[Biology Class Notes] on Ovule Pdf for Exam

The Plant Ovule is a part of the female reproductive organ. It is that part of the plant where female reproductive cells are made and kept, and these cells and, in due course, develop into seeds after fertilisation. These seeds later ripen and turn into adult plants. In plants, ovules are kept in ovaries at the very bottom of a vase type structure known as a carpel, and this has a neck called a style with an opening at the top, which is called a stigma. 

In a flower, the ovule starts to swell after fertilisation, and it starts to toughen up in the due process of becoming a seed. The ovary starts growing around it and becomes a fruit. However, this is not true for all plants. Plants like avocado have a single ovule, while kiwi has many ovules which can develop into fruits. In angiosperms, the ovaries are found inside the ovary, whereas in gymnosperms, the ovules are found on female cones.

Components of an Ovule

The ovule of a plant has many different components, all of which have their purpose. These components are the Nucellus, Integuments, and the female Gametophyte.

Nucellus: This is the largest part of the ovule, and the nucellus contains the embryo sac and nutritive tissue. This nutritive tissue remains present in the plants after fertilisation which provides nutrients to the embryo. It is found at the centre of the flowering plant.

Integuments: This is the tough protective layer in the ovule. Ovule, meaning it protects the seeds, and this integument encloses the nucleus but leaves a small gap known as the micropyle. In gymnosperms, usually, there is one integument in an ovule, whereas angiosperms have two integuments. The integument is the part of the ovule whose components include gamete-producing sex organs critical for the reproduction in plants. The female gametophyte has a single unpaired chromosome which means that it is a haploid, and this is primarily referred to as the embryo sac or megagametophyte.

Types of Ovules

There are six types of ovules in a plant, and these types of ovules are based on their shapes. 

1. Orthotropous or Atropous: The area where the nucellus and integuments merge is known as chalaza. The Orthotropous is where the body of the plant ovule is straight so that the chalaza, the funicle (this attaches the ovule of the plant to the placenta), and the micropyle aligns. 

2. Anatropous: A scar that marks the area where the seed was eventually attached to the fruit by the funicle is called the hilum. In Anatropous, the ovules are completely inverted throughout development so that the micropyle lies close to the hilum. 

3. Hemi-Anatropous: Here, the ovules take the shape of a right angle about the funicle. Its shape eventually looks like the ovule is lying on its side.

4. Campylotropous: The shape of the Campylotropous ovule is bent, and the alignment that occurs between the chalaza and the micropyle ceases to exist; therefore, the embryo sac becomes slightly curved.

5. Amphitropous: The body of the Amphitropous ovule is so curved that the ovule and the embryo sac takes the shape of a horseshoe.

6. Circinotropous: The shape of circinotropous is the most distinct of them all. The funicle in the case of the circinotropous ovule is so long that it creates a full circle around the ovule and its micropyle ultimately points upwards. It is deemed to be the most distinctly shaped out of the six types of ovules.

Functions of the Ovule

The Plant Ovule plays an essential role in the sexual reproduction of plants. The process of reproduction becomes simple with the presence of an ovule. When the pollen grain lands on the stigma of the flower of the same species of flower, the plant brings out the pollen tube down through the style. Next, this pollen tube enters the ovary and reaches the ovule of the plant, and after this, fertilisation can occur as the nucleus of the pollen grain is sent down to merge with the nucleus in the embryo sac. 

Fun Facts

A reproductive organ that contains an egg cell and after fertilisation develops into a seed is called an ovule. Ovules develop inside an ovary in angiosperms, and ovules are attached to the inner surface of the female cone in gymnosperms. The ovules are unprotected in the case of gymnosperms, but in angiosperms, the ovules are protected by the megasporophyll. After the angiosperm ovule matures, it consists of protective layers, integuments, and embryo sac.