Why is double fertilization considered a unique event in the plant kingdom?

Hint: Double fertilization was discovered in liliaceous plants, Lilium martagon, and Fritillaria tenella in 1898 by Sergei Nawaschin. The fusion of one female gamete with two male gametes occurs during double fertilization. Diploid zygote and the primary endosperm nucleus are the two end products of double fertilization. Double fertilization takes place in the ovary.

Complete answer:
In double fertilization, the ovary develops into fruit and the ovule develops into a seed having an embryo and primary endosperm in it. One male gamete merges with the egg cell and forms a diploid zygote during double fertilization, this process is known as syngamy, and another male gamete merges with the two polar nuclei to produce triploid endosperm. This is known as triple fusion. Double fertilization uses both the male gametes formed by the pollen grains. This is a significant part of organisms for evolution.

Additional information:
-Gymnosperms lack double fertilization.
-It is a characteristic feature of angiosperms.
-Angiosperms are plants that bear flowers.
-Double fertilization increases the feasibility of the seeds of angiosperms.
-Double fertilization produces endosperm. The endosperm provides nourishment to the developing embryo.

Note: Double fertilization is the joining of one female gamete with two male gametes. One male gamete joins with the female egg to form a zygote whereas another male gamete joins with two polar nuclei to give rise to triploid endosperm form. When the endosperm is produced a large number of nutrients are transferred from other plant parts to the tissue and allows the endosperm to stockpile starch, lipids, and proteins. Development of ovary into fruit and ovule into a seed having embryo and primary endosperm in it is the only reason that double fertilization is considered a unique event in the plant kingdom.