Sexual Reproduction in Flowering Plants (LAQs)

Botany-1 | 7. Sexual Reproduction In Flowering Plants – LAQs:
Welcome to LAQs in Chapter 7: Sexual Reproduction In Flowering Plants. This page includes the most important FAQs from previous exams. Each answer is explained in simple English, followed by a Telugu explanation, and presented in the exam format. This helps you grasp complex concepts easily and aim for top marks in your final exams.

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LAQ-1 : Write a neat, labelled diagram, describe the parts of a mature angiosperm embryo sac. Mention the role of synergids.

Introduction:

In the lifecycle of angiosperms (flowering plants), the mature embryo sac plays a pivotal role in sexual reproduction. It contains the female gametophytes and is essential for the fusion of male and female gametes. The embryo sac comprises three main parts: the egg apparatus, the central cell, and the antipodals. Additionally, synergids are specialized cells within the embryo sac with important functions in the reproductive process.

Parts of a Mature Angiosperm Embryo Sac:

1. Egg Apparatus: The egg apparatus is located at the micropylar end of the embryo sac and consists of three cells. It includes two synergids and one egg cell (oospore).
   For example, in a pea plant, the synergids have distinctive cellular thickenings at their micropylar ends known as the filiform apparatus. This structure helps guide the pollen tube. The central cell in this group, called the egg or oospore, is the cell that will fuse with a male gamete to form the zygote.
2. Central Cell: The central cell is the largest cell in the embryo sac, situated at the center. It contains two polar nuclei, which are haploid (n) nuclei.
   For example, in a sunflower, these two polar nuclei eventually fuse to form a single diploid (2n) secondary nucleus, which is crucial for the formation of the endosperm, providing nourishment to the developing embryo.
3. Antipodals: Antipodals are located at the chalazal end of the embryo sac. This area consists of three small cells, which may degenerate either before or after fertilization.
   In plants like corn, antipodals are considered vegetative cells of the embryo sac and their role in the fertilization process is less critical compared to the other parts.

Role of Synergids:

1. Absorption of Nutrients: Synergids absorb nutrients from the surrounding nucellus, the tissue of the ovule. These nutrients are essential for nourishing the developing female gametophyte.
   For example, in roses, synergids ensure that the embryo sac receives the necessary nutrients for its growth and function.
2. Nourishing the Female Gametophyte: Synergids provide vital nourishment to the female gametophyte, ensuring its proper development and function.
3. Guiding the Pollen Tube: After pollination, the pollen tube grows from the stigma to the ovule, delivering male gametes (sperm cells) to the egg apparatus. Synergids play a crucial role in guiding the pollen tube towards the egg cell. The filiform apparatus at the micropylar end helps attract and direct the pollen tube, facilitating fertilization.

Summary:

The mature angiosperm embryo sac is a key structure in sexual reproduction, consisting of the egg apparatus, central cell, and antipodals. Synergids, specialized cells within the embryo sac, are essential for nutrient absorption, nourishing the female gametophyte, and guiding the pollen tube towards the egg cell. These processes are vital for the successful fusion of male and female gametes, leading to zygote formation and the initiation of seed development in angiosperms.

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LAQ-2 : Draw a diagram of a microsporangium and label its wall layers. Write briefly about the wall layers.

Introduction:

The microsporangium is an essential structure in the anther of flowering plants where pollen is produced. Each layer of the microsporangium plays a specific role in the development and release of pollen grains. Here’s a detailed explanation of each wall layer:

1. Epidermis: The epidermis is the outermost layer of the microsporangium. It serves as a protective barrier for the internal structures, preventing damage and pest infiltration.
   For example, in a rose flower, the epidermis helps shield the developing pollen grains from environmental threats and harmful insects. The cells of the epidermis are typically thick-walled, providing structural support and resilience.
   Additionally, the epidermis includes a specialized area known as the stomium. The stomium consists of thin-walled cells that facilitate the opening of the pollen sacs. This process is crucial for the release of pollen grains.
   For instance, during the blooming of a hibiscus, the stomium’s role becomes vital. It ensures that the pollen can escape when the flower reaches maturity, ready for pollination.
2. Endothecium: The endothecium lies just beneath the epidermis and is crucial for the dehiscence of the anther. This layer is characterized by cells with radial fibrous thickenings, which play a key role in pollen release.
   In plants like sunflowers, the endothecium contracts as the anther matures. This contraction helps open the pollen sacs and allows the pollen grains to be dispersed effectively.
3. Middle Layers: The middle layers are situated below the endothecium and consist of 1 to 5 layers of thin-walled cells. These layers assist in the process of dehiscence, contributing to the release of pollen grains from the anther.
   For example, in corn plants, the middle layers help push the pollen out of the anther when the flower blooms. This function is essential for ensuring that pollen is released into the air for pollination to take place.
4. Tapetum: The tapetum is the innermost layer surrounding the sporogenous tissue, which develops into pollen grains. It consists of large, thin-walled, and often multinucleate cells.
   The tapetum plays a critical role in nourishing the developing pollen grains. For example, in lily flowers, the tapetum provides essential nutrients that support the growth and maturation of pollen, ensuring that viable pollen grains are produced for successful fertilization.

Summary:

Each layer of the microsporangium contributes to the effective development and release of pollen grains. The epidermis provides protection and facilitates pollen sac opening, the endothecium aids in pollen release, the middle layers support the process of dehiscence, and the tapetum nourishes the pollen grains. These functions are crucial for successful reproduction in flowering plants.gents, such as wind or pollinators, to the female reproductive structure (stigma) for fertilization.

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LAQ-3 : Describe the process of Fertilization in angiosperms.

Introduction:

Fertilization in angiosperms, or flowering plants, is a process where male and female gametes join to form seeds. This is essential for the growth of new plants and the survival of the species.

Process of Fertilization:

1. Pollination: Pollination is the first step, where pollen (the male part) is transferred from the anther (the part of the flower that produces pollen) to the stigma (the female part).
   For example, in apple blossoms, bees carry pollen from one flower to another while collecting nectar, helping in pollination.
2. Germination of Pollen: Once the pollen lands on the stigma, it starts to grow. It forms a pollen tube that extends down through the style (the tube connecting the stigma to the ovary) to reach the ovary.
   In tomato flowers, the pollen germinates on the stigma and sends a tube down the style to deliver the sperm cells to the ovule.
3. Growth of Pollen Tube: The pollen tube carries the sperm cells to the ovule. It follows chemical signals from the ovary to guide it along the style.
   For example, in lilies, the pollen tube grows through the style, directed by signals, to reach the ovule where fertilization happens.
4. Entry into Ovule: The pollen tube enters the ovule through a small opening called the micropyle. The ovule contains the female gametophyte or embryo sac, where fertilization will occur.
   In sunflowers, the pollen tube goes through the micropyle to reach the embryo sac inside the ovule.
5. Double Fertilization: In angiosperms, double fertilization occurs, which means there are two important fusion events:
   • Fertilization of the Egg Cell: One sperm cell fuses with the egg cell to form a zygote (a fertilized egg). This zygote will develop into the embryo inside the seed.
   • For example, in orchids, one sperm cell combines with the egg cell to form the embryo.
   • Formation of Endosperm: Another sperm cell fuses with the central cell (which has two polar nuclei) to form a triploid cell. This cell becomes the endosperm, which provides nutrients to the developing embryo.
   • In wheat plants, this process creates endosperm that feeds the growing embryo inside the seed.
6. Formation of Seed and Fruit: After fertilization, the ovule becomes a seed. The ovary grows into a fruit, which surrounds and protects the seed(s).
   For example, in peach trees, the fertilized ovule turns into a seed within the fruit, which later disperses the seed to grow into a new plant.

Summary:

Fertilization in angiosperms involves pollination, germination of pollen, growth of the pollen tube, entry into the ovule, double fertilization, and the formation of seeds and fruits. Double fertilization is unique to flowering plants, where one sperm cell fertilizes the egg and another forms the endosperm to nourish the embryo. This process ensures the creation of seeds and the continuation of plant species.