The correct answer is option 2: Dioecious species are those which bear male and female flowers on separate plants.

Dioecious plants have separate male and female individuals, meaning that one individual plant will have only male flowers and the other will have only female flowers. This is in contrast to monoecious species, where both male and female flowers are found on the same plant.

Option 1 is incorrect because dioecious plants do not have male and female flowers on the same plant.

Option 3 is incorrect because perfect flowers have both male and female reproductive structures, and dioecious species do not have perfect flowers on all plants.

Option 4 is incorrect because imperfect flowers are either male or female, and while dioecious plants have separate male and female plants, they do not have both perfect and imperfect flowers on separate plants.

Therefore, option 2 is the correct choice for the definition of dioecious species.