|Centaurium erythraea, Karori, New Zealand.|
This separation of male and female parts in a flower is called herkogamy. There are different kinds of herkogamy, classified according to how they function. This looks a bit like movement herkogamy, where one sex is presented first, then moved away to make room for the other sex. Here you might imagine that the stamens were presented in the middle of the flower, then moved to one side as the stigma grew up. However, all the flowers I saw—young ones and old ones—were like this, so it clearly doesn't change as the flower ages. It could be approach herkogamy, which ensures a pollinator contacts one sex first, except that a radially symmetrical flower like this can be approached from any direction. It'll need some observations of how pollinators interact with these flowers, and that's best done in its native range, in Europe.
|Gentianella, Enderby Island.|
Addendum (27 December 2012)
In Europe, centaury is pollinated by hover-flies, with a fail-safe mechanism for delayed self-pollination in the event of pollination failure (link here).