Ovule – Definition, Structure, Types, Diagram, Functions

What is Ovule?

• The ovule is a critical component of the female reproductive system in seed plants, playing an essential role in the production of seeds. Located within the ovary of the carpel, the ovule houses the female gametes and is pivotal for seed development.
• Structurally, the ovule is comprised of three main parts: the integument, the nucleus, and remnants of the megasporangium. The integument is a protective outer layer, while the nucleus contains the developing female gametophyte. The megasporangium is a tissue that supports the development of the ovule.
• Upon fertilization, the ovule undergoes a series of transformations. It begins to swell and its outer layers become more robust, preparing to develop into a seed. Concurrently, the surrounding ovary enlarges and matures into a fruit. This process varies among plant species; for example, avocados typically contain a single ovule, whereas kiwis possess multiple ovules that develop into seeds.
• In angiosperms, or flowering plants, the ovules are situated inside the ovary, which is part of the carpel structure. Gymnosperms, such as conifers, present a different arrangement where ovules are found on the scales of female cones.
• The ovule also includes the female gametophyte, known as the embryo sac in flowering plants. This gametophyte develops from a megagametophyte and is responsible for producing the egg cell necessary for fertilization. The ovule’s position and orientation can vary: it may be anatropous, where the micropyle faces inward, or exhibit other orientations like campylotropous, amphitropous, or orthotropous.
• Following fertilization, the ovule’s transformation into a seed involves the formation of a diploid zygote, which then undergoes cell division to develop into a sporophyte embryo. This process ultimately leads to the production of a new plant.

Definition of Ovule

An ovule is a small structure within the ovary of a plant that contains the female gamete (egg cell) and develops into a seed after fertilization.

Location of Ovule

The location of ovules in flowering plants is intricately tied to their role in reproduction and is defined by various patterns of attachment known as placentation. Understanding these placements is crucial for comprehending how plants develop their seeds and fruits. Here is a detailed overview of the different types of ovule placement:

1. Apical Placentation:
   • Description: In this arrangement, the placenta is situated at the apex or top of the ovary.
   • Occurrence: This can be observed in both simple and compound ovaries.
   • Function: It ensures that ovules are positioned at the uppermost part of the ovary, where they can be readily accessed for fertilization.
2. Axile Placentation:
   • Description: The ovary is divided into radial segments with placentas located in separate locules. The ventral sutures of the carpels converge at the center of the ovary, and placentas are along the fused margins of the carpels.
   • Occurrence: Found in plants with two or more carpels, such as Hibiscus, Citrus, and Solanum.
   • Function: This arrangement supports multiple ovules in separate chambers, facilitating a higher number of ovules within a single ovary.
3. Basal Placentation:
   • Description: The placenta is located at the base of the ovary, often on a protrusion of the thalamus (receptacle).
   • Occurrence: Common in simple or compound carpels and unilocular ovaries, such as Sonchus, Helianthus, and Asteraceae.
   • Function: It provides a stable base for ovule attachment, ensuring that ovules are positioned at the bottom of the ovary.
4. Free-Central Placentation:
   • Description: Derived from axile placentation, this type occurs when partitions are absorbed, leaving ovules attached to a central axis within a unilocular ovary.
   • Occurrence: Seen in compound unilocular ovaries, such as Stellaria and Dianthus.
   • Function: The central placement of ovules optimizes space within the ovary and supports central attachment.
5. Marginal Placentation:
   • Description: The simplest form of placentation, where a single elongated placenta runs along one side of the ovary, and ovules are attached at the fusion line of the carpel’s margins.
   • Occurrence: Common in legumes with simple carpels and unilocular ovaries, such as Pisum.
   • Function: Provides a straightforward arrangement for ovule attachment, facilitating easy placement and development.
6. Parietal Placentation:
   • Description: Placentae are located on the inner walls of a non-sectioned ovary, corresponding to the margins where carpels are fused.
   • Occurrence: Found in ovaries with two or more carpels and unilocular structure, such as Brassica.
   • Function: This arrangement supports ovule attachment along the inner surface of the ovary, ensuring effective nutrient transfer.
7. Superficial Placentation:
   • Description: Similar to axile placentation but occurs on the inner surfaces of a multilocular ovary.
   • Occurrence: Seen in plants like Nymphaea.
   • Function: Allows for the distribution of ovules on the surfaces of multiple locules, optimizing space within the ovary.

In gymnosperms, the placement of ovules is different:

1. Gymnosperms (e.g., Conifers):
   • Description: Ovules are borne on the surface of an ovuliferous scale, usually within an ovulate cone (megastrobilus).
   • Function: This placement supports the ovules in a structure that facilitates fertilization and seed development.
2. Early Seed Ferns:
   • Description: Ovules were borne on the surface of leaves or within a cupule, a modified branch or group of branches.
   • Function: This arrangement reflects early evolutionary adaptations for protecting and supporting ovules.

Structure of Ovule (Megasporangium)

The ovule, also known as the megasporangium, is a crucial component of the plant’s reproductive system, playing a key role in seed development. The structure of an ovule comprises several distinct parts:

1. Funiculus (Funicle):
   • Definition: A stalk-like structure that attaches the ovule to the placenta within the ovary.
   • Function: Provides physical support and facilitates nutrient transport to the ovule.
2. Hilum:
   • Definition: The point where the funiculus attaches to the ovule body.
   • Function: Serves as the junction between the ovule and the funiculus, often marked by a noticeable ridge or indentation.
3. Body of the Ovule:
   • Definition: The central mass of the ovule, consisting mainly of parenchymatous tissue.
   • Components:
     • Nucellus: The inner tissue of the ovule, rich in reserve food materials. It surrounds the embryo sac and is essential for providing nutrients to the developing embryo.
4. Integuments:
   • Definition: One or two protective layers encasing the nucellus.
   • Types:
     • Unitegmic Ovule: Possesses a single integument.
     • Bitegmic Ovule: Enclosed by two integuments.
   • Function: Protects the internal structures of the ovule and contributes to seed coat formation after fertilization. The integuments meet at the base of the ovule, forming the chalaza.
5. Micropyle:
   • Definition: A small opening at the apex of the ovule where the integuments do not completely cover the nucellus.
   • Function: Allows the pollen tube to enter the ovule for fertilization.
6. Chalaza:
   • Definition: The basal region where the nucellus, integuments, and funiculus converge.
   • Function: Acts as the connection point for the ovule’s internal tissues and is crucial for nutrient exchange.
7. Embryo Sac (Female Gametophyte):
   • Definition: A large, sac-like structure located towards the micropylar end of the nucellus.
   • Function: Develops from the functional megaspore and houses the egg cell and other components necessary for fertilization and seed development.
8. Endothelium (Integumentary Tapetum):
   • Definition: A specialized layer of the inner integument in some ovules.
   • Function: Provides nourishment to the embryo sac. This tissue is generally single-layered, with radially elongated cells containing dense cytoplasm.
9. Hypostase and Epistase:
   • Hypostase:
     • Definition: A group of cells at the base of the ovule, between the chalaza and the embryo sac.
     • Function: Acts as a boundary or barrier to limit further growth of the embryo sac.
   • Epistase:
     • Definition: Thick-walled cells located above the micropylar end of the embryo sac.
     • Function: Provides additional structural support to the embryo sac.
10. Tenuinucellate and Crassinucellate Types:
    • Tenuinucellate: Features a hypodermal sporogenous cell with a thin layer of nucellar tissue, leading to a smaller nucellus.
    • Crassinucellate: Has a subhypodermal sporogenous cell surrounded by a thicker layer of nucellar tissue, resulting in a larger nucellus.

Components of Ovule

The structure of an ovule in flowering plants is integral to the reproductive process, as it supports the development of the female gametophyte and ultimately, the seed. The ovule comprises several key components:

1. Integument: The integument is the outer layer of the ovule, often referred to as the seed coat. It serves to protect the inner tissues from damage and desiccation. Depending on the plant species, the integument may be single-layered (unitegmic) or double-layered (bitegmic).
2. Micropyle: The micropyle is a small, specialized opening in the integument. It provides a pathway for the pollen tube to enter the ovule and deliver sperm cells necessary for fertilization.
3. Nucellus: Located centrally within the ovule, the nucellus is a tissue that houses the megasporangium. The megasporangium is a sac-like structure where meiosis occurs, producing haploid megaspores from a diploid megaspore mother cell. One of these megaspores will develop into the female gametophyte.
4. Female Gametophyte (Embryo Sac): The female gametophyte, or embryo sac, arises from the megaspore within the nucellus. It is a multicellular structure that includes the egg cell. During fertilization, the egg cell merges with a sperm cell to form a zygote, which then develops into an embryo.
5. Funiculus: The funiculus is the stalk-like structure that connects the ovule to the placenta. It serves as the conduit for nutrients and water from the ovary to the developing ovule.
6. Hilum: The hilum is the specific area where the ovule attaches to the funiculus. It functions as the entry point for essential resources required by the ovule during development.
7. Chalaza: The chalaza is the basal part of the ovule where the integument, nucellus, and funiculus converge. This region plays a role in anchoring the ovule and facilitating nutrient transfer.

Types of Ovule

The diversity in ovule types reflects various evolutionary adaptations in plants. Each type is categorized based on its orientation relative to the funiculus and the hilum, influencing the ovule’s shape and position. Here are the six primary types of ovules:

1. Orthotropous:
   • Description: The ovule maintains a straight alignment with the micropyle, chalazal end, and funiculus in a single vertical line.
   • Function: This primitive form offers a direct pathway for pollen tube entry.
   • Examples: Found in plants such as Piperaceae and Polygonaceae.
2. Anatropous:
   • Description: The ovule is inverted by 180 degrees, bringing the micropyle and hilum into close proximity at one end.
   • Function: This configuration is the most common and facilitates effective protection of the developing embryo.
   • Examples: Common in many dicots and monocots.
3. Campylotropous:
   • Description: The ovule is curved, resembling a banana, with the micropyle directed towards the chalazal end.
   • Function: This shape reduces the distance between the micropyle and chalaza, although they do not align in a straight line.
   • Examples: Present in the Leguminosae family.
4. Amphitropous:
   • Description: The ovule exhibits a more moderate curvature compared to campylotropous ovules, positioning the micropyle and chalazal end at right angles to the funiculus.
   • Function: This form results in a horseshoe-shaped nucellus, facilitating the development of the embryo sac.
   • Examples: Found in some Alismataceae.
5. Hemianatropous:
   • Description: The ovule body bends at nearly a right angle relative to the funiculus, with the micropyle oriented away from the chalazal end.
   • Function: This shape places the ovule in a transverse orientation, potentially aiding in seed protection.
   • Examples: Seen in the Primulaceae family.
6. Circinotropous:
   • Description: The ovule forms a complete circle around the funiculus, with the micropyle facing upwards.
   • Function: This unique shape is rare and provides a distinctive arrangement of the ovule and its components.
   • Examples: Observed in the Cactaceae family.

Special Structures of ovule

Special structures of ovules exhibit unique adaptations that support the reproductive processes of plants. These specialized structures include the endothelium, obturator, aril, arillode, caruncle, and hypostase. Each has distinct functions and contributions to the ovule’s role in reproduction:

1. Endothelium:
   • Description: In ovules with a single integument and a degenerated nucellus, the innermost layer of the integument transforms into a specialized tissue known as the endothelium.
   • Function: The endothelium performs the nutritive function for the developing embryo sac, providing essential nutrients during early stages of embryo development. It is typically a single layer of radially elongated cells with dense cytoplasm.
   • Significance: This structure is crucial for sustaining the embryo sac when the nucellus has degenerated, thus ensuring proper development.
2. Obturator:
   • Description: The obturator is an outgrowth of the placenta located near the micropyle of some ovules.
   • Function: It plays a significant role in guiding pollen tubes towards the micropyle and nourishing them, facilitating successful fertilization.
   • Significance: By directing and supporting the pollen tubes, the obturator enhances the likelihood of fertilization.
3. Aril:
   • Description: An aril is a specialized outgrowth from the seed that partially or completely covers it. This structure is often fleshy and can be found on various seeds.
   • Function: Arils, such as those in litchi, serve to attract animals, aiding in seed dispersal. They may be edible, which encourages animals to carry and spread the seeds.
   • Significance: Arils contribute to effective seed dispersal, promoting the spread of plant species.
4. Arillode:
   • Description: An arillode is similar to an aril but originates from a different point on the seed coat, rather than from the attachment point (funiculus or hilum).
   • Function: Like arils, arillodes can aid in seed dispersal, although they have a different developmental origin.
   • Significance: They provide additional means for seed dispersal, benefiting plant reproductive strategies.
5. Caruncle:
   • Description: The caruncle is a fleshy outgrowth found in the micropylar region of seeds in the Euphorbiaceae family.
   • Function: It facilitates seed dispersal by ants (myrmecochory) and may also influence seed germination.
   • Significance: By attracting ants, the caruncle aids in the dispersal of seeds, ensuring successful colonization of new areas.
6. Hypostase:
   • Description: The hypostase is a group of irregularly outlined, poorly cytoplasmic cells located at the chalazal region of the embryo sac.
   • Function: It forms a barrier that limits the growth of the embryo sac, preventing overgrowth and ensuring proper development.
   • Significance: The hypostase is crucial for maintaining the structural integrity of the embryo sac and regulating its growth.

Ovule Development Stages

The development of an ovule, particularly in an anatropous ovule with Polygonum-type embryo sac development, involves several key stages. Below is a detailed and sequential explanation of each stage:

1. Initiation of the Ovule
   • Description: At this initial stage, the ovule is just beginning to form. It consists of a single megasporocyte (also known as a megaspore mother cell) within the nucellus.
   • Components:
     • Megasporocyte (ms): A diploid cell that will undergo meiosis.
     • Nucellus (nu): The central part of the ovule that surrounds the megasporocyte.
2. Formation of Integuments
   • Description: The ovule develops two integuments, which are protective layers. At this stage, the megasporocyte has completed its first meiotic division.
   • Components:
     • Inner Integument (ii): The inner protective layer forming around the nucellus.
     • Outer Integument (oi): The outer protective layer surrounding the ovule.
   • Position Changes:
     • The axis of the nucellus is temporarily perpendicular to the axis of the funiculus (fu), which connects the ovule to the ovary.
3. Post-Meiotic Ovule Development
   • Description: After meiosis, the megasporocyte undergoes meiotic division to form four haploid megaspores. Only one of these megaspores becomes functional, while the other three degenerate.
   • Components:
     • Functional Megaspore (fm): The viable haploid cell that will develop into the embryo sac.
     • Degenerate Megaspores (dm): Non-functional haploid cells that eventually disappear.
   • Position Changes:
     • The axis of the nucellus becomes parallel to the funiculus due to the differential growth of the integuments.
4. Megagametogenesis
   • Description: The functional megaspore undergoes several mitotic divisions to form a mature embryo sac. This process results in a structure containing seven cells and eight nuclei.
   • Components:
     • Embryo Sac: The mature structure containing:
       • Egg Cell: The female gamete that will fuse with the male sperm cell.
       • Polar Nuclei: Two nuclei that fuse with a sperm cell to form the triploid endosperm.
       • Antipodal Cells: Cells located at the chalazal end of the embryo sac, which support the embryo sac’s development.

Megasporogenesis

Megasporogenesis is the process through which a megaspore develops from a megaspore mother cell within the ovule. This process is crucial for the formation of the female gametophyte, or embryo sac, which is essential for fertilization and subsequent seed development. Here is a detailed explanation of megasporogenesis:

1. Initiation:
   • Archesporium Formation: During ovule development, a single hypodermal cell in the nucellus differentiates into an archesporium. This cell initiates the process of megasporogenesis.
   • Direct or Indirect Formation: In some plants, the archesporial cell directly becomes the megaspore mother cell. In others, the archesporial cell undergoes a transverse division to produce an outer primary parietal cell and an inner primary sporogenous cell.
2. Formation of Megaspore Mother Cell:
   • Primary Sporogenous Cell: The primary sporogenous cell eventually becomes the megaspore mother cell. The parietal cell may remain undivided or divide further, embedding the primary sporogenous cell within the nucellus.
3. Meiotic Division:
   • Process: The megaspore mother cell undergoes meiosis, resulting in the formation of four haploid megaspores. These megaspores are typically arranged in a linear tetrad.
4. Types of Megaspore Development:
   • Monosporic Development:
     • Definition: Only one of the four megaspores develops into the embryo sac.
     • Outcome: The chalazal megaspore is usually functional, while the other three degenerate.
     • Example: Polygonum.
   • Bisporic Development:
     • Definition: Two of the four megaspores participate in the formation of the embryo sac.
     • Example: Allium.
   • Tetrasporic Development:
     • Definition: All four megaspores contribute to the development of the embryo sac.
     • Example: Peperomia.
5. Functional Megaspore:
   • Development: The functional megaspore, whether arising from monosporic, bisporic, or tetrasporic development, differentiates into the female gametophyte or embryo sac.
6. Summary:
   • General Case: Typically, an ovule contains a single embryo sac. The process and type of megasporogenesis can vary among different plant species, leading to different developmental outcomes and structures.

Development of Monosporic Embryo Sac

The development of a monosporic embryo sac, also known as the female gametophyte, follows a specific sequence of stages. This type of development involves the formation of the embryo sac from a single functional megaspore. Here is a detailed overview of the process:

1. Initiation:
   • Functional Megaspore: The functional megaspore is the first cell of the embryo sac. It is derived from the megaspore mother cell following meiosis, where typically only one megaspore remains functional while the others degenerate.
2. Elongation and Initial Divisions:
   • Elongation: The functional megaspore elongates along the micropylar-chalazal axis.
   • Nuclear Divisions: The nucleus of the megaspore undergoes a mitotic division. At this stage, wall formation does not occur immediately after the nuclear division.
3. Formation of Vacuole and Nuclei Distribution:
   • Vacuole Development: A large central vacuole appears between the two daughter nuclei. This vacuole expands, pushing the nuclei towards opposite poles of the embryo sac.
   • Further Divisions: Both nuclei divide mitotically, resulting in four nuclei at each pole. All eight nuclei are initially within a common cytoplasm, a process known as free nuclear division.
4. Cellular Organization:
   • Elongation and Shape: After the final nuclear division, the cell elongates and assumes a sac-like appearance.
   • Cellular Arrangement: The eight nuclei begin to organize into distinct cellular structures.
5. Differentiation of Cellular Components:
   • Micropylar End:
     • Egg Apparatus: Three nuclei at the micropylar end differentiate into the egg apparatus. This structure includes one central egg cell and two synergids.
     • Synergids: Synergids secrete chemotropic substances that attract pollen tubes. They have specialized cellular structures called the filiform apparatus, which aids in nutrient absorption from the nucellus and guides the pollen tube to the egg cell.
     • Upper Polar Nucleus: The fourth nucleus at the micropylar end remains free in the cytoplasm of the central cell, referred to as the upper polar nucleus.
   • Chalazal End:
     • Antipodal Cells: Three nuclei at the chalazal end form antipodal cells.
     • Lower Polar Nucleus: The fourth nucleus functions as the lower polar nucleus.
6. Formation of Central Cell:
   • Polar Nuclei Fusion: Depending on the plant species, the two polar nuclei (upper and lower) may either remain free or fuse to form a secondary nucleus, creating the central cell.
7. Completion of Embryo Sac:
   • Final Structure: The result is a seven-celled, eight-nucleated embryo sac. The egg apparatus consists of the egg cell and two synergids, while the antipodal cells are positioned at the chalazal end.

Function of Ovule

The ovule is a fundamental component of the plant reproductive system, playing several essential roles that contribute to successful seed formation and plant reproduction. Its functions can be summarized as follows:

1. Protection:
   • Description: The integument, which forms the outer layer of the ovule, serves a protective function. It shields the developing egg cell from physical damage, dehydration, and potential pathogens.
   • Function: By safeguarding the inner tissues, the integument ensures the integrity of the ovule during the critical stages of development and fertilization.
2. Nourishment:
   • Description: Inside the ovule, the nucellus functions as a storage tissue that accumulates nutrients necessary for the developing embryo.
   • Function: The nucellus provides essential nourishment to the embryo, supporting its growth until it can sustain itself post-fertilization.
3. Egg Cell Production:
   • Description: The ovule contains the embryo sac, which houses the female gamete, or egg cell.
   • Function: The primary role of the embryo sac is to produce and house the egg cell, which is crucial for the fertilization process. This cell merges with the sperm cell to form a zygote, marking the beginning of seed development.
4. Seed Development:
   • Description: Following fertilization, the ovule undergoes significant transformation. The integument develops into the seed coat, while the nucellus continues to support the embryo. The embryo sac evolves into the seed’s internal structure.
   • Function: The ovule’s transition into a seed ensures the continuation of the plant’s lifecycle. The seed coat provides protection, the nucellus supplies nutrients, and the embryo develops into a new plant.

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