Process of Double Fertilisation — Definition

Definition

Imagine a tiny seed, the future of a plant, waiting to be formed. In flowering plants, this process is quite special and involves two 'marriages' or fusions, which is why we call it 'double fertilisation'.

It all starts when a pollen grain, which carries the male genetic material, lands on the sticky top part of a flower called the stigma. Think of the stigma as a landing pad. Once landed, the pollen grain gets a signal and starts to grow a long, thin tube, much like a tiny straw, called the pollen tube.

This tube is incredibly important because it's the pathway for the male reproductive cells to reach the female reproductive part, deep inside the flower's ovule. The ovule contains the embryo sac, which is like the 'womb' of the plant, housing the egg cell and other important cells.

The pollen tube, guided by chemical signals, makes its way through the stigma, style, and into the ovule, eventually entering the embryo sac, usually through a small opening called the micropyle. Inside the pollen tube, there are two male reproductive cells, often called male gametes.

These are the 'sperm' of the plant, but they don't swim; they are carried by the pollen tube. Once the pollen tube reaches the embryo sac, it releases these two male gametes. Now, for the 'double' part: The first male gamete rushes to meet the egg cell.

When they fuse, it's like the first marriage, and this fusion is called 'syngamy'. This union creates a single cell called the zygote, which is the very first cell of the new plant embryo. The zygote will then divide and grow into a tiny plantlet inside the seed.

But what about the second male gamete? It doesn't go to waste! It travels to a large central cell within the embryo sac, which already contains two nuclei (called polar nuclei) that have fused to form a diploid secondary nucleus.

This second male gamete fuses with this diploid central cell. This second fusion is called 'triple fusion' because it involves three nuclei (one male gamete nucleus and the two polar nuclei that have already fused).

The product of this triple fusion is a triploid cell called the primary endosperm nucleus (PEN). This PEN then develops into the endosperm, which is a nutritious tissue that acts as food for the developing embryo, much like a packed lunch.

So, in summary, one male gamete forms the embryo, and the other forms the food supply for that embryo. This clever dual process ensures that the developing plant embryo has a ready source of nutrition right from the start, making it a highly efficient and successful reproductive strategy for flowering plants.