Explain double fertilisation with diagram

In angiosperms, flower is the reproductive organ of the plant. Stamen, the male reproductive part of flower, is made up of anther and filament. Carpel is the female reproductive part and is composed of stigma, style and ovary. The flower may be unisexual that is, it contains either stamens or carpels or bisexual which contains both stamens and carpels. Stamen produces pollen grains that are yellowish in colour. The ovary contains ovules and each ovule has an egg cell. When a pollen grain falls on the stigma of the carpel, it bursts open and grows into a pollen tube downwards through the style towards the female gamete in the ovary. A male gamete moves down the pollen tube and enters the ovule in the ovary. The tip of the pollen tube bursts open and male gamete comes out of the pollen tube which combines with the nucleus of the female gamete present in the ovule to form a fertilised egg called zygote. 

The pollen needs to be transferred from the stamen to the stigma. The transfer of pollen grains from the anther of a stamen to the stigma of a carpel is called pollination.

If this transfer of pollen occurs in the same flower, it is referred to as self-pollination but if the pollen is transferred from one flower to another, it is known as cross-pollination. The process of pollination is achieved by agents like wind, water or animals. After the pollen lands on a suitable stigma, it has to reach the female germ-cells which are in the ovary. For this, a tube grows out of the pollen grain and travels through the style to reach the ovary. The fertilised egg divides several times to form an embryo within the ovule which develops a tough coat around it and is gradually converted into a seed. The ovary of the flower develops and becomes a fruit with seeds inside it.

• The process of fusion of one male gamete with the egg along with the union of the second male gamete with the two polar nuclei or the secondary nucleus is called double fertilisation.
• Out of the two male gametes, one fuses with the egg to carry out generative fertilisation or syngamy. It gives rise to a diploid zygote or oosphere.
• The nucleus of the second male gamete fuses with two haploid polar nuclei or diploid secondary nucleus of the central cell to form a triploid primary endosperm nucleus (PEN). The central cell is called primary endosperm cell (PEC). This is called vegetative fertilisation or triple fusion