Double Fertilisation — Definition

Definition

Imagine a tiny seed, waiting to grow into a magnificent plant. For this to happen, a very special event called 'double fertilisation' must occur inside the flower. This process is exclusive to flowering plants, making them unique in the plant kingdom. It's not just one fertilisation, but two, happening almost simultaneously!

It all begins when a pollen grain, carrying the male genetic material, lands on the stigma of a flower. This pollen grain then germinates, growing a long tube called the pollen tube. This tube acts like a microscopic highway, burrowing its way through the style and into the ovule, where the female reproductive structure, the embryo sac, resides.

Inside the pollen tube, there are two non-motile male gametes (sperm cells). As the pollen tube reaches the embryo sac, it typically enters through a small opening called the micropyle. Once inside, the tip of the pollen tube ruptures, releasing the two male gametes into the cytoplasm of one of the synergids (helper cells) within the embryo sac.

Now, the 'double' part comes into play. One of these male gametes travels towards the egg cell, which is the actual female gamete. They fuse together in a process called syngamy or generative fertilisation. This fusion results in the formation of a diploid cell ($2n$) called the zygote. This zygote is the very first cell of the new plant embryo.

The second male gamete doesn't go to the egg. Instead, it moves towards the large central cell of the embryo sac. This central cell contains two special nuclei called polar nuclei. These two polar nuclei often fuse before fertilisation to form a single diploid secondary nucleus.

The second male gamete then fuses with these two polar nuclei (or the secondary nucleus) in a process called triple fusion or vegetative fertilisation. Since one male gamete (n) fuses with two polar nuclei (n+n, or a $2n$ secondary nucleus), the resulting nucleus is triploid ($3n$).

This triploid structure is called the Primary Endosperm Nucleus (PEN).

The PEN then develops into the endosperm, which is a rich, nutritive tissue that provides food for the developing embryo. So, in essence, one fertilisation creates the baby plant (embryo), and the second fertilisation creates its food supply (endosperm).

This coordinated development is highly efficient and ensures the survival and proper growth of the new plant. Without double fertilisation, the embryo would lack its essential nourishment, making this process fundamental to the reproductive success of flowering plants.