SEXUAL REPRODUCTION IN FLOWERING PLANTS | PART – D | 5 MARKS QUESTION WITH ANSWERS | CLASS 12 | PUC 2 YEAR
Before you begin, Read and revise notes here: SEXUAL REPRODUCTION IN FLOWERING PLANTS – Notes
1. Explain the structure of pollen grain.
Answer:
- Pollen grains represent the male gametophyte
- Pollen grains are generally spherical, measuring about 25-50 micrometers in diameter.
- Exine (Outer Wall): A hard outer layer made up of sporopollenin, which is one of the most resistant organic materials known. It has prominent apertures called germ pores where sporopollenin is absent.
- Intine (Inner Wall): The inner wall is a thin and continuous layer made up of cellulose and pectin.
- Vegetative Cell: It is bigger, has an abundant food reserve, and a large irregularly shaped nucleus.
- Generative Cell: It is small and floats in the cytoplasm of the vegetative cell. It is spindle-shaped with dense cytoplasm and a nucleus.
- Mature Pollen grains may shed at the 2 or 3 -celled stage.
2. Schematically represent the development of a female gametophyte in angiosperms.
Answer:
3. Answer the following:
a) Explain different types of pollination based on the source of pollen. (3)
b) List the characteristic features of water pollinated flowers. (2)
Answer:
a) Types of pollination based on source:
- Autogamy: Transfer of pollen grains from the anther to the stigma of the same flower.
- Geitonogamy: Transfer of pollen grains from the anther to the stigma of another flower of the same plant.
- Xenogamy: Transfer of pollen grains from the anther to the stigma of a different plant.
b) Features of water pollinated flowers:
- They are not very colourful
- They do not produce nectar.
- Pollen grains are protected from wetting by a mucilaginous covering.
4. How does pollination occur in Vallisneria and Zostera?
Answer: Pollination in Vallisneria:
- Vallisneria is a submerged, dioecious fresh water plant. It shows Epihydrophily.
- The female flowers reach the surface of water by the long stalk.
- The male flowers or pollen grains are released on to the surface of water.
- Male flowers are carried passively by water current and some of them reach female flowers.
- The anther burst open to release pollen grains. The pollen grains are attached to the large sticky stigma and bring about pollination.
- The pollen grains are protected from wetting by a mucilaginous covering.
Pollination in Zostera:
- Zostera is a marine sea grass. It shows Hypohydrophily.
- The female flowers remain submerged in water.
- The pollen grains are released inside the water.
- The pollen grains are long, ribbon-like and are carried passively inside the water.
- Some of the pollen grains reach the stigma and bring about pollination.
5. Pollen-Pistil interaction is a dynamic process involving pollen recognition followed by promotion or inhibition. Explain.
Answer:
- All the events—from the deposition of pollen on the stigma until the pollen tube enters the ovule—are collectively referred to as pollen-pistil interaction.
- It is a dynamic process involving pollen recognition followed by either the acceptance or rejection of the pollen.
- The pistil has the ability to recognise whether the pollen is of the compatible (right type) or the incompatible (wrong type).
- This continuous dialogue is mediated by chemical components of the pollen interacting with those of the pistil.
- If compatible, the pollen grain germinates on the stigma to produce a pollen tube that grows through the tissues of the stigma and style to reach the ovary.
6. Flowering plants have developed many devices to discourage self-pollination and encourage Cross pollination. Comment.
Answer: Continued self-pollination results in inbreeding depression, so plants developed devices to discourage self-pollination:
- Non-synchronization: Pollen release and stigma receptivity happen at different times.
- Spatial separation: The anther and stigma are placed at different positions so that pollen cannot contact the stigma of the same flower.
- Self-incompatibility: A genetic mechanism that prevents self-pollen from fertilizing the ovules by inhibiting pollen germination or pollen tube growth.
- Unisexual flowers (Dicliny): Producing male and female flowers on separate plants (dioecious) prevents both autogamy and geitonogamy.
7. Answer the following:
a) Double fertilization is a unique event taking place in flowering plants. Briefly explain. (3)
b) Arrange the following terms in the correct developmental sequence:
Pollen grain, Sporogenous tissue, Microspore tetrad, Pollen mother cell, Male gametes. (2)
Answer:
a) Double Fertilization:
- Syngamy: One of the male gametes moves towards the egg cell and fuses with its nucleus to form a diploid zygote (2n).
- Triple Fusion: The second male gamete fuses with the two polar nuclei to produce a triploid Primary Endosperm Nucleus (PEN) (3n).
- Since two distinct types of fusions—syngamy and triple fusion—take place simultaneously within the same embryo sac, the entire phenomenon is termed double fertilisation.
b) Correct developmental Sequence:
Sporogenous tissue → Pollen mother cell → Microspore tetrad → Pollen grain → Male gametes.
VI. Answer the following questions in 200-250 words each, wherever applicable: 5 Marks Each
1. Picture of a mature angiosperm embryo sac is given below. Answer the questions that follow.
a) Which cells/nuclei of the embryo sac produce zygote and primary endosperm nucleus? (2)
b) What is the ploidy of antipodal cells and primary endosperm nucleus? (2)
c) Why the endosperm development precedes embryo development? (1)
Answer:
a) The egg cell produces the zygote. The two polar nuclei produce the primary endosperm nucleus (PEN).
b) Antipodal cells = Haploid (n). Primary Endosperm Nucleus (PEN) = Triploid (3n).
c) Endosperm development always precedes embryo development because it is an adaptation to provide assured nutrition to the developing embryo.
2. Picture related to pollination is given below. Answer the questions that follow.
a) Will this pollination confirm fertilization? Justify your answer with suitable reasons. (2)
b) What are the floral rewards provided by the plants to the insects to revisit? (1)
c) Mention the characteristics of flower in this pollination other than floral rewards. (2)
Answer:
a) No, pollination does not guarantee fertilisation. The pistil must first recognise whether the pollen is of the right type (compatible) or the wrong type (incompatible) during pollen-pistil interaction.
b) The usual floral rewards offered to insects are nectar and pollen grains.
c) Insect-pollinated flowers – Flowers are usually large and colourful.
Flowers are fragrant to attract insects.
Flowers are rich in nectar.
Some flowers also provide safe places for insects to lay eggs.
3. Answer the following:
a) Plan an experiment and prepare a flow chart of the steps that you would follow to ensure that the seeds are formed only from the desired sets of pollen grains. (3)
b) Name the type of experiment that you carried out. (1)
c) Write the importance of such experiments. (1)
Answer:
a) Flow chart of steps:
| A bisexual female flower is taken |
| ↓ |
| Anthers are removed by process called Emasculation |
| ↓ |
| The stigma of emasculated flower is covered with polythene bag/ butter paper (bagging) |
| ↓ |
| When the bagged stigma attains receptivity, the desired set of pollen grains is dusted (pollination) on the stigma of the bagged flower |
| ↓ |
| Rebagging is done. Labelling and tagging also done |
| ↓ |
| Fruits are allowed to develop |
| ↓ |
| Desired seeds are obtained |
b) Type of experiment- Artificial Hybridisation.
c) It is a major approach of crop improvement programmes to combine desirable characters to produce commercially “superior” varieties.
4. Double fertilization is the unique feature of angiosperms and the products of this double fertilization are zygote and PEN. In context of this when a hexaploid plant is pollinated by a tetraploid plant find out the ploidy of zygote and PEN through a schematic illustration.
Answer:
- The female plant is hexaploid (6n) → Its egg cell and polar nuclei will be haploid of that = 3n.
- The male plant is tetraploid (4n) → Its male gametes will be haploid of that = 2n.
- Zygote = Egg Cell (3n) + Male Gamete (2n) = 5n (Pentaploid).
- PEN = Two Polar Nuclei (3n + 3n) + Male Gamete (2n) = 8n (Octaploid).
5. Why is the process of fertilization in angiosperms termed as double fertilization? Draw a diagram of an angiospermic embryo sac where fertilization is just completed and label the following parts:
a) The part that develops into an embryo
b) The part that develops into an endosperm
c) The degenerating cells at the chalazal end.
Answer:
- Since two distinct types of fusions—syngamy (fusion of a male gamete with the egg cell) and triple fusion (fusion of a male gamete with two polar nuclei)—take place simultaneously within the same embryo sac, the entire phenomenon is termed double fertilisation.
6. Answer the following:
a) Double fertilization is reported in plants of both, castor and groundnut. However, the mature seed of groundnut is non-albuminous and castor is albuminous. Explain the post-fertilization events that are responsible for it. (3)
b) Banana is a parthenocarpic fruit whereas oranges show polyembryony. How are they different from each other with respect to seeds? (2)
Answer:
a) Post-fertilization events in seeds:
- Non-albuminous (Groundnut): The endosperm is completely consumed by the developing embryo before seed maturation.
- Albuminous (Castor): The endosperm is not completely used up during embryo development and persists in the mature seed.
b) Difference in seeds:
- Banana (Parthenocarpy): Fruits develop without fertilisation, so these fruits are generally seedless.
- Oranges (Polyembryony): Nucellar cells surrounding the embryo sac start dividing and develop into multiple embryos, meaning the seed contains more than one embryo.
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