Double Fertilisation

Two Gametes, Two Fusions, One Remarkable Outcome

Flowering plants have a trick up their sleeve that no other plant group can match. Once the pollen tube arrives at the embryo sac and enters one of the synergids (the two helper cells at the micropylar end), it releases both male gametes into the synergid’s cytoplasm. What happens next is not one fertilisation event but two, running almost in parallel inside the same tiny structure.

How the Two Fusions Work

Think of the two male gametes as having separate assignments. Each one travels to a different target cell inside the embryo sac, and the result of each fusion is completely different.

First Fusion: Syngamy

One male gamete moves toward the egg cell and merges with its nucleus. This fusion of a male gamete with the egg is called syngamy (the union of two gametes to form a single cell). Because both the male gamete and the egg cell are haploid (carrying a single set of chromosomes, $n$), the resulting cell is diploid (carrying two sets, $2n$). This diploid cell is the zygote, the very first cell of the next generation.

Second Fusion: Triple Fusion

At the same time, the other male gamete heads toward the central cell, which houses two polar nuclei (one from each pole of the embryo sac). This gamete fuses with both polar nuclei simultaneously. Three haploid nuclei ($n + n + n$) merge into one, which is why the event is called triple fusion. The product is a triploid nucleus ($3n$) known as the primary endosperm nucleus (PEN).

Once this triple fusion is complete, the central cell is renamed the primary endosperm cell (PEC).

Why “Double” Fertilisation?

Two distinct types of fusion, syngamy and triple fusion, take place inside the same embryo sac at roughly the same time. That is exactly what the name double fertilisation captures: not one fusion but two, each producing a cell with a very different destiny. This mechanism is exclusive to angiosperms (flowering plants). You will not find it in gymnosperms, ferns, or any other plant group.

What Happens After Double Fertilisation

The two products of double fertilisation set off on very different paths:

• Zygote ($2n$) goes through repeated cell divisions and gradually differentiates into the embryo. The embryo carries the future body plan of the new plant, complete with a radicle (embryonic root), a plumule (embryonic shoot), and cotyledons (seed leaves that store or absorb food).

• Primary endosperm cell ($3n$) divides rapidly to form the endosperm, a nutrient-rich tissue that feeds the developing embryo. In many seeds you eat, such as wheat and rice, the bulk of what you consume is actually endosperm.

Meanwhile, the cells that have finished their jobs begin to break down. The synergids at the micropylar end degenerate after the pollen tube has discharged its contents. The three antipodal cells at the chalazal (opposite) end also degenerate. What remains is a developing embryo cushioned by its personal food supply, the endosperm, all housed within the ovule.

Fig 1.13: (a) Fertilised embryo sac showing the zygote and PEN; (b) Stages in embryo development in a dicot

A Quick Summary Table

Event	What fuses	Product	Ploidy	Develops into
Syngamy	Male gamete + egg cell	Zygote	$2n$ (diploid)	Embryo
Triple fusion	Male gamete + 2 polar nuclei	Primary endosperm nucleus (PEN)	$3n$ (triploid)	Endosperm

Both events together = double fertilisation, a hallmark of angiosperms.