged flower becomes receptive, mature pollen grains from the desired male parent are dusted onto the stigma, and the flowers are re-bagged to allow fertilization and fruit development.
• This technique helps plant breeders ensure that only the desired pollen fertilizes the flower, allowing them to produce hybrid seeds with specific, desired traits.

Question 11.
What is triple fusion? Where and how does it take place? Name the nuclei involved in triple fusion.
Answer:
Triple fusion is the process in which one of the male gametes fuses with the secondary nucleus (formed by the fusion of two polar nuclei) in the embryo sac.

It takes place in the central cell of the embryo sac during fertilization. After syngamy (the fusion of one male gamete with the egg cell), the other male gamete fuses with the secondary nucleus to form the primary endosperm nucleus (PEN).

The nuclei involved in triple fusion are:

1. One male gamete nucleus (haploid, n)
2. Two polar nuclei or secondary nucleus (diploid, 2n)

As a result of triple fusion, a triploid (3n) primary endosperm nucleus is formed, which later develops into the endosperm that nourishes the developing embryo.

Question 12.
Why do you think the zygote is dormant for sometime in a fertilised ovule?
Answer:
The fertilized egg, known as the zygote, gives rise to the embryo. Before beginning development, the zygote undergoes a resting or dormant period.

This dormancy occurs because the zygote waits for the formation of a sufficient amount of endosperm, which serves as nourishment for the developing embryo.

This is an adaptation to ensure assured nutrition and proper development of the embryo within the seed.

Question 13.
Differentiate between:
(a) hypocotyl and epicotyl:
(b) coleoptile and coleorrhiza;
(c) integument and testa;
(d) perisperm and pericarp.

Answer:
(a) Hypocotyl and Epicotyl

(b) Coleoptile and Coleorhiza

(c) Integument and Testa

(d) Perisperm and Pericarp

Question 14.
Why is apple called a false fruit? Which part(s) of the flower forms the fruit?
Answer:
Most fruits develop only from the ovary and are called true fruits. When a fruit develops from other floral parts in addition to the ovary, it is known as a false fruit.
Apple is a false fruit because the thalamus (receptacle) also contributes to the formation of the fruit.

Question 15.
What is meant by emasculation? When and why does a plant breeder employ this technique?
Answer:
Emasculation is the removal of anthers before anthesis (before they release pollen) from a bisexual flower that is to be used as the female parent in hybridization.

During plant breeding, this technique is employed to prevent self-pollination, ensuring that the stigma of the emasculated flower is fertilized only by pollen from the desired male parent.

This helps the plant breeder in producing hybrid plants with desired characteristics.

Question 16.
If one can induce parthenocarpy through the application of growth substances, which fruits would you select to induce parthenocarpy and why?
Answer:
Parthenocarpic fruits are fruits that develop without fertilization and are therefore seedless. Parthenocarpy can be induced through the application of growth hormones such as auxins and gibberellins.

Important fruits like banana, papaya, orange, grapes, guava, and watermelon can be made seedless by applying these growth substances. These are economically important fruits, and when made seedless, they become more valuable and consumer-preferred.

Question 17.
Explain the role of tapetum in the formation of pollen-grain wall.
Answer:
Tapetum is the innermost layer of the microsporangium. It provides nourishment to the developing pollen grains. During microsporogenesis, the cells of the tapetum secrete enzymes, hormones, amino acids, and other nutrients required for pollen development. It also produces sporopollenin, which forms the exine layer of the pollen grain wall. This exine layer is highly resistant and protects the pollen grain from physical and chemical damage.

Question 18.
What is apomixis and what is its importance?
Answer:
Normal sexual reproduction that involves meiosis and fertilisation is called amphimixis. In some plants, this normal process is replaced by an abnormal form of reproduction called apomixis.

The term apomixis was first given by Winkler (1908). It may be defined as a process in which an egg or other cells (such as synergids or antipodals) develop into an embryo without fertilisation, and with or without meiosis.

Importance:

• Hybrid varieties of crops show high yield and productivity.
• However, hybrid seeds need to be produced every year, as the next generation shows segregation and loses hybrid traits.
• Through apomixis, hybrid plants can produce seeds that remain true to the parent type.
• Farmers can reuse hybrid seeds every year, reducing the cost of hybrid seed production.
• Embryos formed through apomixis are usually disease-free.