Bottle Gentian
Gentiana andrewsii
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.

 Fungi and other Parasites

Fungus-growing ants

Fungus-growing ants (which include the Leaf-cutting ants) collect various materials which they feed to a symbiotic fungus that lives in their nests. The ants then feed on special nutritional bodies produced by the fungus. This is an example of mutualism. The ants are dependent on the fungus and vice-versa. This means that the ants have evolved special mechanisms to protect the fungus from parasites and predators

Large blue butterflies
 Large blue butterflies spend most of their larval stage inside ant nests, either eating ant larvae or being fed by the ants as if they were the ants' own brood (like cuckoos). This is an example of parasitism. The butterflies are dependent on the ants for survival, and have evolved special mechanisms to allow them to trick the ants into looking after them, even though it is against the ants own interests to do so.