Reproduction - Sexual Reproduction In Flowering Plants: Development of Embryo Sac

Sexual reproduction is the method through which most plants reproduce.
It involves the formation of male and female reproductive structures.
In flowering plants, the female reproductive structure is the pistil, and the male reproductive structure is the stamen.
The development of the embryo sac is an essential part of the female reproductive process.
Let’s understand the steps involved in the development of the embryo sac in flowering plants.

Step 1: Megaspore Mother Cell

The first step in the development of the embryo sac is the formation of the megaspore mother cell.
The megaspore mother cell is a type of diploid cell present within the ovule of the pistil.
It undergoes meiosis to form four haploid cells called megaspores.

Step 2: Megaspore Formation

Out of the four megaspores formed, only one develops into the embryo sac.
The other three megaspores degenerate.
The surviving megaspore undergoes further divisions to form the embryo sac.
These divisions occur without cytokinesis, resulting in the formation of a multinucleate structure called the embryo sac.

Step 3: Formation of the Embryo Sac

The multinucleate embryo sac consists of several nuclei enclosed within a single cytoplasmic membrane.
The nuclei in the embryo sac are arranged into three regions:
1. The chalazal end, which is the basal part of the embryo sac.
2. The micropylar end, which is the apical part of the embryo sac.
3. The central cell, which is located between the chalazal and micropylar ends.

Step 4: Synergids and Antipodal Cells

In addition to the central cell, the embryo sac also contains two synergids and three antipodal cells.
The synergids are located near the micropylar end and play a role in guiding the pollen tube towards the embryo sac.
The antipodal cells are located near the chalazal end and their function is still not fully understood.

Step 5: The Egg Cell

The central cell in the embryo sac contains two polar nuclei.
The polar nuclei fuse with one of the two male nuclei in the pollen tube to form the primary endosperm nucleus.
The egg cell is present at the micropylar end of the embryo sac.
It is the female gamete and is involved in fertilization with the male gamete.

Step 6: Synergid Degeneration

After the pollen tube enters the embryo sac, the synergids degenerate.
This step ensures that only one male gamete fuses with the egg cell.
The degeneration of the synergids also helps in preventing polyspermy, which could lead to abnormal development of the embryo.

Step 7: Double Fertilization

Double fertilization is a unique characteristic of flowering plants.
One male nucleus fuses with the egg cell to form the zygote, which develops into the embryo.
The other male nucleus fuses with the two polar nuclei to form the triploid endosperm.
The endosperm provides nourishment to the developing embryo.

Step 8: Embryo Development

After fertilization, the zygote undergoes divisions to form an embryo.
The embryo develops into various tissues and structures like the shoot and root systems.
The endosperm also develops and provides nutrients for the growing embryo.
The ovule, which develops into a seed, contains the embryo and endosperm.

Step 9: Seed Formation

After fertilization and embryo development, the ovule matures into a seed.
The seed contains the embryo, endosperm, and protective seed coat.
It is the reproductive structure that ensures the dispersal and survival of the plant species.
Seeds can remain dormant until favorable conditions for germination are present.

Step 10: Germination

Germination is the process by which a seed begins to grow into a new plant.
It involves the emergence of the embryo from the seed coat and the development of a root and shoot system.
Factors like water, temperature, and oxygen influence the germination process.
Once the seed germinates, the plant starts its life cycle and continues the process of sexual reproduction.

Slide 11: Components of the Embryo Sac

The embryo sac is composed of several important components:
- Polar nuclei: Two nuclei in the central cell that fuse with a male nucleus during double fertilization to form the endosperm.
- Egg cell: Female gamete located at the micropylar end, involved in fertilization with the male gamete.
- Synergids: Located near the micropylar end, they play a role in guiding the pollen tube towards the embryo sac.
- Antipodal cells: Located near the chalazal end, their function is still not fully understood.

Slide 12: Development of the Endosperm

Endosperm development occurs through double fertilization:
- One male nucleus fuses with the polar nuclei to form the primary endosperm nucleus.
- The primary endosperm nucleus undergoes several rounds of mitotic division to form the endosperm tissue.
- The endosperm provides nourishment to the developing embryo.
Examples of seeds with endosperm: Corn, coconut, and rice.

Slide 13: Role of the Synergids

The synergids play a crucial role in ensuring the successful fertilization process:
- They secrete a chemical attractant to guide the pollen tube towards the embryo sac.
- The pollen tube grows between the synergids until it reaches the micropyle.
- This guidance is essential for achieving fertilization and preventing abnormal development.
The degeneration of the synergids after successful pollen tube guidance prevents polyspermy.

Slide 14: Importance of Double Fertilization

Double fertilization has several important implications for flowering plants:
- It ensures efficient use of resources by producing both embryo and endosperm tissues.
- The endosperm provides nourishment to the developing embryo, promoting its growth and development.
- This mechanism gives flowering plants a reproductive advantage over other plant groups.

Slide 15: Significance of Endosperm

The endosperm has multiple roles in flowering plants:
- It provides a source of nutrients and energy for the developing embryo.
- The endosperm tissue is rich in starch, proteins, and other essential substances.
- Some plants absorb the endosperm completely, while others store it as part of the seed for future use.
- Endosperm also serves as a reserve tissue for seed germination.

Slide 16: Seed Development

After successful fertilization, the ovule develops into a seed:
- The outer layer of the ovule becomes the seed coat, which provides protection to the embryonic plant.
- The embryo, including the shoot and root systems, develops inside the seed.
- The endosperm, if present, also develops and provides nourishment to the embryo.

Slide 17: Structure of a Seed

A typical seed has the following three main parts:
1. Seed coat: The protective outer layer that encloses the embryo and endosperm.
2. Embryo: The young plant in its early stage of development, consisting of the radicle, plumule, and cotyledons.
3. Endosperm: A storage tissue rich in nutrients, providing nourishment for the developing embryo.

Slide 18: Importance of Seeds

Seeds play a crucial role in the life cycle of flowering plants:
- They ensure the dispersal and survival of the plant species.
- Seeds can be dispersed by wind, water, animals, or other means.
- They can remain dormant until favorable conditions for germination are present.
- Seeds have the ability to withstand periods of unfavorable climate or environmental conditions.

Slide 19: Germination Process

Germination is the process by which a seed starts to grow into a new plant:
- It involves the activation of the embryo and the emergence of the radicle, followed by the shoot system.
- The radicle develops into the primary root, providing anchorage to the growing plant.
- The shoot system includes the plumule, which develops into leaves and stem.
- Germination is influenced by factors like water, temperature, and oxygen availability.

Slide 20: Germination Examples

Various examples of plant seeds that undergo germination:
- Bean seeds: Germinate when provided with water, warmth, and suitable soil conditions.
- Mustard seeds: Require moist soil and a temperature range of 15-30°C for germination.
- Coconut seeds: Can germinate after being carried by currents and deposited on an island or shore.
Different plants have specific germination requirements based on their ecological niches.

Slide 21: Pollination and Fertilization

Pollination is the transfer of pollen from the anther to the stigma of a flower.
Fertilization is the fusion of the male and female gametes to form a zygote.
In flowering plants, both pollination and fertilization are necessary for successful seed production.
Pollination can occur through various means like wind, water, or by animals.
Fertilization involves the fusion of the male gamete from the pollen with the female gamete in the embryo sac.

Slide 22: Types of Pollination

Self-pollination: Occurs when the pollen from the anther of a flower is transferred to the stigma of the same flower or another flower on the same plant.
Cross-pollination: Occurs when pollen is transferred from the anther of one flower to the stigma of a flower on a different plant of the same species.
Cross-pollination promotes genetic variability, whereas self-pollination maintains the characteristics of the parent plant.

Slide 23: Agents of Pollination

Wind pollination: Involves the transfer of pollen grains through air currents. Examples: Grasses, corn, and conifers.
Insect pollination: Insects like bees, butterflies, and moths are attracted to flowers for nectar and inadvertently transfer pollen from one flower to another.
Animal pollination: Birds, bats, and some mammals also play a role in pollination by transferring pollen.
Water pollination: Occurs in aquatic plants where pollen is dispersed through water currents.

Slide 24: Steps of Pollen Tube Formation

Pollination: Pollen grains are transferred to the stigma.
Germination: The pollen grain germinates on the stigma and forms a pollen tube.
Growth: The pollen tube grows down through the style towards the ovule.
Penetration: The pollen tube enters the ovule through the micropyle.
Fertilization: The male gamete from the pollen tube fuses with the female gamete in the embryo sac.

Slide 25: Significance of Pollen Tube

The pollen tube plays a crucial role in delivering the male gamete to the embryo sac:
- It provides a direct route for the male gamete to reach the female gamete, ensuring fertilization.
- The pollen tube grows through the style towards the ovule, guided by chemical signals from the ovule and synergids.
- The growth of the pollen tube is a tightly regulated process, governed by various molecular and cellular mechanisms.

Slide 26: The Embryo Sac and Fertilization

The embryo sac is a specialized structure within the ovule of a flowering plant.
It contains the female gamete, or egg cell, which is involved in fertilization.
Fertilization occurs when the male gamete from the pollen tube fuses with the egg cell in the embryo sac.
This fusion results in the formation of a zygote, which develops into an embryo.

Slide 27: Mechanism of Double Fertilization

Double fertilization is a unique characteristic of flowering plants.
It involves the fusion of two male gametes with two female gametes within the embryo sac:
1. One male gamete fuses with the egg cell to form the diploid zygote.
2. The other male gamete fuses with the two polar nuclei to form the triploid endosperm.
Double fertilization ensures the coordinated development of the embryo and endosperm.

Slide 28: Importance of Double Fertilization

Double fertilization has several advantages for flowering plants:
- It prevents wastage of resources by ensuring that both the embryo and endosperm develop only when fertilization is successful.
- The endosperm provides nourishment to the growing embryo, promoting its growth and development.
- The coordinated development of both embryo and endosperm optimizes the reproductive success of flowering plants.

Slide 29: Examples of Double Fertilization

Double fertilization occurs in various flowering plants:
- Wheat: The embryo sac of wheat contains two polar nuclei, which fuse with a male nucleus to form the endosperm.
- Pea plants: Double fertilization occurs in pea plants, resulting in the formation of a zygote and endosperm.
- Orchids: Orchids also undergo double fertilization, leading to the development of an embryo and endosperm.
These examples illustrate the widespread occurrence and significance of double fertilization in flowering plants.

Slide 30: Recap and Summary

Sexual reproduction in flowering plants involves the development of the embryo sac and subsequent fertilization.
The embryo sac is a specialized structure within the ovule, containing the female gamete.
The pollen tube plays a crucial role in delivering the male gamete to the embryo sac.
Double fertilization is a unique feature of flowering plants, resulting in the formation of both embryo and endosperm.
Double fertilization ensures efficient use of resources and promotes the reproductive success of flowering plants.