An ovule is the structure present inside the ovary of flowering plants where the female gametophyte develops. It contains the integuments, nucellus, and embryo sac, and after fertilisation it develops into a seed.

Ovule: An Overview

An ovule is the structure present inside the ovary of seed plants where the female gametophyte develops. It is the structure that later becomes the seed after fertilisation. In flowering plants, the ovule contains the tissues needed for megaspore formation, embryo sac development, and ultimately seed formation. This makes the ovule one of the most important reproductive structures in angiosperms.

The ovule contains:

• protective coverings called integuments

• the nucellus

• the female gametophyte or embryo sac

After fertilisation, the ovule develops into a seed containing the embryo, endosperm, and seed coat. Under suitable conditions, this seed can grow into a new plant.

What is Ovule? Understand the Basics

An ovule is the female reproductive structure found in the ovary of a flower. It is the site where the female gametophyte develops and where fertilisation takes place. In simple words, the ovule is the seed precursor.

This is why, in plant reproduction, the ovule is highly important:

• Before fertilisation, it contains the embryo sac

• During fertilisation, it receives the male gametes

• After fertilisation, it transforms into the seed

Structure of Ovule

The structure of the ovule includes three major parts:

1. Integuments

2. Nucellus

3. Female gametophyte or embryo sac

The embryo sac lies in the centre, enclosed by the nucellus, which in turn is covered by one or two integuments.

Main Components of Ovule Structure

In angiosperms, the female gametophyte is called the embryo sac. It usually develops from a single functional megaspore.

Ovule Diagram and the Parts

An ovule diagram generally includes the following labelled parts:

• funicle

• hilum

• integument

• micropyle

• chalaza

• nucellus

• embryo sac

Parts of Ovule

1. Funicle

The funicle is the stalk-like structure by which the ovule is attached to the placenta inside the ovary. It serves as the connecting stalk of the ovule.

2. Hilum

The hilum is the region where the body of the ovule joins the funicle. It represents the junction between the ovule and the funicle.

3. Integument

The integuments are protective envelopes surrounding the ovule. Each ovule may have one or two integuments. These coverings protect the inner tissues of the ovule and later develop into the seed coat after fertilisation.

4. Micropyle

The micropyle is the small opening left at the tip of the ovule where the integuments do not completely cover the nucellus.

Functions of Micropyle

• Allows pollen tube entry during fertilisation

• During germination, the radicle emerges through it

• helps in the entry of water and gases

This makes the micropyle a very important part of the ovule structure.

5. Chalaza

The chalaza is the basal part of the ovule opposite to the micropyle. It is the region where the integuments and nucellus meet at the base.

6. Nucellus

The nucellus is the central mass of cells present inside the integuments. It contains abundant reserve food material and surrounds the embryo sac.

Important facts:

• It is a diploid tissue

• It is structurally and functionally equivalent to the megasporangium

• In immature ovules, it contains the megaspore mother cell (MMC)

After fertilisation, the nucellus may persist as perisperm, which can nourish the developing embryo.

7. Embryo Sac or Female Gametophyte

The embryo sac is the female gametophyte present inside the nucellus. In angiosperms, an ovule generally has a single embryo sac formed from one megaspore.

A common question is: how many embryo sacs are present in an ovule?
The standard answer is: Usually, one embryo sac is present in one ovule

Interesting Fact: In Ovule, meiosis occurs in the megaspore mother cell (MMC), which is present in the nucellus. The MMC undergoes meiotic division to produce megaspores.

Megasporogenesis

The process of megaspore formation from the megaspore mother cell is called megasporogenesis.

Important Points

• Usually, a single megaspore mother cell differentiates in the nucellus

• It is generally present in the micropylar region

• It is a large cell with dense cytoplasm and a prominent nucleus

• It undergoes meiosis to form megaspores

This process is a key part of ovule development.

Embryo Sac Development

The embryo sac or female gametophyte develops from a megaspore. A common pattern in angiosperms is:

• One megaspore remains functional

• This functional megaspore undergoes three rounds of mitosis

• The embryo sac is formed

Thus, the embryo sac is the mature female gametophyte found inside the ovule.

Location of Ovule

In flowering plants, the ovule is found inside the ovary, which is part of the gynoecium, the female reproductive organ of the flower.

The ovule is attached to the placenta of the ovary by a stalk-like structure called the funiculus or funicle. After fertilisation:

• The ovary becomes the fruit

• The ovule becomes the seed

Types of Ovules

The types of ovule can be classified on two main bases:

1. Based on the number of integuments

2. Based on the position of the micropyle, chalaza, hilum, and orientation of the funicle

Types of Ovule Based on Number of Integuments

1. Unitegmic Ovule

Ovules having one integument are called unitegmic ovules. This type is common in many gymnosperms.

2. Bitegmic Ovule

Ovules having two integuments are called bitegmic ovules. This is the most common condition in angiosperms.

3. Ategmic Ovule

Ovules having no integuments are called ategmic ovules. Examples: 

• Loranthus

• Olax

• Liriosma

• Santalum

Types of Ovule Based on Position and Orientation

This classification depends on the relationship between:

• micropyle

• chalaza

• hilum

• funiculus

These are the main types of ovules in angiosperms.

1. Orthotropous Ovule or Atropous Ovule

The orthotropous ovule is also called the atropous ovule.

Features

• The ovule is straight and upright

• micropyle, chalaza, and hilum lie in one straight line

• Raphe is absent

Occurrence

It is found in primitive angiosperms and gymnosperms, including:

• betel

• piper

• Polygonum

This is an important type because questions often ask about the difference between orthotropous and anatropous ovules.

2. Hemitropous or Hemianatropous Ovule

Features

• ovule bends at about 90°

• The body of the ovule becomes horizontal

• micropyle and chalaza remain in one line

• Micropyle stays away from the hilum

Examples

• Ranunculus

• Primula

3. Anatropous Ovule

The anatropous ovule is the most important and the most common type.

Features of an anatropous ovule

• The ovule gets inverted by 180°

• This happens due to unilateral growth of the funiculus

• The micropyle comes to face downward

• chalaza and micropyle come close to each other

• Raphe is present

Importance

• It is the most common type of ovule

• found in about 80% of angiosperm families 

• Exceptions include Capsella

Examples

• Malvaceae

• Cucurbitaceae

• Solanaceae

• Compositae

Because it is so common, the anatropous ovule diagram is the standard diagram shown in most biology textbooks.

4. Amphitropous Ovule

Features

• Curvature affects the nucellus

• The embryo sac becomes horseshoe-shaped

• Micropyle comes close to the hilum

• also called a transverse ovule

Examples

• Mirabilis

• Lemna

• Papaver

5. Campylotropous Ovule

Features

• The ovule body is curved

• The micropyle and chalaza do not lie in one straight line

• Nucellus is curved

• The embryo sac remains straight

• The micropyle lies close to the hilum

Examples

• Leguminosae

• Cruciferae

• Capsella

6. Circinotropous Ovule

Features

• The ovule first becomes inverted and then straightens again

• The funiculus grows around the ovule in a coiled manner

• also called a coiled ovule

• Micropyle remains away from the hilum

Example

• Opuntia of the family Cactaceae

Types of Ovule at a Glance

Orthotropous Ovule vs Anatropous Ovule

This comparison is very useful for board exams and entrance exams.

• The nucellus is structurally and functionally equivalent to the megasporangium

• The ovule as a whole is the megasporangium together with its protective coverings and internal gametophytic tissues

So, when asked in either objective or conceptual form, the accepted technical equivalence is usually linked to the megasporangium via the nucellus.

Important Points to Remember About Ovule

• An ovule is the structure inside the ovary where the female gametophyte develops.

• After fertilisation, the ovule becomes the seed.

• The main parts of the ovule are the integuments, the nucellus, and the embryo sac.

• Meiosis in the ovule occurs in the megaspore mother cell.

• Usually, one embryo sac is present in an ovule.

• Nucellus is equivalent to the megasporangium.

• Anatropous ovule is the most common type in angiosperms.

• The orthotropous ovule is straight, and the raphe is absent.

• Bitegmic ovule is a common condition in angiosperms.

Related Topics for Interlinking of Ovule