Fertilization in Plants

Fertilization is the fusion of a male gamete carried by pollen with a female gamete present in the ovule to form a diploid zygote, which later develops into an embryo. The process of Fertilization involves some pre-steps like pollination and germination On reaching the stigma, the pollen grain containing two nuclei germinates to produce the pollen tube. The pollen tube grows down into the style, deriving nutrition from it. The generative nucleus divides mitotically into two male gametes. The male gametes fuse with the ovules to form the zygote. In this article, we will study fertilization in plants in detail including its process, types, and Post fertilization development.

What is Fertilization?

When pollen from the anther of the stamen joins with the ovules present in the ovary, it is called fertilization. Strasburger discovered fertilization for the first time in 1884. In angiosperms, the female gametophyte (embryo sac) is located in the ovule. As a result, pollens that reach the stigma form a pollen tube, which helps the delivery of male gametes deep into the embryo sac from the stigma. The developing pollen tube penetrates the stigmatic tissue, moves through the style, and then goes down the ovarian wall.

Reproductive Parts of the Plant

The main reproductive part of the plant is the flower. It contains both the male and female reproductive parts. The male reproductive part of the plant is the androecium. It is made up of a bilobed structure called the anther and a stalk leading to it, known as the filament. The anther contains the pollen grains, which act as the male gametophytes. To know more about it, please refer to the article ‘Androecium‘.

The female reproductive part of the plant is called the Gynoceium. It is also called the ‘pistil’. It is further made up of Style, Stigma and Ovary. It contains the ovules, which are the female gametophytes. To know more about it, please refer to the article ‘The Structure and Functions of Pistil‘.

Process of Fertilization

The following steps occur for the process of fertilization to occur:

Pollination

Pollination is the process by which pollen from one plant is transported to the stigma of another plant. The pollen is carried to another flowering plant by the wind or water. Now that the pollen grains have been transferred onto the stigma, a pollen tube grows through the style until it reaches the ovary. To know more, please refer to the article ‘Pollination‘.

Germination

It is the stage at which the seed sprouts to become a seedling. Inside each seed is a plumule, which develops into the plant shoot, and a radicle, which develops into the root. Seeds have cotyledons, which operate as a food store for the developing plant until it can perform photosynthesis, as well as a seed coat, which forms a protective shell. To know more, please refer to the article ‘Seed Germination‘.

Fertilization

Fertilization begins with the arrival of suitable pollens at the stigma and ends with the merging of male and female gametes in the embryo sac. The stigma stores pollen collected by the female reproductive organ, the carpel. Since the fusing within the embryo sac occurs twice, this event is known as double fertilization and is a distinct property of angiosperms.

Types of Fertilization

The pollen tube makes its way into the ovule after entering the ovary. The pollen tube can penetrate the ovule in three ways.

• via the micropyle
• via the chalaza (basal part of the ovule) end
• via the integument or the funicle

Three names are defined based on the mechanisms of entry of the pollen tube into the ovule:

1. Porogamy: Porogamy occurs when the pollen tube reaches the ovule through the micropyle. This is the most typical way for pollen tubes to enter the ovule.
2. Chalazogamy: This condition occurs when the pollen tube penetrates the ovule by the chalazal end. Casuarina, Betula, and Juglans regia have this form of pollen tube entrance into the ovule.
3. Mesogamy: This state occurs when the pollen tube reaches the ovule through the integument or the funicle. This sort of pollen tube penetration into the ovule has been reported in Cucurbita (via integument) and Pistacia (via funicle).

Double Fertilization

The following points summarise the double fertilization process:

• S.G. Nawaschin (1898) was the first to demonstrate double fertilization. He demonstrated that both sperm produced by a pollen tube and sperm released by a pollen tube are engaged in fertilization. They fertilize two separate parts of the embryonic sac.
• The occurrence, known as twofold fertilization, is unique to angiosperms. The nucleus of one of the sperm joins with the nucleus of the egg, completing syngamy.
• This results in the creation of a diploid cell, called the zygote. The other male gamete goes towards the two polar nuclei in the central cell and unites with them to form a triploid endosperm nucleus.
• The fusion of three haploid nuclei is known as triple fusion. Because two types of fusion, syngamy and triple fusion, occurs in the embryo sac, the occurrence is known as double fertilization, and it is unique to flowering plants.
• After triple fusion, the central cell becomes a Primary Endosperm Cell (PEC), while the zygote develops into an embryo.

Diagram of Fertilization

Here is a diagram highlighting the mechanism of Fertilization in plants:

Post Fertilization Development

Following fertilization, the embryo and endosperm grow concurrently within the embryo sac. The embryo grows from the oospore (zygote), while the endosperm develops from the primary endosperm nucleus. Other nuclei or cells within the embryo sac (synergids, antipodal cells) eventually disorganize.

To know more refer to the article Double Fertilization: Process & Significance.

Importance of Fertilization

The following points highlight the importance of fertilization:

• Double fertilization is a unique feature of angiosperms; it ensures that the parent plant envests a seed with food storage only if the egg is fertilized.
• Zygote formed as a result of fertilization develops into an embryo, and the embryo develops into a new plant.
• Fertilization restores diploid conditions by fusion of haploid male gametes and female gametes to produce zygote (through syngamy).
• Fertilization also helps to avoid polyembryony.

FAQ’s – Fertilization in Plants

1. What happens after a Plant is Fertilized?

After fertilization, the cells of the zygote begin dividing, and the zygote develops into an embryo, or developing plant. The embryo is stored in a seed capsule, where it remains dormant until environmental conditions support its development into a new plant.

2. Which Types of plants show Double Fertilization?

Double fertilization is unique to flowering plants (angiosperms) and is responsible for the formation of both the embryo and its potential food source in the seed. Ephedra and Gnetum are exceptions to gymnosperms as they show double fertilization.

3. Which Plants show Self-Fertilization?

Orchids, peas, sunflowers, and tridax are just a few of the plants that may self-pollinate. The majority of self-pollinating plants have small, inconspicuous blooms that release pollen straight onto the stigma before the bud opens.

4. What happens to Plants without Fertilization?

The pollination of a flower triggers the female reproductive organs to mature into fruit. However, without fertilization, as we know, there will be no seed development. Production of the plant without fertilization is done by vegetative propagation.

5. Why is Fertilization Important for Plants?

Double fertilization provides stimulus to the plant, resulting in the ovarian development of fruits and the development of ovules into the seed. When the haploid male gametes and female gametes fuse, the diploid state of the plant is restored.