Double Fertilisation

In angiosperms, the haploid, diploid and triploid structures of a fertilised embryo sac sequentially are: Synergids, Zygote and Primary endosperm nucleus.

Double fertilization is a unique phenomenon that occurs in angiosperms only. Syngamy + Triple fusion = Double fertilization

Double fertilization is a characteristic feature exhibited by angiosperms. It involves syngamy and triple fusion.

The egg apparatus at the micropylar end of the embryo sac consists of the egg cell and two synergids, which have a filiform apparatus at their micropylar tips.
Before or at the time of arrival of the pollen tube, one of the two synergids degenerates.
The pollen tube, guided by the filiform apparatus, enters the embryo sac through this degenerated synergid and discharges the two male gametes into its cytoplasm.
One male gamete then fuses with the egg (syngamy) and the other with the two polar nuclei of the central cell (triple fusion).
The other synergid persists for some time and degenerates later.
So the pollen tube enters the embryo sac through the degenerated synergid.

Double fertilisation (one male gamete fuses with the egg and the other with the polar nuclei) occurs in all angiosperms; it is a unique and universal feature of this group.
Vessels are absent in some primitive angiosperms (e.g. Wintera, Trochodendron).
Secondary growth is absent in most monocots.
Some angiosperms are not autotrophic, e.g. the parasite Cuscuta and saprophyte Monotropa.
So double fertilisation is without exception in angiosperms.

The pollen tube grows through the style and enters the ovule through the micropyle.
Guided by the filiform apparatus of the synergids, it enters one of the synergids and releases its two male gametes into the cytoplasm of that synergid.
From there, one male gamete moves to the egg (syngamy) and the other to the central cell (triple fusion).
So the pollen tube liberates its male gametes into the synergids.

In double fertilisation, one male gamete fuses with the egg (syngamy) to form the diploid zygote.
The second male gamete fuses with the two polar nuclei (or the secondary nucleus) of the central cell. This is triple fusion, forming the triploid (3n) primary endosperm nucleus, which develops into the endosperm.
So endosperm is formed by two polar nuclei and one male gamete.