Ovule: Definition, Meaning, Types, Components, Description, & Facts

What Is An Ovule?

A structure in seed plants, the ovule consists of the female gamete, otherwise known as the egg cell. It is considered to be that part that is going to develop into a seed after fertilization. An ovule is located in the ovary inside the plant's flower. The nature of ovules is made up of diverse parts, which work in coordination for the fertilization and seed development processes.

Characteristics Of Ovules

1. Location: The ovules lie in the ovary of a flower in the female reproductive organ—the pistil.

2. Structure: A typical ovule comprises:

3. Integuments: These are protective layers around the ovule, which develop into the seed coat after fertilization.

4. Nucellus: Tissue in the ovule in which the megasporangium is present; inside the megasporangium lies the megasporocyte, otherwise known as the megaspore mother cell.

5. Micropyle: A small opening in the integuments for the entering pollen tubes at the time of fertilization.

6. Funiculus: The stalk that attaches the ovule to the wall of the ovary.

7. Development: From the primordium of the ovule, the ovules develop during the process of ovule development, an event initiated in the ovary.

8. Role in Reproduction: Ovules are regarded as an essential part of the sexual reproduction of seed plants because they contain a female gamete and contribute to forming seeds after fertilization has occurred.

Types Of Ovules

Ovules can be divided into several types based on the various orientations and structures they may possess. These include:

Orthotropous Ovule :

In an orthotropous ovule, the body of the ovule is straight, allowing the micropyle, the funicle attaching the ovule to the placenta, and the chalaza (where the integuments and nucellus merge) to lie in a single vertical line.

Example: This type of ovule is found in families such as Piperaceae and Polygonaceae.

Anatropous Ovule

Anatropous ovule: It is that ovule which becomes inverted during development and the micropyle lies close to the hilum. The hilum scar marks the area where the funicle attached the seed to the fruit wall.

Example: Common examples include many flowering plants like sunflowers and legumes.

Hemianatropous Ovule

In a hemianatropous ovule, the body of the ovule is oriented transversely and at right angles to the funicle. This gives it a unique orientation.

Example: This type occurs in the Primulaceae family.

Campylotropous Ovule

Campylotropous Ovule: The body is curved at the micropylar end and thus forms more or less of a bean-shaped structure. The embryo sac is curved slightly, and the hilum, micropyle and chalaza lie adjacent to each other with the micropyle directed towards the placenta.

Example: This type of ovule is common in the Leguminosae family, the legumes.

Amphitropous Ovule

In an amphitropous ovule, the distance between the hilum and chalaza is reduced and, as a result, the nucellus becomes horse-shoe in shape. This unique arrangement can influence the development of the ovule.

Example: Amphitropous ovules are found in some members of the Alismataceae family.

Circinotropous Ovule

In a circinotropous ovule the funiculus is long embracing the ovule. This type of ovule has a unique structure that influences fertilization and seed development.

Example: The Cactaceae family consists of circinotropous ovules.

Components Of The Ovule

The ovule consists of various components that help in functioning. These are:

Integuments

• The integuments are protective membranes about the ovule that grow into the seed coat after fertilization.

• Gymnosperms are usually unitegmic, whereas angiosperms are usually bitegmic.

• The point where the nucellus and integuments merge is opposite to the micropyle and is called chalaza.

• The integuments do not surround the nucellus; there is a small opening at the top that is called the micropyle, via which the male gametophyte, the pollen, enters to fertilize the ovule.

Nucellus

• The nucellus is the most essential constituent of the ovule.

• It contains nutritive tissue and the embryo sac and in some flowering plants, it persists until maturity after fertilization as a source of nutrition for the embryo.

• The nucellus forms a layer of sporophytic or diploid cells near the integuments.

• Nucellar embryony is the process of asexual reproduction in some plants whereby the diploid nucellus tissue can give rise to the embryo inside the seed.

• After fertilization, the perisperm that will nourish the embryo may develop from the nucleus.

Female Gametophyte

• The female gametophyte is also known as the megagametophyte or embryo sac, a haploid structure that contains the content of gamete-producing components essential for sexual reproduction.

• It develops from the surviving megaspore and contains the egg cell, with other supporting cells that are essential for fertilization.

Functions Of Ovule

The ovule carries out several important functions in sexual reproduction:

1. Pollination: A pollen grain extends a pollen tube down the style, following its landing on the stigma of the same species of flower. This tube extends eventually into the ovary and then into the ovule.

2. Fertilization: Fertilization takes place when the nucleus of the pollen grain moves down the tube to merge with the one at the end of the embryo sac, once the pollen tube reaches the ovule.

3. Seed Development: The development of the ovule, after fertilization, to a seed containing the embryo and surrounding tissues that provide nourishment.

Conclusion

The ovule is a vital structure that is present in seed plants; it has a very significant role in reproduction and seed formation. It enables the learner to comprehend more of the plant's biology and its strategies for reproduction. Therefore, the processes involving ovules are those that cannot be skipped if plant species are to be perpetuated, whose populations contribute to the biodiversity in ecosystems.

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