Explain Double Fertilization and Triple Fusion

Double Fertilization and Triple Fusion

The pollen tube enters the synergids and releases two male gametes into their cytoplasm. One of the male gametes fuse with the nucleus of an egg cell. Which is called syngamy, The zygote or diploid cell is formed during syngamy.  The other male gamete fuse with two polar nuclei to produce a triploid endosperm nucleus. As it involves the fusion of 3 haploid nucleus , it is called as triple fusion. As two type of fusion  one is syngamy and another is triple fusion takes place in an embryosac, this phenomenon is called double fertilization. The double fertilization is unique character of a flowering plants. 
The process of double fertilization in angiosperm is studied in following steps -

Germination of pollen  After pollination the pollen or microspore are identified by a enzyme present in stigma and allow the germinate of pollen which are belonging to the same species. The identification process is due to pollen- pistil interaction and the pollen which are belonging to the same species is termed as compatible pollen. The pollen developed pollentube which move towards the ovule by Chemotrophic moment. For the Chemotrophic moment of pollen tube the chemical signal released by the synergids.
Entry of pollen tube in ovule Normally, the pollen tube enters in the ovule through the micropyle called prorogamy fertilization.  some times the entry of pollentube into the ovule occurs through integument called mesogamy. When the pollen tube entres through chalaza end the fertilization is known as chalazogamy. 
Entry of pollen tube into the embryosac The entry of pollentube into the embryosac is always through Micropyle because the micropylar end contains egg cell. Normally pollentube reach into the anyone of the synergids through Filliform apparatus. when the pollentube reach into the synergids the tube nucleus degenerate, the pollentube burst and released both the male gametes in the cytoplasm of synergids. But sometimes the pollentube reach below the egg cell passing in between anyone of the synergids and egg cells. 
Double fertilization
One male gamete fused with egg nucleus to form the zygote which is diploid in nature ( genrative nucleus) . Later on the zygote developes into an embryo. Another male gamete fused with secondary nucleus to form primary endosperm nucleus, which is triploid in nature ( vegetative fertilization) . Later on which develops into endosperm. since 1 male gamete fused with egg nucleus and another male gamete fused with secondary nucleus ( hence called double fertilization). Two haplopid nuclei fused together to form secondary nucleus which is again fused with male gamete called triple fusion. Double fertilization and triple fusion is an unique characteristics features of an angiosperm.