Bihar Board - Class 12 Biology - Chapter 2: Sexual Reproduction in Flowering Plants Long Answer Question

Long Questions Answers

Q.1. Explain the different  Characteristics of different pollinating agents

A. Characteristics of wind-pollinated flowers:

1. The pollen is smooth, light can be blown easily by wind and are in large numbers.

2. The stigma is feathery and can catch pollen from the wind.

Characteristics of water-pollinated flowers:

1. They possess small male flowers that are not clearly visible.

2. A large number of pollens are released in water that is caught by large, feathery stigma of female flowers.

3. This pollen keeps floating on the water surface until they are caught by female flowers.

Characteristics of insect-pollinated flowers:

1. They are large with bright-coloured petals to attract insects.

2. The flowers have nectar and a pleasant fragrance.

3. The pollen grains are sticky and can easily stick to the insect’s body.

Q.2. Explain the structure of the pollen.

A. Pollen grains are microscopic structures that carry the male reproductive cells of a plant. It is a double-walled structure with a thin inner wall known as endospore composed of cellulose and a thick outer wall known as exospore, composed of sporopollenin.

The exospore protects the male genetic material during transportation from anther to stigma. The waxes and proteins present on the pollen surface repel moisture and interact with the stigma.

Q.3. What are the functions of a flower?

A.The flowers are the reproductive organs of plants and are mainly involved in the reproduction process. The essential functions of flowers are mentioned below:

1. It provides the beauty of the plant.

2. Gametophytes developed in the flowers.

3. Helps in the development of fruit with a seed.

4. Involved in the union of male and female gametes.

5. It accommodates the sex organs of the plant.

6. Flowers provide nectar to certain birds and insects.

7. It protects the reproductive organs of a plant.

8. The flowers can produce diaspores without fertilization.

9. Flowers also promote the union of sperm and eggs from the same flower or different flowers.

    10. It helps in pollination by attracting insects and other animals prior to transferring pollen grains.

Q.4. Explain the basic structure of the dicot and monocot embryo.

A. Embryo develops at the micropylar end of the embryo sac where the zygote is situated. Most zygotes divide only after a certain amount of endosperm is formed. This is an adaptation to provide assured nutrition to the developing embryo. Though the seeds differ greatly, the early stages of embryo development (embryogeny) are similar in both monocotyledons and dicotyledons. The zygote gives rise to the proembryo and subsequently to the globular, heart-shaped and mature embryo.

A typical dicotyledonous embryo consists of an embryonal axis and two cotyledons. embryonal axis above the level of cotyledons is the epicotyl. which terminates with the plumule or stem tip. The cylindrical portion below the level of cotyledons is hypocotyl that terminates at its lower end in the radicle or root tip. The root tip is covered with a root cap.

Embryos of monocotyledons possess only one cotyledon. In the grass family the cotyledon is called scutellum that is situated towards one side (lateral) of the embryonal axis. At its lower end, the embryonal axis has the radical and root cap enclosed in an undifferentiated sheath called colcorrhiza. The portion of the embryonic axis above the level of few leaf primordia enclosed in a hollow follar structure, the coleoptile. attachment of scutellum is the epicotyl. Epicotyl has a shoot apex and a Soak a few seeds in water (say of wheat, maize, peas, chickpeas, ground nuts) overnight. Then split the seeds and observe the various parts of the embryo and the seed.

Q.5. What is endosperm? 

A.The cells of this tissue are filled with reserve food materials and are used for the nutrition of the developing embryo. In the most common type of endosperm development, the PEN undergoes successive nuclear divisions to give rise to free nuclei. This stage of endosperm development is called free-nuclear endosperm. Subsequently cell wall formation occurs and the endosperm becomes cellular. The number of free nuclei formed before cellularization varies greatly. The coconut water from tender coconut that you are familiar with. is nothing but free-nuclear endosperm (made up of thousands of nuclei) and the surrounding white kernel is the cellular endosperm.

Q.6 What is double fertilization? How does it occurs ?

A.DOUBLE FERTILISATION: After entering one of the synergids, the pollen tube releases the two male gametes into the cytoplasm of the synergid. One of the male gametes moves towards the egg cell and fuses with its nucleus thus completing the syngamy. This results in the formation of a diploid cell, the zygote." Another male gamete moves towards the two polar nuclei located in the central cell and fuses with them to produce a triploid primary endosperm nucleus (PEN). As this involves the fusion of three haploid nuclei it is termed triple fusion. Since two types of futons, syngamy and triple fusion take place in an embryo sac the phenomenon is termed double fertilization, an event unique to flowering plants. The central cell after triple fusion becomes the primary endosperm cell (PEC) and develops into the endosperm while the zygote develops into an embryo.

Q.7 What are outbredding devices?

A.Outbreeding Devices: Flowering plants have developed many devices to discourage self- pollination and to encourage cross-pollination. In some species, pollen release and stigma receptivity are not synchronized. Either the pollen is released before the stigma becomes receptive or stigma becomes receptive much before the release of pollen. In some other species, the anther and stigma are placed at different positions so that the pollen cannot come in contact with the stigma of the same flower. Both these devices prevent autogamy. The third device to prevent inbreeding is self-incompatibility. This is a genetic mechanism and prevents self-pollen (from the same flower or other flowers of the same plant) from fertilizing the ovules by inhibiting pollen germination of pollen tube growth in the pistil. Another device to prevent self-pollination is the production of unisexual flowers. If both male and female flowers are present on the same plant such as castor and maize (monoecious), it prevents autogamy but not geitonogamy. In several species such as papaya, male and female flowers are present on different plants, that is each plant is either male or female (dioecy). This condition prevents both autogamy and geitonogamy.