Question

What do you mean by double fertilization?

Answer 1

Hint:
Fertilization is the process of fusion of gametes which initiate the formation of a new individual organism. It can also be termed as conception, fecundation, syngamy and impregnation. In plants, based on the entry of pollen tubes in the ovule, it is of three types.
1) Porogamy
2) Chalazogamy
3) Mesogamy

Complete answer:
In order to solve this question we need to know about fertilization in plants.
In plants, fertilization is a process for sexual reproduction. It takes place after the process of pollination and germination. It is a physicochemical process, which can be explained as the fusion of the pollen with the ovum to form a diploid zygote.
Flowers play an important role as they are referred to as the reproductive structures of angiosperms (flowering plants).
Porogamy- It is the most common type of fertilization which occurs in all flowering plants. In this, the pollen tube enters the ovule through a microscopic opening called micropyle.
Double Fertilization- It is a process of fertilization characterized by the fusion of a female gametophyte with two male gametes, thus there are two fertilizations taking place therefore referred to as double fertilization.
In this mechanism, one sperm cell fuses with the egg which produces zygote, and the other sperm fuses with the two polar nuclei to form the primary endosperm nucleus (PEN).
All flowering plants undergo this double fertilization process.
It is a chief trait of angiosperms.
Double fertilization provides a signal to the plant which results in the development of ovaries to fruits and development of ovules into the seed.
When the haploid sperm cell and female gametophyte fuse, the diploid state of the plant is regained.
Zygote formed is diploid (2n) and PEN is triploid (3n).
As endosperm is a result of the fusion of three haploid nuclei, it is referred to as triple fusion. The primary endosperm nucleus (PEN) further forms a primary endosperm cell (PEC) and then the endosperm is formed.
The zygote transforms into an embryo following many cell divisions.
This is all you need to know about double fertilization.


Note:
Chalazogamy- It is a type of fertilization which is mostly carried out in all Casuarina species of plants. In this, the pollen tube enters the ovule through the pollen tube.
Mesogamy- In this type of fertilization, the pollen tube enters the ovule from its center part or from integuments. It is seen in all Cucurbit plant species, example pumpkin, ridge gourds, bitter gourd etc.