Gametophyte development

Pollen (microgametophyte)

A pollen mother cell in sporogenous tissue of anther undergoes meiosis to form a radial quartet of haploid microspores
Each microspore undergoes mitosis to form a pollen grain containing a vegetative cell and generative cell.
The vegetative cell is transcriptionally and metabolically active, the generative cell is quiescent
The generative cell divides again to form 2 sperm cells.

A megaspore mother cell within the ovule undergoes meiosis to form a linear quartet of megaspores
In monosporic development (the most common type, also known as Polygonum type), three megaspores degenerate
The surviving megaspore undergoes 3 mitotic divisions to form an 8-nucleate cell. Cellularization produces the mature embryo sac with an egg cell, 2 synergids, a binucleate central cell and 3 antipodals.

A pollen grain lands on the stigma of a flower and germinates, producing a pollen tube. Germination involves active hydration of the pollen grain.
The pollen tube enters the cell wall of the stigmatic (papillar) cells and grows between cells through the style to the ovary.
In the ovary, the pollen tube exits the stylar tissue and grows on the surface of the ovary cavity. It grows to an ovule and enters the micropyle.
The pollen tube penetrates a synergid and ruptures, releasing the 2 sperm nuclei.
The synergids have regions of direct membrane contact with the egg cell and central cell.
One sperm cell fuses with the egg cell membrane, allowing the nucleus to enter the egg and fuse with the egg nucleus, forming the diploid zygote.
The other sperm fuses with the central cell membrane and fuses with the 2 nuclei to form the triploid primary endosperm cell.