NCERT Solutions Class 12 Biology Chapter 1 - Sexual Reproduction in Flowering Plants

Q1: Name the parts of an angiosperm flower in which development of male and female gametophyte take place.
Ans: The male gametophyte (pollen grain) develops inside the pollen chamber (microsporangium) of the anther. The female gametophyte (embryo sac) develops inside the nucellus of the ovule, arising from the functional megaspore.

Q2: 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.
Ans:

Both processes involve meiosis (reduction division), which produces haploid cells from diploid mother cells.
Microsporogenesis: A diploid microspore mother cell (pollen mother cell) undergoes meiosis to form a microspore tetrad. The microspores usually separate and each develops into a pollen grain (male gametophyte).
Megasporogenesis: A diploid megaspore mother cell undergoes meiosis to produce a megasporic tetrad of four haploid megaspores. Typically, three megaspores degenerate and one functional megaspore remains, which develops into the embryo sac (female gametophyte).

Q3: Arrange the following terms in the correct developmental sequence:
Pollen grain, sporogenous tissue, microspore tetrad, pollen mother cell, male gametes.
Ans: The correct development sequence is as follows:
Sporogenous tissue → pollen mother cell → microspore tetrad → Pollen grain → male gamete

• During development of the microsporangium, cells of the sporogenous tissue differentiate into pollen (microspore) mother cells. Each pollen mother cell undergoes meiosis to form a microspore tetrad.
• The microspores separate and develop into pollen grains. Each mature pollen grain produces the male gametes (sperm cells) on germination.

Q4: With a neat, labelled diagram, describe the parts of a typical angiosperm ovule.
Ans: An ovule is the female megasporangium where megaspore formation and subsequent embryo sac development occur.
The various parts of an ovule are:

Structure of an Ovule

• Funiculus: A stalk-like structure that attaches the ovule to the placenta of the ovary.
• Hilum: The point on the ovule where it is attached to the funiculus.
• Integuments: One or two protective layers surrounding the nucellus; they develop into the seed coat after fertilisation.
• Micropyle: A small opening formed by the projection of integuments; it is the usual entry point for the pollen tube during fertilisation.
• Nucellus: A mass of parenchymatous tissue inside the integuments that nourishes the developing embryo sac; the embryo sac is located within the nucellus.
• Chalaza: The basal swollen region of the nucellus from which the integuments and nucellus tissue arise; it is opposite the micropyle.

Q5: What is meant by the monosporic development of female gametophyte?
Ans: 

• Monosporic development means the female gametophyte (embryo sac) develops from a single functional megaspore.
• During megasporogenesis, the megaspore mother cell undergoes meiosis to produce four haploid megaspores. In monosporic development, only one of these megaspores remains functional; the other three degenerate.
• The single functional megaspore then undergoes mitotic divisions to form the mature embryo sac.

Q6: With a neat diagram explain the 7-celled, 8-nucleate nature of the female gametophyte.
Ans: 

• The embryo sac develops from one functional megaspore, which undergoes three successive mitotic divisions to give eight nuclei within the sac.
• After the first mitosis there are two nuclei; one migrates to the micropylar end and the other to the chalazal end.
• Each of these nuclei divides twice more so that there are four nuclei at each pole (four micropylar, four chalazal = eight nuclei).
• At the micropylar end, three of the nuclei differentiate into two synergids and one egg cell (together called the egg apparatus), while at the chalazal end three nuclei become antipodal cells.
• The remaining two nuclei, one from each pole, migrate to the centre and become the polar nuclei in the large central cell. Thus there are seven cells (egg cell + two synergids + three antipodals + one central cell) but eight nuclei (because the central cell contains two polar nuclei).

Q7: What are chasmogamous flowers? Can cross-pollination occur in cleistogamous flowers? Give reasons for your answer.
Ans:

(a) Chasmogamous Flowers
(b) Cleistogamous Flowers

• Certain plants (for example Oxalis and Viola) produce two kinds of flowers: chasmogamous and cleistogamous flowers.
• Chasmogamous flowers open normally, exposing anthers and stigma; they are adapted for cross-pollination by external agents (insects, wind etc.).
• Cleistogamous flowers remain closed and never open; the anthers and stigma are held very close together inside the closed flower. Because the flower does not open, pollen is transferred within the same flower and only self-pollination normally occurs. Therefore cross-pollination cannot occur in cleistogamous flowers under natural conditions.

Q8: Mention two strategies evolved to prevent self-pollination in flowers.
Ans: Self-pollination is the transfer of pollen from the stamen to the pistil of the same flower.
Two strategies that have evolved to prevent self-pollination in flowers are as follows:

• Self-incompatibility: A genetic mechanism by which a flower's stigma can recognise and inhibit pollen from the same plant or genetically similar plants. The incompatible pollen fails to germinate or the pollen tube growth is prevented, so fertilisation does not occur.
• Dichogamy (Protandry and Protogyny): Temporal separation of male and female maturity. In protandry the anthers mature before the stigma; in protogyny the stigma matures before the anthers. This reduces the chance of pollen from the same flower fertilising its own ovules.

Q9: What is self-incompatibility? Why does self-pollination not lead to seed formation in self-incompatible species?
Ans:

• Self-incompatibility is a genetically controlled mechanism in many angiosperms that prevents self-fertilisation by recognising and rejecting pollen from the same plant or genetically similar plants.
• In self-incompatible species the stigma or style prevents the germination of self-pollen or halts pollen-tube growth. Because the pollen tubes do not reach the ovule, fertilisation (fusion of male and female gametes) does not occur and therefore no seed formation follows.

Q10: What is bagging technique? How is it useful in a plant breeding programme?

Ans:

• In artificial hybridisation, the anthers are removed from bisexual flowers (a process called emasculation) so that a flower cannot self-pollinate.
• After emasculation, the flower is covered with a bag to exclude unwanted pollen; this is called bagging. Bagging prevents pollination by foreign or undesirable pollen and ensures that only the chosen pollen (applied later) fertilises the ovule.

Bagging Technique

• This technique is essential in plant breeding because it allows breeders to control parentage and produce hybrids with desired traits by ensuring fertilisation occurs only with selected pollen.

Q11: What is triple fusion? Where and how does it take place? Name the nuclei involved in triple fusion.
Ans: 

• Triple fusion is the fusion of one male gamete nucleus with the two polar nuclei in the central cell of the embryo sac; it occurs inside the embryo sac of the ovule.
• After pollen germination, the pollen tube releases two male gametes into a synergid. One male gamete fuses with the egg cell nucleus to form the zygote (syngamy). The other male gamete fuses with the two polar nuclei in the central cell to form the triploid primary endosperm nucleus.
• The nuclei involved are one male gamete nucleus and two polar nuclei (one from each pole), and their fusion produces the triploid endosperm nucleus that gives rise to endosperm.

Q12: Why do you think the zygote is dormant for some time in a fertilized ovule?
Ans: 

• The zygote remains dormant briefly because the developing embryo requires nourishment from the endosperm. Endosperm formation begins after triple fusion and produces food reserves.
• Zygote dormancy allows time for the primary endosperm nucleus to develop and store reserves so that the embryo can grow successfully. Thus embryonic development is coordinated with endosperm formation.

Q13: Differentiate between:
(a) Hypocotyl and Epicotyl
(b) Coleoptile and Coleorrhiza
(c) Integument and Testa
(d) Perisperm and Pericarp
Ans: 

Q14: Why is apple called a false fruit? Which part(s) of the flower forms the fruit.
Ans: Botanically ripened ovary is called a true fruit. The fruits in which thalamus and other floral parts develop along with the ovary are called false fruits. For example - apple, strawberry, cashew etc. In apple the main edible portion of the fruit is the fleshy thalamus. Ovary forms the fruit after fertilization or without fertilization in parthenocarpic fruits.

Q15: What is meant by emasculation? When and why does a plant breeder employ this technique?
Ans: Emasculation is the removal of anthers from bisexual flowers before they dehisce, so that the flower cannot self-pollinate.

Emasculation

• Plant breeders perform emasculation in order to control pollination and produce hybrids. It is done before anther dehiscence and is usually followed by bagging to exclude unwanted pollen.
• Later, selected pollen from the desired male parent is applied to the emasculated stigma to carry out artificial pollination and obtain offspring with chosen characteristics.

Q16: If one can induce parthenocarpy through the application of growth substances, which fruits would you select to induce parthenocarpy and why? 
Ans: Parthenocarpic fruits are seedless. They develop from ovary without fertilization. Banana, grapes, oranges, Pineapple, Guava, Watermelon, lemon are selected because these seedless of units are of high economic importance. The fruits in which seeds or seed part form edible portion (e.g.,Pomegranate) are not selected to induce parthenocarpy.

Q17: Explain the role of tapetum in the formation pollen-grain wall.
Ans: 

• Tapetum is the innermost nutritive layer of the microsporangium (anther) that surrounds developing microspores.
• Its cells supply nutrients, enzymes and precursors required for pollen development and maturation.
• The tapetum also contributes materials (including sporopollenin precursors) required for formation of the outer pollen wall called the exine, which makes pollen grains resistant and species-specific.

Q18: What is apomixis and what is its importance?
Ans: 

• Apomixis is the formation of seeds without meiosis and fertilisation, so offspring are genetically identical to the mother plant.
• Its importance lies in preserving desirable hybrid characteristics without segregation that occurs in sexual reproduction. Apomixis allows clonal seed production, which is cost-effective for farmers and useful in maintaining uniform, high-value crop traits.