The "bottle gentian" (or "closed gentian") is an unusual woodland flower -- its common name derives from the fact that its petals never open! Mature flowers look just like large buds. Although these flowers produce a rich source of pollen and nectar, most insect pollinators are not able to get inside.
Bottle gentians are pollinated almost exclusively by large bumblebees that are strong enough to force the petals open and crawl inside. This is an example of a mutualistic association -- the bees benefit by having exclusive access to a bountiful nectar supply, and the plants benefit by attracting "loyal" pollinators that improve the chances for cross pollination.