Why is fertilization in flowering plants called double?

In flowering plants, fertilization takes place in the ovary of the pistil. There is an egg cell – a female gamete with a haploid (single) set of chromosomes and a large diploid cell located in the center. The pollen grain germinates in the form of a pollen tube, which contains 2 male gametes – sperm. During fertilization, 1 sperm fuses with the egg, forming a zygote, and 2 – with the central cell, forming a cell with a triple set of chromosomes. Since there are two mergers, fertilization is called double.