Tuesday, 25 December 2012

Wednesday wildflower: centaury

Plants classified in the gentian family, Gentianaceae, have mostly radially symmetrical flowers, so when I looked closely at this common pink-flowered weed I got a bit of a surprise.  Its calyx and corolla are indeed radially symmetrical, but the stamens and style curve away to opposite sides of the flower, keeping out of each other's way.
Centaurium erythraea, Karori, New Zealand.
It shows nicely in a photo on this blog from Spain too, and in this case the flower colour is more intense.  The stigma is two-lobed, and the five stamens are bent away from it.

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.
New Zealand has many native gentians, mostly white-flowered.  They're classified in the genus Gentianella.  On the subantarctic islands, flower colour is very variable, from white through to deep red.

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).

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