Fertilization in Plants & Double Fertilization Notes - NEET

Fertilization in Plants

A plant is nature’s gift as they provide us with the food, oxygen, shelter, clothing, etc. They are also known as universal or primary producers. Like all other living things, plant respire, grow, develop, excrete and reproduce. All higher plants reproduce by fertilization. Fertilization in flowering plants was discovered by Ralph B. Strasburger in the year 1884. Let us have a brief discussion on fertilization.

What is Fertilization?

In plants, fertilization is a process of sexual reproduction, which occurs after pollination and germination.

Fertilization can be defined as the fusion of the male gametes (pollen) with the female gametes (ovum)  to form a diploid zygote. It is a physicochemical process which occurs after the pollination of the carpel. The complete series of this process takes place in the zygote to develop into a seed.

In the fertilization process, flowers play a significant role as they are the reproductive structures of angiosperms (flowering plants). The method of fertilization in plants occurs when gametes in haploid conditions fuse to produce a diploid zygote.

In the course of fertilization, male gametes get transferred into the female reproductive organs through pollinators (honey bees, birds, bats, butterflies, flower beetles) and the final product will be the formation of the embryo in a seed.

➤ Fertilization Process
In flowers, the pollen grain germinates after the pollination of the carpel and grows into the style by creating the pathway for the pollen grain to move down to the ovary.

The pollen tube opens into the ovule through the micropyle and bursts into the embryo sac. Here, the male nucleus unites with the nucleus of an egg inside the ovule forming a diploid zygote, which later swells up and develops into a fruit.

➤ Types of Fertilization
Fertilization process can be grouped into three types and are classified mainly based on the entry of the pollen tube into the ovule.

• Porogamy
  It is the common type of fertilization carried out in all angiosperms or flowering plants. In this type of fertilization, the pollen tube enters the ovule through the micropyle.
• Chalazogamy
  This type of fertilization is carried out on all Casuarina species of plants. In this condition, the pollen tube enters the ovule through the pollen tube.
• Mesogamy
  This type of fertilization is seen in all Cucurbit plants, such as pumpkin, ridge gourds, bitter gourd and other gourd plants. In this type of fertilization, the pollen tube enters the ovule through its middle part or through the integuments of the ovule.

➤ Double Fertilization
Double fertilization is a process of fertilization characterized by the fusion of a female gametophyte with two male gametes. In this mechanism, one sperm cell fuses with the egg-producing zygote, and the other fuses with the two polar nuclei to make the endosperm. All angiosperm plants undergo double fertilization process.

➤ Definition:
“Double fertilization is a complex process which involves the fusion of one female gametophyte with two male gametes”

➤ What is Double Fertilization?
Double fertilization is a chief trait of flowering plants. In the phenomena, one female gamete unites with two male gametes. One of the male gametes fertilizes the egg resulting in the formation of a zygote and the other unites with 2 polar nuclei for the formation of an endosperm.

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

➤ Double Fertilization Process
The process of double fertilization is explained below:

Double Fertilization in Angiosperms
Angiosperms are flower-bearing plants and are the most diverse group of terrestrial plants. The flowers form the reproductive part of angiosperms with separate male and female reproductive organs. Each contains gametes – sperm and egg cells, respectively.

Pollination helps the pollen grains to reach stigma via style. The two sperm cells enter the ovule-synergid cell. This proceeds to fertilization.

In angiosperms, fertilization results in two structures, namely, zygote and endosperm, hence the name “double fertilization.”

Double fertilization is a complex process where out of two sperm cells, one fuses with the egg cell and the other fuses with two polar nuclei which result in a diploid (2n) zygote and a triploid (3n) primary endosperm nucleus (PEN) respectively.

Since endosperm is a product of the fusion of three haploid nuclei, it is called triple fusion. Eventually, the primary endosperm nucleus develops into the primary endosperm cell (PEC) and then into the endosperm.

The zygote becomes an embryo after numerous cell divisions.

Double Fertilization 

Development of Embryo in Angiosperms
Once fertilization is completed, embryonic development starts and no more sperms can enter the ovary. The fertilized ovule develops into a seed, and ovary tissues develop a fleshy fruit which encloses the seed.

After fertilization, the zygote divides into the upper terminal cell and lower basal cell. The basal cell develops into suspensor, which helps in the transport of nutrients to the growing embryo. The terminal cell develops into pro-embryo.

Following are the different stages involved in the development of an embryo.

➤ Stages of Embryonic Development In Angiosperms

1. In the first stage of development, the terminal cell divides forming a globular pro-embryo. The basal cell also divides, into a suspensor.
2. The developing embryo attains a heart shape due to the presence of cotyledons.
3. The growing embryo gets crowded and begins to bend.
4. The embryo fills the seed completely.

➤ Significance of Double Fertilization

The significance of double fertilization is as follows:

1. Two products are obtained as a result of double fertilization.
2. There are chances of polyembryony, and the plant has better chances of survival.
3. Double fertilization gives rise to an endosperm that provides nourishment to the developing embryo.
4. It increases the viability of the seeds of angiosperms.
5. It utilizes both the male gametes produced by the pollen grains.

Frequently Asked Questions

Q. What is double fertilization?
Ans: Double fertilization refers to the fusion of one female gametophyte with two male gametophytes.

Q. What is the importance of double fertilization?
Ans: Double fertilization provides stimulus to the plant as the ovary develops into a fruit. Due to the fusion of male and female haploid gametes, a diploid zygote is formed. The zygote develops into an embryo, giving rise to a new plant.

Q. What are the end products of double fertilization?
Ans: The end products of double fertilization are:

1. Diploid Zygote
2. Primary Endosperm Nucleus

Q. Why is double fertilization absent in gymnosperms?
Ans: Gymnosperms are devoid of ovaries, and the male and female gametophytes are present on cones. Whereas, in angiosperms, the gametophytes are a part of the flower. Hence, pollination in gymnosperms occurs with the help of wind that blows the pollen to land on the female cones. That is why they do not exhibit double fertilization.

Q. Who discovered double fertilization?
Ans: Double fertilization was discovered by Nawaschin in 1898 in liliaceous plants, Lilium martagon and Fritillaria tenella.