Flowering plants are characterised by their use of double fertilisation. A female gamete unites with two male gametes to create a new gamete. A zygote is formed when one of the male gametes fertilises an egg, and endosperm is formed when the other male gamete connects two polar nuclei.

Double fertilisation accelerates plant growth, resulting in ovules becoming fruits and ovarian development into seeds. The plant becomes diploid after the haploid male and female gametes join.

Double Fertilisation Process

After being placed on the stigma, pollen must develop and evolve through the style to reach the ovule. The pollen tube cell and the generative cell make up the pollen or microspores. A pollen tube cell develops into a pollen tube through which the growing cell flows. The pollen tube needs oxygen, water, and particular chemical signals to evolve. The style’s tissues help the pollen tube develop through the embryo sac. It dissociates to form two sperm cells if the generative cell hasn’t yet divided into two. 

The synergids in the embryo sac release substances that allow the pollen tube to pass. The pollen tube enters the ovule sac via the micropyle. The alternative sperm cell connected with the other two secondary nuclei (polar nuclei) develops a triploid cell that transforms into the endosperm. Double fertilisation occurs when these two angiosperm fertilisation mechanisms are combined. After the fertilisation process is over, no more sperms can enter. The fertilised ovule generates the seed, while the ovarian tissues develop into the fruits that generally contain the seeds. 

Embryo development begins after conception. The zygote separates into two cells, the terminal cell and the basal cell. The suspensor is formed by the basal cell dissociation. The suspensor allows food from the parent plant to reach the growing embryo. Pro-embryos are formed when the terminal cell dissociates. Due to the presence of two rudimentary cotyledons, the growing embryo of eudicots or dicots resembles a heart. Non-endospermic dicots, like Capsella bursa, grow endosperm first, then digest it. The nutrients are delivered into the two cotyledons. As the seed grows, the cotyledons and embryos get crowded and forced to twist. The cotyledons and embryos eventually inhabit the seed completely, allowing it to be dispersed. The embryo’s development is halted and resumes only after the seed expands. Unless the first leaves start photosynthesis, the seedling relies on the cotyledons for sustenance.

Double Fertilisation in Angiosperms

• Angiosperms are flower-bearing flora that is the most structurally and functionally diversified of terrestrial plants. Angiosperm flowers have separate male and female reproductive systems. Their gametes are sperm and egg cells, respectively.

• Pollination helps pollen grains reach the stigma via style. The sperm cells enter the ovule synergid cell. Then comes fertilisation.

• Fertilisation produces two structures, the zygote and the endosperm, hence the name “double fertilisation.”

• Doubling the number of sperm cells produces a triploid endosperm nucleus (PEN) and two diploid zygotes (2n). One polar nucleus is united with the egg mobile, while the other two are merged.

• The term “triple fusion” refers to the process by which three haploid nuclei are fused to form endosperm. Several endosperm nuclei develop into the primary cell (p.c.) and finally into the endosperm during the last stages of development.

• After several cell divisions, the zygote will develop into an embryo.

• The embryo begins to develop, becomes congested, and eventually begins to bow.

• During the last stage, the embryo fills the seed’s volume.

Significance of Double Fertilisation

• Two points need to be mentioned to fully understand the relevance of double fertilisation in angiosperms.
• Only two products are generated as a result of the two fusions.
• After fertilisation, the triploid primary endosperm nucleus (PEN) forms into a nutritive tissue called the endosperm, which feeds the growing embryo with nutrients.
• The outcome of the fusion process retains the diploid state that existed earlier in the life cycle. The diploid zygote develops into an embryo, which then develops into a new plant as a result of the process.
• Double fertilisation uses both male and female gametes produced by a pollen grain.
• Poly-embryology increases the chances of the new plant surviving and reproducing.
• Double fertilisation encourages the establishment of reproductive organs that turn the ovary into fruits and ovules into seeds.
• Angiosperm seeds are easily accessible due to their double fertilisation.
• Double fertilisation causes feature re-fusion, resulting in variation in the progeny.

Conclusion

There is a possibility of polyembryony, and the plant will have a better chance of surviving. Double fertilisation results in the formation of an endosperm, which is responsible for providing sustenance to the developing embryo. It improves the viability of angiosperm seeds by increasing their germination rate. It is important to note that in angiosperms, double fertilisation results in the production of an embryo as well as an endosperm, both of which are necessary for the generation of viable seeds. Diploid plants produce diploid embryos and triploid endosperm tissue, while triploid plants produce triploid endosperm tissue. The endosperm has two mother genomes and one paternal genome.