3 Most FAQ’s of Sexual Reproduction in Flowering Plants Chapter in Inter 1st Year Botany (TS/AP)

8 Marks

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

Introduction

In the life cycle of angiosperms, the mature embryo sac plays a crucial role in sexual reproduction. It contains female gametophytes and facilitates the fusion of male and female gametes. The embryo sac consists of three main parts: the egg apparatus, the central cell, and the antipodals. Additionally, synergids are specialized cells within the embryo sac that have essential roles in the reproductive process.

Parts of a Mature Angiosperm Embryo Sac:

1. Egg Apparatus:
   • Three cells are grouped together at the micropylar end to form the egg apparatus.
   • The egg apparatus consists of two synergids and one egg cell (oospore).
   • The synergids have unique cellular thickenings at their micropylar ends, known as the filiform apparatus.
   • The middle and largest cell in the egg apparatus is called the egg or oospore.
2. Central Cell:
   • The central cell is the largest cell in the embryo sac and is located at the center.
   • It contains two polar nuclei, which are haploid (n) nuclei.
   • These two polar nuclei eventually fuse together to form a single diploid (2n) secondary nucleus.
3. Antipodals:
   • The antipodals are found at the chalazal end of the embryo sac.
   • This region consists of three small cells, and they may degenerate either before or after fertilization.
   • Antipodals are considered as vegetative cells of the embryo sac.

Role of Synergids

1. Absorption of Nutrients: Synergids absorb nutrients from the surrounding nucellus, which is the tissue of the ovule. These nutrients are crucial for nourishing the developing female gametophyte.
2. Nourishing the Female Gametophyte: Synergids provide essential 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 the male gametes (sperm cells) to the egg apparatus. The synergids play a vital 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 critical structure in sexual reproduction. It consists of the egg apparatus, central cell, and antipodals. Synergids, specialized cells within the embryo sac, play crucial roles in nutrient absorption, nourishing the female gametophyte, and guiding the pollen tube towards the egg cell. These processes are essential for the successful fusion of male and female gametes, leading to the formation of the zygote 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

Please note that as an AI text model, I am unable to draw diagrams directly. However, I can provide a description of the microsporangium and its wall layers.

Description of Wall Layers of Microsporangium:

The microsporangium is the structure within the anther of a flowering plant where microspores are produced, which eventually develop into pollen grains. The wall layers of the microsporangium are as follows:

1. Epidermis:
   • The epidermis is the outermost layer of the microsporangium.
   • It is thick and serves as a protective covering.
   • The cells present between the two pollen sacs are thin-walled, forming a region called the stomium.
   • The stomium is crucial for the dehiscence (opening) of the pollen sacs during pollen release.
2. Endothecium:
   • The endothecium is located below the epidermis.
   • It consists of cells that expand radially with fibrous thickenings.
   • At maturity, these cells lose water and contract, contributing to the opening of the pollen sacs during dehiscence.
3. Middle Layers:
   • Below the endothecium, there are 1 to 5 layers of thin-walled cells forming the middle layers of the microsporangium.
   • These cells also play a role in the dehiscence of the anther, allowing the release of pollen grains.
4. Tapetum:
   • The innermost wall layer of the microsporangium is the tapetum.
   • It surrounds the sporogenous tissue, which gives rise to the microspores.
   • Tapetal cells are large, thin-walled, and multinucleate.
   • One of the main functions of the tapetum is to nourish and support the development of pollen grains, providing essential nutrients for their maturation.

Summary

The coordinated actions of these wall layers are crucial for the proper development, maturation, and release of pollen grains during the reproductive process of angiosperms. The mature pollen grains, after being released from the microsporangium, are carried by various agents, 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, also known as flowering plants, is a complex process that involves the fusion of male and female gametes. This process is crucial for the formation of seeds and the continuation of the species.

Process of Fertilization

1. Pollination:
   • The process begins with pollination, where pollen grains are transferred from the anther (male part) to the stigma (female part) of a flower.
   • Pollination can occur through various agents like wind, insects, or birds.
2. Germination of Pollen:
   • After landing on the stigma, the pollen grain germinates.
   • It produces a pollen tube that grows down through the style towards the ovary.
3. Growth of Pollen Tube:
   • The pollen tube carries the male gametes (sperm cells).
   • It navigates through the style by following chemical signals emitted by the ovary.
4. Entry into Ovule:
   • The pollen tube enters the ovule through the micropyle.
   • The ovule contains the female gametophyte or embryo sac.
5. Double Fertilization:
   • Angiosperms undergo a unique process called double fertilization.
   • One sperm cell fuses with the egg cell to form a zygote (2n), initiating the development of a new plant.
   • The other sperm cell fuses with the central cell (which contains two polar nuclei) to form a triploid cell (3n). This cell develops into the endosperm, providing nutrients to the developing embryo.
6. Formation of Seed and Fruit:
   • Following fertilization, the ovule develops into a seed.
   • The ovary grows into a fruit, which encloses and protects the seed(s).

Summary

Fertilization in angiosperms is a vital process that ensures the production of seeds for the next generation of plants. This process includes pollination, germination of pollen, growth of the pollen tube, entry into the ovule, double fertilization, and the formation of seed and fruit. The unique feature of double fertilization, where one sperm cell fertilizes the egg and another fertilizes the central cell, is characteristic of angiosperms.