Double Fertilization

Once a pollen tube has completed its journey through the style and ovary and has released its two

male gametes into a synergid of the embryo sac, flowering plants perform a reproductive event found

in no other group of plants: double fertilisation. The phenomenon was first described in 1898 by

the Russian botanist S. G. Nawaschin, and it is now recognised as one of the defining, unique

features of angiosperm reproduction.

The 'double' in double fertilisation refers not to two separate acts of fertilising two separate

eggs, but to two genuinely different types of nuclear fusion, occurring together, within the same

embryo sac, immediately after the pollen tube discharges its two male gametes.

The first of these two fusions is syngamy: one of the two male gametes fuses directly with the

haploid egg cell of the egg apparatus. Because both the male gamete and the egg cell are haploid

(n), their fusion restores the normal diploid (2n) chromosome number characteristic of the

sporophyte generation, and the resulting cell is the zygote -- the single cell from which the whole

embryo, and ultimately the new sporophyte plant, will develop.

The second fusion is triple fusion, so named because it involves the coming together of three

haploid nuclei rather than two. The second male gamete released by the pollen tube migrates from

the synergid to the large central cell of the embryo sac, where it fuses with the two polar nuclei

that occupy that cell (the two polar nuclei may already have fused with one another into a single

diploid secondary nucleus before this point, or all three nuclei -- the two polar nuclei and the

male gamete -- may fuse together essentially simultaneously). Because three haploid nuclei

contribute to it in total, the resulting nucleus is triploid (3n), and it is called the primary

endosperm nucleus, or PEN. The PEN subsequently undergoes repeated division to build up the

endosperm, the nutritive tissue that will feed the developing embryo -- a process described in

detail in the following section.

These two fusion events -- syngamy, which sets the embryo in motion, and triple fusion, which

simultaneously sets in motion the formation of a fresh, dedicated food supply specifically for that

embryo -- occur essentially together, within minutes of one another, inside a single embryo sac.

This is precisely what makes double fertilisation unique to flowering plants: no other plant group

combines these two distinct fusion events in this way. In gymnosperms, for comparison, the tissue

that nourishes the embryo (sometimes loosely also called endosperm) develops from the haploid

female gametophyte before fertilisation even takes place, rather than arising from a second, …