Pollen grain contains two male gametes , on reaching to te pollen tube, one of the male gametes fuses with egg to form zygote, other male gamete fuses with polar nuceli to each endosperm , hence it involved two fertilisation, then after this, ovary change into fruit, and ovule change into seed
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Double fertilization is takes place in flower.

When pollen grains are stuck to stigma of plants then it takes sugar and water from the stigma of flower .then pollen tube started forms and grow.then two male gametes forms inside the pollen tube in which one male gamete fertilize with ovule and forms seed of plant and second male gamete fertilize with nucleas and forms endosperm this whole process of fertilization is known as double fertilization

The fusion during syngamy and during fertilization are collectively called double fertilization

In plants, fertilisation takes place twice. one gemete fuse with egg to form zygote called syngamy while other male gamete fuse with two polar nuclei called triple fission to form zygote.

The process of fusion of 1 male gamete with 2 polar nuclei
The process of fusion one of the male gamate with egg and another male gamete with polar nuclei is called double fertilization, it occurs in flowering plants………….

After pollination by pollinating agents, the pollen sticks on to the sticky surface of the stigma. Some chemical processes trigger the formation of a pollen tube. Each pollen grain releases two male gametes. The two male gametes travel down the pollen tube and reach the ovary. After reaching the ovary, one of the gametes fuses with the egg (ovum), called syngamy, and the other fuses with the polar nuclei, which are two in number and contain two chromosomes (2n) . On combination with the haploid male gamete(n) they become triploid (3n). This fusion of the male gamete with the polar nuclei is known as triple fusion, due to formation of triploid zygote. Thus, the processes of syngamy and triple fusion together is known as double fertilisation. Refer to diagram from text.
On reaching the ovary, two male gametes are formed in the pollen tbe. One male gametes fuses with the eggs, the process is syngamy, the product is zygote . The other male gametes fuses with two polar nuclei , one from each end of embroy sac, to form endosperm nucles, thus two fertilisation occurs . This is double fertilisation
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