Why do so many flowers have five petals?

— Ajith Kizhakkethil

Many flowers are indeed pentamerous — but across flowering plants as a whole, the petal number varies widely. Monocots often have flower parts in threes. Eudicots have four or five. Many species also have fused petals, others have several petals, and yet others lack them altogether.

In the flower bud, new organs begin as small bumps on a ring of tissue, and the eventual number is equal to the number of slots this tissue lays down in a whorl, given its size and shape and the organs’ spacing needs.

Early in the evolution of angiosperms, different major clades ‘opted’ for different numbers of slots. The monocots typically evolved three slots per whorl. The eudicots opted for four to five per whorl.

Importantly, the plant’s genes don’t control the exact number; instead they only control the dynamics of growth. For example, if the meristem — i.e. the population of undifferentiated cells that develop into new tissues per the planet’s needs — is bigger, more organs form with more parts. If an organ develops sooner, it will have more time to initiate its parts, leading to more of them. And so on.