Embryo Sac

Embryo Sac is a type of structure that is found in an ovule of a female flowering plant. An ovule is the part of the female flowering plant reproductive organ known as the pistil or carpel, and that ovule is present in the ovary of the female flower. Megasporogenesis and megagametogenesis is a process through which Embryo Sac forms inside the ovule. Embryo Sac is known to be important for the reproduction of female flowers. The function of an Embryo Sac is to do fertilization when male pollen approaches the female ovule and forms a pollen tube that initiates male sperm cells to reach an Embryo Sac.

Definition of Embryo Sac

Embryo Sac is structure which is present in female flower ovule and where sexual reproduction occur after fusing male sperm cell and female ovule.

What is an Embryo Sac?

A seed plant generates female germ cells in a sac with thin walls called an Embryo Sac which is also known as gametophyte. The structure of an Embryo Sac composed of three main parts known as egg apparatus, central cell, polar nuclei and antipodal cells. Seed coat development arises from surrounding tissues, and the fertilised egg grows into an Embryo that finally produces the seed.

Structure of an Embryo Sac

The structure of an Embryo Sac is composed of seven cells and eight nuclei. Mature Embryo Sac is also known as female gametophyte. An Embryo Sac is made up of egg apparatus, central cell, polar nuclei and antipodal cells.

1. Egg Apparatus: It has two components an egg cell and two synergids. An egg cell is also known as female ovum and are present at the end of an Embryo Sac. Synergids are the cells which aids in flanking of the egg cell.
2. Central cell: As their name suggest these are present in the middle of the an Embryo Sac and has two nuclei known as polar nuclei.
3. Antipodal cells: They might provide structural support to an Embryo Sac and are present just opposite to the egg apparatus.
4. Polar nuclei: These are present in central cell and fuse with sperm cell at the time of fertilization.

Types of Embryo Sac

The Embryo Sac type is basically the numbers of cell in the Embryo Sac of female flower. There two types of Embryo Sac including

Monosporic Embryo Sac

• It has four megaspores.
• After mitotic division megaspores form seven cells and eight nuclei Embryo Sac.
• Every mature monosporic Embryo Sac have egg apparatus, central cell and antipodal cells

Bisporic Embryo Sac

• It also has four megaspores.
• In bisporic Embryo Sac each two megaspores goes into mitosis division and forms a different structure known as synergistic Embryo Sac and vegetative Embryo Sac.
• Synergistic Embryo Sac take part in fertilization and have egg apparatus and central cell.
• While vegetative Embryo Sac does not take part in fertilization.

Formation of an Embryo Sac

There are two processes known as megasporogensis and megagametogenesis through which an Embryo Sac form. Below is the overview of complete process.

Megasporogenesis

1. This process occurs within the part of flower carpel known as ovule.
2. In ovule, there is diploid cell present known as megasporocyte which go through meiosis and form four large haploid cells known as megaspores.
3. In four megaspores three haploids are degenerative only one is functional.

Megagametogenesis

1. The one functional haploid megaspore produces numerous cells through mitotic division.
2. And these numerous cells form a mature Embryo Sac.
3. The structure of an Embryo Sac might be different depending upon the species of the plant.
4. The mature Embryo Sac is having egg appratus, central cell and antipodal cell.

Fertilization within the Embryo Sac

1. A pollen tube is formed when pollen settles on the flower’s stigma.
2. This pollen tube forms a route towards ovule and reaches to the Embryo Sac.
3. Then male sperm cell fuse with egg cell and forms a diploid zygote.
4. There are other sperm cells also which start fusin with polar nuclei and forms a triploid cell.
5. Triploid cell mature into endopserm which provide nutrition to the developing Embryo.

Function of an Embryo Sac

For flowering plants to reproduce sexually, the embryo sac is an essential component. Among its functions are

1. Egg cells formation: Egg cells are produced within the Embryo Sac and these egg cells are crucial for zygote formation by fusing with male sperm cells.
2. Center of Fertilization: Fertilization is process in which male and female reproductive cells fuse together to form a zygote which develops into a mature Embryo.
3. Endosperm formation: Endosperm is form as a second fertilization of sperm cell with polar nuclei that results into triploid cell and that matures into endosperm. This endosperm provides nutrition to the developing Embryo.
4. Development of seeds: Ater fertilization, there is coat formation around seed which having Embryo and endosperm that leads to the formation of complete plant.

FAQs- Embryo Sac

1. What is the Meaning of Embryonic Sac?

The entity responsible for generating female germ cells in the sexually reproductive phase of a seed plant is an individual structure characterized by a delicate, membranous Sac. This sac houses the egg nucleus and additional nuclei, collectively forming tissue that serves as a nutrient source upon fertilization.

2. What is seed Embryo Sac?

A rounded structure nestled within the ovule of an angiosperm, the Embryo Sac cradles the egg. In concert with the fertilized egg, it matures into a seed.

3. Why is an Embryo Sac Female?

The female gametophyte, known as the Embryo Sac (FG), is responsible for producing female gametes. Typically concealed within the maternal, sporophytic ovule, the fusion of male and female gametes takes place during double fertilization. This crucial event unfolds after the delivery of sperm cells to the Embryo Sac through the pollen tube.

4. Where is Embryo Sac Located?

The Embryo Sac, nestled within a plant ovule, cradles the essential egg cell. Originating from the megaspore, this structure evolves to encompass both the embryonic plant and the endosperm post-fertilization.

5. What is main Difference between Embryo sac or endosperm?

The primary distinction between Embryo and endosperm lies in their roles: the Embryo signifies the result of fertilization, while the endosperm serves as the nutritive tissue within the seed. The ovule is the origin and housing for the Embryo Sac, forming from the primary endosperm nucleus.