Asked by: Rosemary Whittington, East Lothian
This behaviour is called nyctinasty, and although it is by no means universal, there are lots of other flowers besides poppies that do it. Most of them instigate the closure by pumping water out of cells at the base of the petals so that they wilt themselves shut.
But the genus Kalanchoe of tropical succulents actually grow new cells on the outside of the petal base to force them shut and then grow more cells on the inside to open them again in the morning. The most likely explanation for this behaviour is to protect the pollen. At night the insects that pollinate most flowers are dormant and closing the flower keeps the pollen dry and contained. Some flowers that are pollinated by moths or bats do the opposite and only open at night.