The essence of double fertilization in flowering plants is that two sperm participate in it. One of them fertilizes the egg, and a zygote is formed. The second sperm merges with the central cell, from which storage tissue (endosperm) develops.
In the zygote, a double set of chromosomes is formed, and in the future endosperm, a triple set.
Fertilization in flowering plants is preceded by the formation of gametophytes.
Male gametophyte (pollen grain) is formed in the pollen chambers of the anthers of the stamen from the microspore.
A pollen grain consists of two haploid cells: vegetative and generative, covered with a membrane.
The formation of the female gametophyte (embryo sac) occurs in the ovary of the pistil in the ovule from the megaspore.
The embryo sac consists of seven cells: a haploid egg, a central diploid cell, and five auxiliary haploid cells.
When a pollen grain hits the stigma of the pistil, the vegetative cell begins to divide and forms a pollen tube. The pollen tube grows through the pistil column and enters the ovule through the pollen duct.
The generative cell of the pollen grain divides and forms two sperm. Through the pollen tube, sperm penetrate into the ovule. One sperm fuses with the egg and forms a diploid zygote. The second sperm is fused with the central cell and forms a triploid cell.
The zygote divides and develops into the embryo of a new plant. The endosperm is formed from the triploid cell. The walls of the ovule become the seed coat. Thus, the ovule becomes a seed.