CBSE Class 12 Biology Sexual Reproduction in Flowering Plants Question Bank

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Biology: Chapter 2 – Sexual Reproduction in Flowering Plants (Question Bank)

Why are pollen grains produced in enormous quantity in maize?

Answer: Maize is wind pollinated, and a very large number of pollen grains are lost during the transfer by the wind. The large quantity compensates for these losses, so that some pollen grains reach the stigma.

What is the ploidy of the cells in the microspore tetrad?

Answer: Haploid (n). The microspores are formed by the meiosis of the microspore mother cell.

What is the ploidy of PEN?

Answer: Triploid (3n).

How many eggs are present in an embryo sac?

Answer: One.

Even though each pollen grain has two male gametes, why are at least 10 pollen grains and not 5 pollen grains required to fertilise 10 ovules present in a particular carpel?

Answer: Because of double fertilisation. Both the male gametes of a pollen grain are used in the same ovule: one fuses with the egg (syngamy) and the other fuses with the two polar nuclei (triple fusion). So one pollen grain can fertilise only one ovule, and 10 pollen grains are needed for 10 ovules.

What are parthenocarpic fruits?

Answer: Fruits formed without fertilisation are called parthenocarpic fruits. They are seedless, e.g. banana.

What is scutellum?

Answer: The scutellum is the single cotyledon of the embryo of the monocot (grass family, e.g. maize) seeds. It lies towards one side of the embryonal axis.

What is a pollen bank?

Answer: A pollen bank is a place where the pollen grains are stored (preserved) in liquid nitrogen at −196 °C, for future use, e.g. in crop breeding programmes. They can be used for years, like the seed banks.

Identify the given figure and label the parts. (

Answer: The figure shows a stamen (the male reproductive part of the flower), with the anther (in which the pollen sacs are seen).

1. a: pollen sac (microsporangium)
2. b: pollen grains
3. c: line of dehiscence
4. d: filament

Of the eight nuclei of the embryo sac in flowering plants, three are at the micropylar end. How many are there at the chalazal end and how many nuclei are located in the central cell?

Answer: Three nuclei are at the chalazal end (the antipodal cells), and two nuclei (the polar nuclei) are located in the central cell. The remaining three at the micropylar end are the egg apparatus (one egg and two synergids).

How could pollen grains be well preserved as fossils?

Answer: The outer wall of the pollen grain, the exine, is made of sporopollenin, which is one of the most resistant organic materials known. It can withstand the high temperature, strong acids and alkali, and no enzyme can degrade it. So the pollen grains are well preserved as fossils.

What are the effects of pollen in some people, and how are pollen grains being used as food supplements?

Answer: In some people, pollen causes severe allergies and bronchial afflictions, which may even lead to chronic respiratory disorders (asthma, bronchitis). Pollen grains are rich in nutrients, so pollen products are available in the market as tablets and syrups (food supplements), and they are consumed by the athletes and the race horses to increase their performance.

How many haploid nuclei and haploid cells are present in the female gametophyte of an angiosperm?

Answer: The female gametophyte (embryo sac) is 8-nucleate and 7-celled, so there are 8 haploid nuclei and 7 cells. (The two polar nuclei lie in a single central cell, which makes 7 cells.)

Why is the process of fertilisation in flowering plants referred to as double fertilisation? Explain.

Answer: Two fusions take place in the embryo sac: syngamy, in which one male gamete fuses with the egg and forms the zygote, and triple fusion, in which the other male gamete fuses with the two polar nuclei to form the triploid primary endosperm nucleus. As two types of fusion take place in the same embryo sac, it is called double fertilisation.

How does geitonogamy differ from xenogamy in plants? (

Answer: In geitonogamy the pollen grains are transferred from the anther to the stigma of another flower of the same plant; the plants are genetically similar, as it is the same plant, although it is functionally cross-pollination. In xenogamy the transfer is to the stigma of a flower of a different plant, and so it is the only type that brings genetically different types of pollen to the stigma.

What are the stages of embryogeny in a dicotyledonous embryo?

Answer: The zygote divides to form the proembryo, which passes through the globular stage, then the heart-shaped stage, and then develops into the mature embryo.

How long do the seeds remain alive after they are dispersed?

Answer: It varies from species to species. In a few species the seeds lose their viability within a few months, while in other species the seeds remain alive for several years. Some seeds are viable for hundreds or even thousands of years, e.g. the seed of the lupine Lupinus arcticus germinated after 10,000 years and that of the date palm Phoenix dactylifera after 2000 years.

Draw a diagram of the following and label any six / three parts: (i) T.S. of a mature anther. (ii) Mature pollen grain. (iii) A diagrammatic view of a typical anatropous ovule. (iv) A diagrammatic representation of the mature embryo sac. (v) Fertilised embryo sac. (vi) Stages in embryo development in a dicot. (vii) Dicot embryo. (viii) L.S. of an embryo of grass. (ix) A longitudinal section of a flower showing the growth of the pollen tube.

The labelled diagrams for (i), (ii), (iii), (iv), (vii) and (viii) are given below.

(i) T.S. of a mature anther:

(ii) Mature pollen grain:

(iii) Anatropous ovule:

(iv) Mature embryo sac:

(vii) Dicot embryo:

(viii) L.S. of the embryo of a grass:

Explain the structure of a microsporangium.

Answer: A typical angiosperm anther is bilobed, and each lobe has two thecae (dithecous). In a transverse section, the anther shows four microsporangia (pollen sacs), located at the corners, two in each lobe. The microsporangium is surrounded by four wall layers:

1. the epidermis (outermost, protective)
2. the endothecium (the fibrous layer, which helps in the dehiscence of the anther)
3. the middle layers
4. the tapetum (the innermost layer, which nourishes the developing pollen grains)

In the centre of the microsporangium is the sporogenous tissue, which gives rise to the microspore mother cells and then the pollen grains.

Explain the structure of a pollen grain.

Answer: The pollen grain is generally spherical, about 25–50 micrometres in diameter. It has a two-layered wall. The outer layer, the exine, is hard and made of sporopollenin, which is highly resistant; it has prominent apertures called germ pores where sporopollenin is absent. The inner layer, the intine, is thin and continuous, made of cellulose and pectin. The cytoplasm of the pollen grain is surrounded by a plasma membrane. A mature pollen grain contains two cells: the bigger vegetative cell (with abundant food reserve and a large irregular nucleus) and the small generative cell (spindle-shaped, with dense cytoplasm and a nucleus), which divides to form two male gametes.

Explain the artificial hybridisation technique.

Answer: Artificial hybridisation is the technique in which only the desired pollen grains are used for pollination, and the stigma is protected from contamination by unwanted pollen. It is a major approach in crop improvement. The steps are:

1. Emasculation: the anthers are removed from the bud of the female parent (bisexual flower) before they dehisce, using forceps.
2. Bagging: the emasculated flower is covered with a bag of suitable size (generally of butter paper), to prevent contamination of its stigma with unwanted pollen.
3. Dusting: when the stigma attains receptivity, the mature pollen grains collected from the anthers of the male parent are dusted on the stigma, and the flower is rebagged, and the fruits are allowed to develop.

Explain double fertilisation schematically.

The pollen tube releases the two male gametes into the cytoplasm of the synergid. One gamete moves towards the egg cell and fuses with its nucleus (syngamy) to form the diploid zygote. The other male gamete moves towards the two polar nuclei in the central cell and fuses with them (triple fusion) to form the triploid primary endosperm nucleus (PEN).

What is pollination? Explain its types.

Answer: Pollination is the transfer of the pollen grains from the anther to the stigma. The types, on the basis of the source of pollen, are:

Endosperm development precedes embryo development. Why?

Answer: After the triple fusion, the primary endosperm cell divides repeatedly to form the endosperm tissue, which is filled with the reserve food materials. These are used for the nutrition of the developing embryo. So the endosperm must develop first, so that food is available to the embryo.

Explain free nuclear endosperm development.

Answer: It is the most common type of endosperm development. The primary endosperm nucleus (PEN) undergoes successive nuclear divisions to give rise to free nuclei (this stage is called free-nuclear endosperm). Later, cell wall formation occurs, and the endosperm becomes cellular. The tender coconut water is the free-nuclear endosperm (it has thousands of nuclei), and the surrounding white kernel is the cellular endosperm.

What are the advantages of seeds to angiosperms?

Answer:

1. Pollination and fertilisation are independent of water (unlike in bryophytes and pteridophytes), so seed formation is more dependable.
2. Seeds have better adaptive strategies for dispersal to new habitats, and so help the species to colonise other areas.
3. The hard seed coat provides protection to the young embryo, and as the seeds have the reserve food, the young seedlings are nourished until they are capable of photosynthesis. They also provide the means for the storage and for the new varieties through the genetic recombination.

What do you mean by seed dormancy? What is its significance?

Answer: Seed dormancy is the state of inactivity of a mature seed, when its metabolic activity slows down and the seed becomes dry because of the reduction in the water content. It is significant as the seeds can be stored as food throughout the year, and they remain alive until the conditions (water, temperature, oxygen) are favourable for germination, so the crop can be raised in the next season.

Continued self-pollination results in inbreeding depression. What are the outbreeding devices developed in flowering plants to discourage self-pollination? Which type of pollination is seen in papaya?

The outbreeding devices are:

1. Pollen release and stigma receptivity are not synchronised (the stigma becomes receptive before or after the pollen is released).
2. The anther and the stigma are placed at different positions, so the pollen cannot come in contact with the stigma of the same flower.
3. Self-incompatibility: a genetic mechanism that prevents the self pollen from fertilising the ovules, by inhibiting the pollen germination or the pollen tube growth in the pistil.
4. Production of unisexual flowers: if the male and female flowers are present on the same plant (monoecious, e.g. castor and maize) it prevents autogamy but not geitonogamy; if they are on different plants (dioecious, e.g. papaya) it prevents both autogamy and geitonogamy.

In papaya (a dioecious plant), only cross-pollination (xenogamy) takes place.

What are the characteristics of wind-, water- and insect-pollinated flowers?

Answer:

Explain the pollen–pistil interaction with a sketch showing the entry of the pollen tube into a synergid.

Pollen–pistil interaction is a dynamic process, which involves the recognition of the pollen by the pistil, and the promotion or inhibition of the pollen. It takes place through the chemical components of the pollen and the pistil (a 'dialogue').

1. If the pollen is of the right type (compatible), the pistil accepts it and promotes the post-pollination events.
2. The pollen grain germinates on the stigma to produce the pollen tube through the germ pore.
3. The contents of the pollen grain move into the pollen tube, which grows through the tissues of the stigma and the style and reaches the ovary.
4. The generative cell divides into two male gametes during the growth of the pollen tube (if not already divided).
5. The pollen tube enters the ovule through the micropyle, and then enters one of the synergids through the filiform apparatus, which guides the entry of the pollen tube, and releases the two male gametes.

Define triple fusion. What is the product of this process? What does the product develop into?

Answer: Triple fusion is the fusion of one male gamete with the two polar nuclei present in the central cell of the embryo sac (three haploid nuclei in all). The product is the triploid primary endosperm nucleus (PEN). The PEN develops into the endosperm, which provides the nutrition to the developing embryo.

Trace the development of a mature female gametophyte from a megaspore mother cell.

Answer:

1. The megaspore mother cell (MMC), a large diploid cell in the nucellus of the ovule, undergoes meiosis to form four haploid megaspores.
2. Three of the megaspores degenerate and only one (the functional megaspore) remains.
3. The nucleus of the functional megaspore divides mitotically to form two nuclei, which move to the opposite poles (2-nucleate stage).
4. Two more mitotic divisions follow, first giving the 4-nucleate and then the 8-nucleate stage of the embryo sac (monosporic development).
5. Cell walls are laid down: three cells at the micropylar end form the egg apparatus (one egg and two synergids), three cells at the chalazal end form the antipodals, and the two polar nuclei lie in the large central cell. So the mature embryo sac is 8-nucleate and 7-celled.

Describe the development of the male gametophyte in angiosperms.

Answer:

1. In the microsporangium, the pollen mother cell (microspore mother cell, diploid) undergoes meiosis to form a microspore tetrad (four haploid microspores).
2. As the anther matures, the microspores separate and develop into the pollen grains. The pollen grain wall has an outer exine of sporopollenin and an inner intine, with germ pores.
3. The nucleus of the pollen grain divides unequally by mitosis to form the large vegetative cell and the small generative cell. This is the male gametophyte.
4. The generative cell divides mitotically to form two male gametes, either in the pollen grain or in the pollen tube.

Explain the following terms: non-albuminous, albuminous, perisperm, pericarp, seed dormancy.

Answer:

1. Non-albuminous seeds: have no residual endosperm, as it is completely consumed during the embryo development, e.g. pea and groundnut.
2. Albuminous seeds: retain a part of the endosperm, as it is not completely used up during the embryo development, e.g. wheat, maize, barley, castor and sunflower.
3. Perisperm: the residual persistent nucellus in a seed, e.g. in black pepper and beet.
4. Pericarp: the fruit wall, formed from the ovary wall; it may be dry or fleshy.
5. Seed dormancy: the state of inactivity of the mature seed, which remains till the conditions are favourable for its germination.

Explain the following terms: false fruit, true fruit, parthenocarpic fruit, apomixis, polyembryony.

Answer:

1. False fruit: a fruit in which other parts of the flower, such as the thalamus, also take part in the fruit formation (e.g. apple, strawberry, cashew).
2. True fruit: a fruit that develops only from the ovary (e.g. mango, orange).
3. Parthenocarpic fruit: a fruit that develops without fertilisation, and is seedless (e.g. banana).
4. Apomixis: the formation of seeds without fertilisation (a form of asexual reproduction that mimics the sexual reproduction).
5. Polyembryony: the occurrence of more than one embryo in a seed (e.g. citrus and mango).

Mr Y, a farmer, has kept heaps of seeds to be packed for future use in a room. His children, while playing, poured water on to the seeds. (A) As a student of biology, suggest the measures to be taken before storage and why. (B) What is the significance of seeds for a farmer?

Answer:

1. The wet seeds should be spread in the sun or in a shade and dried again before storage, because the moisture activates the seeds (they will germinate or become infected by fungi and decay). Dry seeds are dormant, with a slow metabolism, so they can be stored for a long time. The seeds can also be treated with the pesticides (or a layer of neem leaves added) and stored in a cool, dry place.
2. Seeds are the source of the next crop for the farmer, and also the source of his food. The seeds can be stored throughout the year, and they can be sold for the income. Good seeds also help to preserve the superior variety.

9. identify the given figure and lable the parts.

TWO MARKS

20. Explain the structure of pollen grain.

21. Explain artificial hybridization technique.

22. Explain double fertilization schematically.

23. What is pollination and explain its types?