Complex sentence: The majority of plant species are monoecious, and hence have either bisexual flowers or possesses both male and female flowers on the same plant; but still a significant number of plant species are dioecious.
Split into simpler sentences: The majority of plant species are monoecious and hence have either bisexual flowers or they possess both male and female flowers on the same plant. Still, a significant number of plant species are dioecious.