What is Triple Fusion?

Triple fusion is defined as fusion taking place between two polar nuclei and sperm nucleus. This happens during double fertilization in a seed plant, and therefore gives origin to a triploid nucleus known as the Primary Endosperm Nucleus(PEN). 


The PEN later evolves into an endosperm. This fusion primarily takes place in sexually reproducing plants called angiosperms. 

To understand reproduction in angiosperms and to know more about what triple fusion is, we must first understand what is the morphology of the reproductive parts of the male and female plants. And further understanding, where, in the process of fertilization, do we come across triple fusion, its formation, and the fate of the gamete formed after triple fusion.

Morphology of a Male Angiosperm Flower.

Most of us know, that in the plant kingdom there are unisexual flowers and bisexual flowers. Unisexual flowers are flowers that are either male or female. These flowers only contain stamen or they contain carpel whereas, in bisexual flowers, both stamen and carpel are present.

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The stamen is the male reproductive part of a flower. It is also called Androcoeium. It consists of anther and filament. The filament is an elongated long and slender stalk. The anther is bilobed and each lobe has two thecas. Hence, it is called dithecous. Each lobe has another sac called microsporangia. It contains the male cells responsible for fertilization called the pollen grains. The generation of pollen grains is through cell division of meiosis type by the MMC (microspore mother cell). This takes place in the inside of microsporangia. Exine is the outer layer which is made up of sporopollenin that is a polysaccharide of complex variety in the pollen grains. There are two main cells that are present within the pollen grain. These cells are the main cells involved in the fertilization process and both of their nuclei are haploid. They are the germ cells and the tube cells. The tube cells are responsible for generating into a structure called pollen-tube through which another cell called the germ cells passes through during fertilization. The male germ cell further divides into two cells by the process of mitosis. This procedure may take place prior to or after fertilization process. 

Morphology of Female Angiosperm flower

The female reproductive part of a flower is called Gynoecium. If it contains a single pistil it is called monocarpellary and if it is more it is called multicarpellary. It consists of the stigma, style, and ovary. It is to be understood that the stigma and style are just the mediators to trap the pollen grains and transfer them to the ovaries. The ovaries can contain one to many ovules. The stigma is receptive surface and pollen grain land on the stigma in various ways. The style is a portion which connects the stigma and the ovary. The style keeps the stigma and ovary at a distance so that they are not close to each other. This distance between them is specific to different classes of angiosperms, it could be useful in tracking the class of angiosperm species. 

The ovaries are the main part in the female flower where all the process of fertilization takes place. The ovaries contain ovarian cavity called locules, a placenta is located inside the locule. It contains one to many ovules. The ovule is a small structure attached to the placenta by means of a funicle. The body of ovule fuses with the funicle in a region known as the hilum. The ovules have protective layers called integuments that encircle the ovule except at a place called micropyle. The end that is opposite to the end of micropyle is called chalaza. Within the integuments the consists of a mass of cells called nucellus which have abundant food materials. And inside the nucellus exists an embryo sac which is called the female gametophyte. The female gametophyte is formed by megaspore mother cells by a process of megasporogenesis. The megaspore mother cell undergoes meiotic division and forms four megaspores. Three of them degenerate and one of them becomes the embryo sac.  This embryo sac 8 cells in total which are also called haploid cells, one egg cell which fuses with the male germ cell, towards the end in the embryo sac (also called micropyle end) there are two synergid cells, 2 polar nuclei which are at the embryo sac (towards the centre) as well as 3 antipodal cells which are the end opposite to the egg cell and synergid cells.

Pollination 

Pollination is the process by which the male anther transfers the pollen grain on to the stigma in the female flower. Pollination can occur either by wind or air(abiotic agents) or by the animals(biotic agents). In wind pollination, the pollen grains are taken by the wind and if the pollen lands on a suitable stigma, the pollination process takes place. This process requires more amount of pollen grain production and also most of the pollen grains go unfertilized as the probability of landing on a stigma remains a problem. Insect driven pollination is more specific as flowers producing nectars are attractive and only a few pollen grains stick to the insects which land on the stigma of another plant when the insect takes nectar from another plant. In this case, the flowers are more attractive and produce nectar for different insects.

Double Fertilization


As the pollen grains land on the stigma, the tube cell of the pollen starts growing which causes the pollen-tube germination. The pollen-tube penetrates the stigma and grows towards the style and reach the base of the ovule. The two germ cells travel through the course of the pollen-tube formation and reach the ovule.


In the ovule, one germ cell fertilizes the egg cell of the female flower and forms the zygote. Both the nuclei are haploid and become diploid after fertilization. The zygote later forms the embryo and is the main product of the fertilization.

However, another fertilization takes place in angiosperms, where 2 polar nuclei and a sperm cell fuse together to form a triploid cell. Since there are three cells fusing together to form another cell, the phenomenon is called Triple Fusion.


The product of the fusion of secondary nuclei results in the formation of Primary Endosperm Nucleus. 


Endosperm


Endosperm development is faster than the development of embryo so that the embryo has a hostile environment for its growth. It divides and forms a triploid endospermic tissue. It has reserve food materials and is used for the nutrition of the embryo. The Primary endosperm nucleus forms a successive nuclear division to give rise to free nuclei. This stage of development is called free nuclear endosperm. It eventually forms the cell wall. In the example of tender coconut, the white kernel is the cellular endosperm and the water is free nuclear endosperm.

The Function of Endosperm 

The primary function of the endosperm is to provide nutrition for the developing embryo after double fertilization. 


Embryo

The fertilization that takes place between the egg cell and the male germ cell of the plant results in the formation of zygote. The zygote later proliferates into the embryo which is the main cell that carries the information of the parent plants.