a most particular flower

Walking around Tamarack Nature Center in White Bear Lake, MN the other day, I saw some unusual plants in wetter patches of the prairie.

This is the white form of Bottle Gentian – the more typical flower color is lilac to deep purple color.

Although the flowers look like buds that aren’t yet open, this is the typical mature flower presentation with its petals closed up tight.

The petals are tightly closed at their tips.

You have to wonder how or whether such a flower can get pollinated.  But it turns out that this plant is very particular about which pollinators it allows to perform the pollination service.  In this case, it requires large-bodied bumblebees that are strong enough to separate the tips of the petals so they can crawl into the flower to pick up the pollen and nectar within.

The bumblebee is about as big as the flower, and uses its front legs to separate the flower tips and push its head into the flower, with the body following.  Photos from naturallycuriouswithmaryholland.wordpress.com .

As the bumblebee enters the flower, it pushes its thorax against the flower’s reproductive parts, rubbing pollen onto the exposed stigma, pollinating it.  By vibrating its wings and body inside the flower, the bee causes that flower’s anthers to release pollen onto the surface of the bee, which it combs off into pollen sacs later.  The nectar is located at the base of the flower, so again, it requires a large bee with a long tongue to reach the nectar source.

In the video below, you can see how hard the bee works to get into the flower and hear the buzzing while the bee is completely encased by the flower. (Video by NaturalistDave Nature Video)

Typically, the bumblebee will visit just one flower of the group at the tip of the plant, moving to the next plant after exiting a particular flower. This ensures the cross-pollination which is required for seed set. When pollinators are excluded from these flowers, only 4% of the flowers were able to produce seed, but when pollinators were allowed, 96% of the flowers produced seed.

Why would bumblebees go to all this work to get into such a tightly closed flower, when there are so many other flowers with nectar and pollen to harvest in the prairie?  Or are there?

Bottle Gentian times its flowering for periods when there are fewer bumblebee pollinated flowers available.  Bee Balm (center) has finished flowering on this prairie and goldenrod has not yet bloomed, so the rich nectar and pollen resources in Bottle Gentian represent a good alternative.