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. But this is tricky. Ovules are rich in chemistry that bacteria and mycota would like to eat, and they are blowing around on the wind too.

It would be good if the ovule could be less exposed, and the sperm cells make their own way inside.

Your cells have long since lost their flagella, as they were 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?