The structure in a flower where sperm cells are formed is called the pollen grain.

Here's why:

* Pollen grains are produced within the anther of a flower, which is part of the stamen (the male reproductive organ).

* Inside the pollen grain, two sperm cells are produced through a process called mitosis.

* These sperm cells are eventually released when the pollen grain lands on the stigma of a flower, initiating the process of fertilization.