Embryo Sac – Structure, Types, Development, Function

What is an Embryo Sac?

  • The embryo sac, also referred to as the female gametophyte, plays a critical role in the reproductive process of flowering plants. This structure, located within the ovule, is formed from the haploid megaspore nucleus through a series of cellular divisions. The embryo sac is an oval, thin-walled structure that is essential for the development of the seed following fertilization.
  • In a typical flowering plant, the embryo sac develops through the Polygonum type, which is the most common form of development. During this process, one of the four megaspores produced by meiosis survives, while the others degenerate. This surviving megaspore undergoes three successive rounds of mitosis, resulting in an embryo sac with eight nuclei. These eight nuclei eventually organize into seven distinct cells, forming the mature embryo sac.
  • Among the cells within the embryo sac, there are two critical components known as the polar nuclei, which often fuse to form the central cell or the primary endosperm nucleus. This fusion is a preparatory step for fertilization. During fertilization, one male nucleus fuses with the egg cell to form a zygote, which will develop into the embryo. Simultaneously, the second male nucleus fuses with the primary endosperm nucleus to form the endosperm, a tissue that provides nourishment to the developing embryo.
  • The embryo sac, therefore, is central to the formation of both the embryo and the endosperm, highlighting its importance in the reproductive cycle of seed plants. The intricate structure and function of the embryo sac ensure the successful development of seeds and, consequently, the propagation of the plant species.

Definition of Embryo Sac

The embryo sac is the female gametophyte in flowering plants, located within the ovule, where fertilization occurs, leading to the development of the seed and endosperm.

Structure of Embryo Sac

The structure of the embryo sac, also known as the mature female gametophyte, is integral to the reproductive process in flowering plants. The embryo sac is a highly organized structure composed of seven cells and eight nuclei, each serving a specific role in fertilization and subsequent seed development. Below is a detailed breakdown of Structure of Embryo Sacs:

  1. Egg Apparatus:
    • Egg Cell: Located at the micropylar end of the embryo sac, the egg cell is the female gamete. It is crucial for fertilization, as it fuses with the male sperm cell to form the zygote, which will eventually develop into the embryo.
    • Synergids: Two synergid cells flank the egg cell. These cells play an essential role in guiding the pollen tube into the embryo sac during fertilization, ensuring the proper delivery of sperm cells.
  2. Central Cell:
    • The central cell occupies the middle region of the embryo sac and contains two nuclei known as polar nuclei. These polar nuclei are critical during double fertilization, where they fuse with one of the sperm cells to form the triploid endosperm, which provides nourishment to the developing embryo.
  3. Antipodal Cells:
    • There are three antipodal cells located at the chalazal end, opposite the egg apparatus. These cells might assist in nutrient absorption and provide structural support, although they often degenerate after fertilization.
  4. Polar Nuclei:
    • The two polar nuclei reside within the central cell. During fertilization, these nuclei merge with one of the sperm cells to form the diploid secondary nucleus, which gives rise to the endosperm.
  5. Overall Organization:
    • The embryo sac is polarized along its chalazal-micropylar axis. This polarity is essential for the functional differentiation of the cells, ensuring successful fertilization and subsequent seed development.
  6. Cell Ploidy:
    • All the cells within the embryo sac, except for the central cell, are haploid. The central cell becomes diploid after the fusion of the two polar nuclei.

Type of embryo sac (female gametophyte)

Embryo sacs, or female gametophytes, can be classified into three main types based on the number of megaspores involved in their formation, the number of nuclear divisions that occur, and the organization of nuclei within the mature structure. Below is a detailed explanation of each type:

  1. Monosporic Embryo Sac
    • Basis for Classification: This type involves only one megaspore in the formation of the embryo sac.
    • Development: The functional megaspore undergoes three rounds of mitotic divisions, resulting in an eight-nucleate structure.
    • Organization: The mature embryo sac consists of seven cells, including the egg apparatus, central cell with polar nuclei, and antipodal cells. A common example is the Polygonum type, which is the most prevalent among flowering plants.
  2. Bisporic Embryo Sac
    • Basis for Classification: Two megaspores participate in the development of the embryo sac.
    • Development: After meiosis, two of the four nuclei fuse, leading to the formation of a bisporic embryo sac. This fusion results in a diploid nucleus, which undergoes subsequent divisions to form the embryo sac.
    • Organization: The final structure typically consists of eight nuclei arranged in a way similar to the monosporic type, though the origin of these nuclei differs due to the bisporic nature.
  3. Tetrasporic Embryo Sac
    • Basis for Classification: All four megaspores formed during meiosis contribute to the development of the embryo sac.
    • Development: No cell walls are formed after meiosis, allowing all four nuclei to participate in subsequent mitotic divisions. This results in a structure with multiple nuclei, often leading to a more complex organization.
    • Organization: The mature tetrasporic embryo sac may contain more than eight nuclei and can have varying arrangements depending on the species. The complexity of this type often leads to diverse forms of fertilization and seed development.

1. Monosporic Embryo Sac

The monosporic embryo sac represents a type of female gametophyte development that originates from a single functional megaspore. As a result, all the nuclei within this embryo sac are genetically identical. This type is fundamental to the reproductive process in many flowering plants, ensuring uniformity in the genetic material passed on to the next generation. Below are the key types of monosporic embryo sacs:

Polygonum type of Embryo Sac
Polygonum type of Embryo Sac
  1. Polygonum Type:
    • Nuclear Count: The Polygonum type embryo sac is characterized by eight nuclei.
    • Prevalence: It is the most common type, found in approximately 81% of plant families.
    • Discovery: First described in Polygonum divaricatum by Strasburger in 1879.
    • Developmental Process:
      • Megasporogenesis: The process begins with the development of the embryo sac from the chalazal megaspore, the one closest to the base of the ovule.
      • Megagametogenesis: The nucleus of this megaspore undergoes three rounds of mitotic division, resulting in an eight-nucleate embryo sac. These nuclei then organize into a typical structure, with one egg cell, two synergids, three antipodal cells, and a central cell containing two polar nuclei.
  2. Oenothera Type:
    • Nuclear Count: The Oenothera type has a four-nucleate embryo sac.
    • Cellular Composition: It includes an egg apparatus consisting of three cells (one egg cell and two synergids) and a central cell with a single polar nucleus.
    • Developmental Process:
      • Origin: This embryo sac develops from the micropylar megaspore, the one closest to the opening of the ovule.
      • Unique Features: Notably, the Oenothera type lacks antipodal cells, a deviation from the typical structure seen in the Polygonum type.
    • Example: This type is commonly found in the Onagraceae family.
Oenothera type
Oenothera type

2. Bisporic Embryo Sac

The bisporic embryo sac is a unique form of female gametophyte development in flowering plants, characterized by its origin from two nuclei of a functional dyad formed after the first meiotic division. Below are the key features and types of bisporic embryo sacs:

Development of bisporic embryo sac
Development of bisporic embryo sac
  1. Development Process:
    • Origin: The bisporic embryo sac develops from one of the two dyads produced after the first meiotic division (Meiosis I) of the megaspore mother cell.
    • Nuclear Participation: Both nuclei within the functional dyad contribute to the formation of the embryo sac.
    • Mitotic Divisions: Each nucleus undergoes two mitotic divisions, leading to the formation of an eight-nucleate embryo sac.
    • Nuclear Organization: The eight nuclei within the mature embryo sac are arranged into three antipodal cells, an egg apparatus (comprising one egg cell and two synergids), and two polar nuclei, similar to the organization in the Polygonum type embryo sac.
    • Genetic Diversity: The four nuclei derived from one of the dyad’s megaspore nuclei are genetically distinct from the four derived from the other, contributing to genetic variation within the embryo sac.
  2. Types of Bisporic Embryo Sacs:
    • Allium Type:
      • Dyad Origin: The Allium type develops from the chalazal dyad, which is the one closer to the base of the ovule.
      • Significance: It is commonly observed in species like Allium, where the chalazal megaspore plays a crucial role in embryo sac formation.
    • Endymion Type:
      • Dyad Origin: The Endymion type develops from the micropylar dyad, located near the opening of the ovule.
      • Characteristics: This type is less common and is typically found in specific plant species where the micropylar dyad is more functionally active in gametophyte development.

3. Tetrasporic Embryo Sac

The tetrasporic embryo sac represents a distinct type of female gametophyte formation in flowering plants, characterized by its development from a coeno-megaspore, where the initial meiotic division does not involve cytokinesis. As a result, the four haploid nuclei from the megaspore mother cell remain within a single cell. Below are detailed insights into the structure and types of tetrasporic embryo sacs:

  1. General Characteristics:
    • Coeno-megaspore Formation: Unlike other types, the tetrasporic embryo sac originates from a coeno-megaspore where all four haploid nuclei are contained within one cell.
    • Nuclear Participation: All four nuclei contribute to the development of the embryo sac.
    • Genetic Diversity: This type is generally more genetically heterogeneous compared to the bisporic embryo sac due to the involvement of multiple nuclei.
  2. Development Process:
    • Meiotic Division: The initial meiotic division of the megaspore mother cell occurs without cytokinesis, resulting in a coeno-megaspore containing all four haploid nuclei within a single cell.
    • Nuclear Participation: All four nuclei participate in the formation of the embryo sac.
    • Genetic Diversity: The tetrasporic embryo sac is genetically more heterogeneous compared to bisporic types due to the presence of multiple haploid nuclei.
  3. Types of Tetrasporic Embryo Sacs:
    • Adoxa Type:
      • Nuclear Arrangement: This type features eight nuclei, resulting from the mitotic division of the four haploid nuclei of the coeno-megaspore.
      • Organization: The arrangement of these nuclei resembles that of the Polygonum type embryo sac.
      • Examples: Adoxa, Sambucus, Ulmus, Tulipa, Erythronium.
    • Plumbago Type:
      • Characteristics: Distinguished by the absence of synergids and antipodals.
      • Nuclear Distribution: One nucleus migrates to the micropylar end, one to the chalazal end, and two to the lateral sides. These nuclei then divide to form four groups of two nuclei each.
      • Formation: One nucleus from each group moves to the center to form polar nuclei. The micropylar nucleus becomes the egg cell, while remaining nuclei may form accessory egg cells.
      • Examples: Plumbaginaceae family.
    • Penaea Type:
      • Nuclear Divisions: The four haploid nuclei undergo two successive mitotic divisions, resulting in sixteen nuclei.
      • Organization: These nuclei are organized into four groups of four, with one group at each end and two lateral groups. One nucleus from each group migrates to the center, forming polar nuclei. The three nuclei at the micropylar end form the egg apparatus.
      • Polyploidy: A highly polyploid primary endosperm nucleus is formed post-fertilization.
      • Examples: Penaeaceae, Malpighiaceae, Euphorbiaceae.
    • Peperomia Type:
      • Egg Apparatus: Characterized by a single synergid.
      • Nuclear Divisions: The coeno-megaspore’s four nuclei undergo two mitotic divisions, forming sixteen nuclei. Two nuclei at the micropylar end form the egg and synergid, while eight fuse in the center to form a polar nucleus, and six at the chalazal end form antipodals.
      • Examples: Peperomia, Gunnera.
    • Drusa Type:
      • Nuclear Organization: Features sixteen nuclei with a high number of antipodal cells.
      • Structure: Three nuclei form the egg apparatus, two act as polar nuclei, and eleven nuclei are segregated into antipodal cells.
      • Variation: The number and organization of nuclei can vary due to irregular divisions.
      • Examples: Drusa, Rubia, Chrysanthemum, Ulmus.
    • Fritillaria Type:
      • Nuclear Groups: The four haploid nuclei organize into two groups—three at the chalazal end forming a triploid nucleus and one haploid at the micropylar end.
      • Divisions: Both the triploid and haploid nuclei undergo mitotic divisions, forming a specific arrangement of nuclei.
      • Organization: Three haploid nuclei form the egg apparatus, three triploid nuclei become antipodal cells, and the remaining nuclei fuse to form a tetraploid polar nucleus.
      • Examples: Fritillaria, Lilium, Piper, Gaillardia.
    • Plumbagella Type:
      • Initial Development: Similar to Fritillaria type, involving a triploid nucleus at the chalazal end and a haploid nucleus at the micropylar end.
      • Divisions: Each nucleus undergoes a single mitotic division, forming two groups of two nuclei each.
      • Formation: A triploid nucleus and a haploid nucleus fuse at the center to form a tetraploid polar nucleus. The micropylar nucleus forms the egg, and a chalazal nucleus becomes an antipodal cell. Synergids are absent.
      • Examples: Specific plants exhibiting this type are less common but include certain species within the same families as Fritillaria.
ADOXA TYPE
ADOXA TYPE
PLUMBAGO TYPE
PLUMBAGO TYPE
PENAEA TYPE
PENAEA TYPE
PEPEROMIA TYPE
PEPEROMIA TYPE
DRUSA TYPE
DRUSA TYPE
FRITILLARIA TYPE
FRITILLARIA TYPE
PLUMBAGELLA TYPE
PLUMBAGELLA TYPE

Formation of an Embryo Sac

Here is the step by step process for Formation of an Embryo Sac:

1. Megasporogenesis

  • Location: Megasporogenesis occurs within the ovule of a flower’s carpel. The ovule is the structure where the female gametophyte will develop.
  • Initial Cell: Inside the ovule, a diploid cell called the megasporocyte (or megaspore mother cell) initiates the process.
  • Meiosis: The megasporocyte undergoes meiosis, a type of cell division that reduces the chromosome number by half, resulting in four haploid cells known as megaspores.
  • Functional Megaspore: Of the four megaspores produced, typically three degenerate and only one remains functional. This functional megaspore will continue to develop into the embryo sac.

2. Megagametogenesis

  • Mitotic Divisions: The functional haploid megaspore undergoes several rounds of mitotic divisions. This process involves:
    • First Division: The megaspore undergoes mitosis to form two nuclei within the cell.
    • Second Division: Each of these nuclei divides again, resulting in four nuclei.
    • Third Division: The nuclei continue to divide, leading to the formation of eight nuclei.
  • Organization of Nuclei: These eight nuclei are arranged and organized to form the mature embryo sac. The final organization includes:
    • Egg Apparatus: Comprising the egg cell and typically two synergid cells, located at the micropylar end of the embryo sac.
    • Central Cell: Contains two polar nuclei, which will eventually fuse with a sperm cell to form the triploid endosperm.
    • Antipodal Cells: Positioned at the chalazal end of the embryo sac, these cells usually consist of three nuclei.
  • Variation by Species: The structure of the mature embryo sac can vary significantly depending on the plant species. While the general organization is consistent, the number and arrangement of cells can differ among different types of plants.

Cellular Anatomy of the Mature Embryo Sac

Diagram of Embryo Sac
Diagram of Embryo Sac

The mature embryo sac, particularly in the Polygonum-type development, is a highly specialized structure within the ovule, composed of distinct cellular components that play crucial roles in fertilization, embryogenesis, and nutrition. Here is a detailed examination of the cellular anatomy of the mature embryo sac:

  1. Egg Cell
    • Location and Function:
      • Positioned at the micropylar end of the embryo sac, adjacent to the synergids.
      • It is the female gamete that will fuse with a sperm cell to form a zygote.
    • Structural Features:
      • Vacuole: A large vacuole located at the micropylar end creates a polarized distribution of the cytoplasm. This setup restricts the nucleus and the majority of the cytoplasm to the chalazal end of the cell.
      • Cytoplasm: The distribution and organization of the cytoplasm contribute to the egg cell’s readiness for fertilization.
  2. Synergids
    • Location and Function:
      • Two synergid cells are situated on either side of the egg cell.
      • They play a critical role in facilitating fertilization by interacting with the pollen tube.
    • Interaction with Pollen Tube:
      • The pollen tube releases its contents into one of the synergids, allowing the sperm nuclei to enter the egg cell and central cell.
      • This process is essential for the successful fusion of gametes and formation of the zygote.
  3. Central Cell
    • Location and Function:
      • Located centrally within the embryo sac.
      • Contains two polar nuclei that migrate from both ends of the coenocytic megagametophyte and eventually merge.
    • Structure:
      • Nuclei: The polar nuclei may partially fuse before the fertilization event.
      • Vacuole and Organelles: The central cell contains a large vacuole and numerous organelles, which support the development of the triploid endosperm nucleus after fertilization.
      • Endosperm Formation: Upon fertilization by a sperm nucleus, the central cell produces the triploid primary endosperm nucleus, which will mature into endosperm, providing nourishment to the developing embryo or seedling.
  4. Antipodal Cells
    • Location and Function:
      • Three antipodal cells are situated at the chalazal end of the embryo sac, opposite the egg cell.
      • Their exact role during reproduction is not well-defined; however, they are believed to be involved in nutrient transfer to the embryo sac.
    • Metabolic Activity:
      • Antipodal cells exhibit significant metabolic activity, as indicated by the presence of ribosomes and mitochondria.
  5. Cytological Characteristics
    • Embryo Sac Cells:
      • Synergids: Rich in ribosomes and mitochondria, indicating high metabolic activity.
      • Central Cell: Contains a dense array of organelles and is central to endosperm development.
      • Egg Cell: Relatively quiescent with fewer organelles, focused on its role in fertilization.
  6. Research and Future Directions
    • Current Knowledge:
      • The study of ovule and embryo sac development is ongoing. There is limited data on the regulatory mechanisms and gene expression patterns governing these processes.
    • Areas of Interest:
      • Understanding the overlap between the genetic programs of the female gametophyte and the sporophyte.
      • Investigating the expression of gametophyte-specific genes and regulatory factors influencing embryo sac development.

Embryo Sac Development Process

Embryo sac development, a critical process in plant reproduction, encompasses two main stages: megasporogenesis and megagametogenesis. These stages culminate in the formation of the mature embryo sac, a highly specialized structure essential for fertilization and subsequent seed development. This process exhibits notable variation among different plant species, reflecting diverse evolutionary strategies and developmental patterns.

  1. Megasporogenesis
    • Definition and Process:
      • Megasporogenesis refers to the formation of megaspores from a diploid megasporocyte through meiosis.
    • Meiotic Division:
      • The megasporocyte undergoes meiosis, resulting in the production of four megaspore nuclei.
      • These nuclei are arranged in a dyad (two cells) after the first meiotic division, followed by a transverse wall formation.
      • In many species, one of the dyad cells degenerates, leaving three megaspores.
    • Patterns of Megaspore Arrangement:
      • Linear Arrangement: Commonly observed, where the megaspore closest to the chalaza enlarges and undergoes further mitotic divisions, while the remaining megaspores degenerate.
      • Tetrahedral and T-shaped Arrangements: Seen in some species like Arabidopsis and maize, respectively.
  2. Callose Deposition
    • Function:
      • Callose, a β-1,3-glucan, plays a crucial role in the selection of the functional megaspore.
    • Accumulative Patterns:
      • During megasporogenesis, callose accumulates in the walls of the megasporocyte and megaspores.
      • After meiosis, callose is typically reduced or absent in the functional megaspore, which ensures its sole access to nutrients from the nucellus.
  3. Megagametogenesis
    • Overview:
      • This stage involves the transformation of the functional megaspore into a mature embryo sac through a series of mitotic divisions and cellular differentiation.
    • Stages of Division:
      • First Mitotic Division: The functional megaspore enlarges and undergoes three free nuclear divisions, resulting in an eight-nucleate stage.
      • Formation of Central Vacuole: After the first mitosis, the nuclei migrate to opposite poles, and small vacuoles merge to form a large central vacuole.
      • Subsequent Divisions: Each nucleus undergoes two additional divisions, resulting in a coenocytic structure with eight nuclei. These nuclei then become organized into a mature embryo sac.
  4. Alternative Modes of Development
    • Apomixis:
      • In some species, embryo sacs can develop without typical meiosis, a process known as apomixis.
      • Mechanisms:
        • Diploid Embryo Sacs: May arise from aberrations in meiosis or directly from non-meiotic cells.
        • This alternative mode of development reveals that embryo sac formation can be independent of traditional meiotic processes.
  5. Embryo Sac Polarity
    • Importance of Polarity:
      • The embryo sac exhibits polarity along the chalazal-micropylar axis of the ovule, influencing the positioning and function of its components.
    • Distribution of Organelles:
      • Chalazal End: Rich in organelles and cytoplasmic components, which are inherited by the functional megaspore.
      • Micropylar End: Features a high concentration of plasmodesmata and plastids, affecting nutrient flow and development.
  6. Mechanisms Influencing Development
    • Nutrient and Hormonal Interactions:
      • Intercellular contacts between the embryo sac and the ovule are crucial for nutrient transfer and developmental guidance.
    • Mechanical Interactions:
      • The physical interaction between the nucellus and integuments can influence embryo sac development, particularly in terms of spatial constraints and nutrient access.
  7. Consequences for Early Embryogenesis
    • Polarity in Zygote:
      • The polarity established in the embryo sac is mirrored in the zygote and early embryo, affecting subsequent embryogenesis.
    • Impact on Patterning:
      • Asymmetric division of the zygote and uneven distribution of cytoplasmic determinants can lead to pattern defects in the embryo.
 Patterns of Embryo Sac Developmen
Patterns of Embryo Sac Developmen

Fertilization Steps within the Embryo Sac

Here is step by step process of Fertilization within the Embryo Sac;

  1. Pollen Tube Formation
    • Pollination: The process begins when pollen grains, deposited on the stigma of a flower, germinate. This germination triggers the formation of a pollen tube.
    • Tube Growth: The pollen tube elongates and grows down the style towards the ovule, which houses the embryo sac within the ovary of the flower. The tube’s primary function is to create a pathway for sperm cells to reach the embryo sac.
  2. Sperm Cell Fusion
    • Entry into Embryo Sac: Upon reaching the embryo sac, the pollen tube penetrates the sac through the micropyle, an opening at the end of the embryo sac.
    • Fertilization of Egg Cell: One of the sperm cells from the pollen tube fuses with the egg cell, resulting in the formation of a diploid zygote. This zygote will eventually develop into the embryo of the seed.
  3. Formation of Endosperm
    • Second Fertilization Event: Simultaneously, another sperm cell fuses with the two polar nuclei located in the central cell of the embryo sac. This fusion creates a triploid cell.
    • Endosperm Development: The triploid cell then differentiates into the endosperm. The endosperm is a crucial tissue that provides nourishment to the developing embryo during seed development.
  4. Developmental Outcomes
    • Embryo Formation: The diploid zygote undergoes cell divisions to form the embryo, which will later develop into a new plant.
    • Nutrient Supply: The endosperm, now fully formed, ensures the embryo receives adequate nutrients necessary for its growth and development until it can photosynthesize independently.

Function of Embryo Sac

Here is the important Function of Embryo Sac;

  • Egg Cell Formation
    • Production of Egg Cells: Within the embryo sac, egg cells are formed from a single functional megaspore. These egg cells are vital for fertilization as they fuse with male sperm cells to create a diploid zygote.
    • Role in Zygote Formation: The fusion of an egg cell with a sperm cell results in the formation of a zygote, which undergoes subsequent development to form the embryo.
  • Center of Fertilization
    • Facilitates Fertilization: The embryo sac acts as the site where fertilization occurs. Upon receiving pollen, the embryo sac enables the fusion of male and female gametes, leading to the formation of the zygote.
    • Zygote Development: Following fertilization, the zygote develops into an embryo, which is the precursor to a new plant.
  • Endosperm Formation
    • Second Fertilization Event: A second fertilization event involves the fusion of a sperm cell with two polar nuclei in the central cell of the embryo sac, forming a triploid cell.
    • Nutrient Provision: This triploid cell differentiates into endosperm, a nutrient-rich tissue that supplies essential nourishment to the developing embryo within the seed.
  • Seed Formation
    • Development of Seed Components: Post-fertilization, the embryo sac contributes to the formation of the seed. This includes the development of the embryo, endosperm, and seed coat.
    • Seed Coat Formation: The seed coat forms around the embryo and endosperm, providing protection and aiding in seed maturation.
  • Reception of Pollen
    • Pollen Reception: The mature embryo sac is responsible for receiving pollen grains. These grains contain male gametes, which are crucial for the fertilization process.
    • Pollen Tube Penetration: The pollen tube grows towards the embryo sac, ensuring that sperm cells reach the egg cells for fertilization.
  • Genetic Variation
    • Meiotic Processes: During its development, the embryo sac undergoes meiosis, which contributes to genetic diversity by shuffling genetic material.
    • Genetic Diversity: This process results in genetically varied offspring, enhancing the adaptability and evolution of plant species.
  • Protection of Developing Embryo
    • Embryo Protection: The embryo sac provides a protective environment for the developing embryo, ensuring its survival until conditions are favorable for germination.
    • Support for Germination: By maintaining a secure and nutrient-rich environment, the embryo sac plays a crucial role in seed viability and subsequent plant growth.

FAQ

What is an embryo sac?

An embryo sac is the female gametophyte within the ovule of a flowering plant. It houses the egg cell and is the site where fertilization occurs.

Why is the embryo sac important for plant reproduction?

The embryo sac is crucial for plant reproduction as it facilitates fertilization, provides nourishment to the developing embryo, and ensures the formation of seeds that will grow into new plants. It plays a central role in the continuity and genetic diversity of plant species.

What happens after fertilization in the embryo sac?

After fertilization, the zygote develops into an embryo, the endosperm nourishes the embryo, and the ovule develops into a seed. The seed is encased in a protective seed coat and is ready for germination.

What is the difference between monosporic, bisporic, and tetrasporic embryo sacs?

Monosporic embryo sacs develop from a single megaspore and are the most common. Bisporic embryo sacs develop from two megaspores, and tetrasporic embryo sacs develop from four megaspores, with specific variations in nuclear arrangement and division.

How does fertilization occur in the embryo sac?

Fertilization occurs when pollen grains land on the stigma of a flower, forming a pollen tube that grows toward the embryo sac. Sperm cells travel through this tube to fertilize the egg cell and the polar nuclei.

How does the embryo sac develop?

The embryo sac develops from a single functional megaspore after meiosis. It undergoes mitotic divisions to form a mature structure containing the egg cell, polar nuclei, and antipodal cells.

What are the different types of embryo sacs?

There are three main types: monosporic, bisporic, and tetrasporic. Each type varies based on the number of megaspores involved and the arrangement of nuclei within the sac.

What roles do antipodal cells play in the embryo sac?

Antipodal cells are located at the chalazal end of the embryo sac. They are thought to have roles in the nutrition and support of the developing embryo, although their specific functions can vary among plant species.

How is the endosperm formed in the embryo sac?

The endosperm is formed through the fusion of a sperm cell with two polar nuclei, resulting in a triploid cell. This cell develops into endosperm, which provides nourishment to the embryo.

What is the function of the egg cell in the embryo sac?

The egg cell is essential for fertilization. It fuses with a male sperm cell to form a diploid zygote, which eventually develops into an embryo.

Reference
  1. https://mgcub.ac.in/pdf/material/202004300509276c475952aa.pdf
  2. https://www.geeksforgeeks.org/embryo-sac-structure/
  3. https://www.pw.live/exams/neet/female-gametophyte/
  4. https://www.indiabiologyneet.com/2020/09/sexual-reproduction-in-flowering-plants_5.html
  5. https://www.ias.ac.in/public/Volumes/secb/001/06/0279-0282.pdf
  6. https://www.uv.mx/personal/tcarmona/files/2016/08/Reiser-y-Fisher-1993.pdf
  7. https://collegedunia.com/exams/embryo-sac-biology-articleid-8448

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