Gymnosperms

Without insects, relying mostly on the wind, pollen are lucky to land in the right place.  This means that plants of your type will inevitably be more likely to live together in large groups, and also to grow higher and higher, in order to drop the pollen out with the greatest possible time in the wind, which provides the greatest possible range.

You slowly evolve into tall trees, existing in great forests.  Your reproductive system works under these conditions and you produce pollen cells which mature into sperm cells on arrival at the female plant.

Your ovules are as exposed as possible to maximise the chance of a pollen cell landing on an ovule.

The sperm cells will still need to migrate down into the egg cell, to complete the job.  Your cells have long since lost their flagella, as they are not needed any more, however they still carry the genes, they just never get turned on.  Perhaps it would be useful if, under these very specific circumstances, the genes for growing a flagellum turned back on?  Or you might try something else?

Tried and tested: bring back the flagellum.

Let's try something new.