as, for example an elm or maple? a random mutation may have provided a small spur to a seed. the spur changed the trajectory of the falling seed and may have allowed it to fall further from the parent plant and into a more ideal growing environment or simply further afield, which would allow it to slightly out-complete others plants.
if this change proved sucessful over longer periods of time, then the mutation would likely get more pronounced as long as the mutation continued to improve the plants fitness for its environment. fast forward 100,000 years later and environment pressure combined with inheritability and mutation have given you seeds with glide wings.