Double Fertilization: Process and Significance

Double fertilization is a process in flowering plants (angiosperms) by which they reproduce sexually. Fertilization, in general, involves fusion of male and female gamete. A double fertilization takes place when male gamete fuses with female egg cell and 2 polar nuclei at the same time. This simply means that two types of fertilization or fusion are occurring simultaneously where one female gamete fuses with two male gametes.

• The fusion of male gamete and polar nuclei leads to development of an endosperm.
• Endosperm plays role in suppliment of nutrients.
• It supports the embryonic growth and protects embryo during seed development.

Read More: Reproduction in Plants

Key Terms : Double fertilazation, Pollination, Germination, Endosperm, Angiosperm, Embryo

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Reproductive structures of a flower

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An angiosperms or flowering plants reproduce themselves by sexual reproduction. Hence, male and female reproductive parts are present in the flower at the time of reproduction. Together, these parts undergo fertilization and develop an embryo that eventually become a seed.

Male Reproductive Part

A flower has a male reproductive part called Stamen. Androecium is the formation of the number of stamens.

• Each stamen consists of a long and thin tube-like filament.
• It carries a bilobed structure called anther at its terminal end.
• Anther contains the pollen sacs packed with pollen grains.
• A matured pollen grain contains one vegetative cell and one generative cell.
• Most angiosperms disperse their pollen grains for pollination at this 2-celled stage.
• In others, the generative cell undergoes mitosis to produce 2 male gametes.

Read More: Difference Between Cross-Pollination and Self-Pollination

Female Reproductive Part

The female reproductive part of a flower is called the gynoecium. It is made up of one or multiple structures called carpels or pistils. Each carpel has 3 parts – the uppermost stigma where the pollen grains land, the long pipe-like style, and the ovary at the base.

• The part inside the ovarian cavity (locule) where the female gamete is present is called ovules.
• Ovules are also called microsporangia.
• Each ovule has a tiny opening at the tip, called the micropyle.
• The ovule contains an embryo sac or female gametophyte that is formed from a megaspore.
• The nucleus of this megaspore undergoes sequential mitosis to form the final 8-nucleate stage of the embryo sac.
• Inside the embryo sac, 6 nuclei are made into cells.
• 3 of which are at the micropylar end.
• 2 of them are synergids and 1 is egg cell.
• The other 3 cells, called the antipodal cells, are located at the opposite end.
• The 2 remaining nuclei, called polar nuclei, lie in the central cell in between these two sets of cells.
• Thus, a matured angiosperm embryo sac has 7 cells although it has 8 nuclei.

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Double Fertilization

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One of the male gametes fuses with the egg cell. This fusion, called syngamy, produces a zygote. The male and female gametes are haploid that is, it has a single set of chromosomes (say n number of chromosomes). Thus, the zygote is diploid that is, it has two sets of chromosomes, one from each parent cell (n + n).

• The other male gamete fuses with the two polar nuclei of the central cell to form a primary endosperm nucleus.
• This fusion is called triple fusion as it involves three haploid nuclei fusing together to form a triploid nucleus(3n).
• Since two types of fusion take place in the embryo sac, the overall process is known as double fertilization.
• After triple fusion, the central cell becomes the primary endosperm cell which ultimately develops into the endosperm.
• The zygote develops into the embryo.

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Post-Fertilisation Events

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The endosperm develops before the embryo. The primary endosperm cell undergoes repeated divisions to form a tissue. The cells in this tissue store food materials for the developing embryo. The zygote undergoes cell division to form the proembryo first and then ultimately the embryo.

• The final product of double fertilization is seed.
• The fertilized ovule is this seed coat.
• Seed consists cotyledon(s), and an embryo axis
• Mature seeds may be albuminous or non-albuminous.
• Non-albuminous have no residual endosperm because it is completely consumed during embryo development.
• Example of such seeds are peas and groundnut.
• The seeds that are albuminous, retain a part of the endosperm which is not completely used up.
• Such type of seeds can be seen in wheat, maize, barley, castor, etc.
• Integuments of ovules transform into tough protective seed coats.
• A small pore that stays in the seed coat is the micropyle.
• This acts as an allowance for the entry of oxygen and water into the seed during germination.
• The water content reduces and it becomes dry with the maturation of the seed.
• It might happen that the embryo may become dormant if it does not get the favourable conditions for germination.
• Required condition includes adequate moisture, oxygen, and suitable temperature.
• The ovary develops into the fruit only after proper fertilization.
• The ovarian wall is called pericarp that later becomes the fruit wall. 

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Significance of Double Fertilization

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Double fertilization produces two immediate products – the endosperm and the embryo. The final product is the seed.

• The endosperm provides nourishment to the developing embryo.
• Even in the young seedlings, there is sufficient food reserve to sustain them till they are capable of performing photosynthesis.
• The seed coat protects the young embryo.
• Seed formation is water independent, hence favoured by nature.
• This is because the reproductive processes like pollination and fertilization do not require water.
• Seeds are better suited for dispersal to new habitats.
• Thus, the species can colonize in other areas.
• Dehydration and dormancy of mature seeds help in their storage.
• Double fertilization being a process of sexual reproduction, leads to new genetic combinations resulting in variations.
• Polyembryony (more than one embryo from a fertilized egg) is possible.
• Hence, the viability of the seeds is increased.

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Things to Remember

• Double fertilization is a event takes place during sexual reproduction in angiosperms.
• The flower that reprouces sexually involves male and female reproductive parts in it.
• Male gamete from anther fertilizes to female gamete in ovary.
• Fusion of male gamete occurs with egg as well as 2 polar nuclei.
• Two fusion event is the reason the fertilization is called as double fertilization.
• The 2 intermediate products are endosperm and embryo.
• Endosperm supplies the nutrients required for germination.
• Embryo further germinates into seed.