Because the gametes are haploid. Whereas the Zygote are diploid.
The process of fertilization consist of fusion of male cell and the egg cell inside the ovary. The stigma of each flower secretes a liquid containing sugar. The pollen grains respond to this liquid and start growing. However, the pollen grain start growing only if the liquid is produced by a flower of a different species. After pollination, a thin tube called the pollen tube grows down from the pollen grain through the pistil. The pollen tube carries the make cell. It grows until it reaches the ovule and enters it. The make cell moves into the ovule and fuses with the egg to form zygote.
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