An orchid uses a finger-like appendage to pollinate itself

A species of fungus-eating orchid has an ingenious self-pollinating method. The secret lies in the orchid’s mysterious finger-like appendage.

“I knew there had to be more to it than just an odd-looking quirk,” says Kenji Suetsugu at Kobe University in Japan.

Suetsugu had long been fascinated by the Stigmatodactylus sikokianus orchid because it lives in shady Japanese forests and feeds on soil fungi throughout its life, rather than relying on photosynthesis. The orchid also has a little finger-like appendage under its stigma, the sticky part that receives pollen during mating.

To investigate the appendage’s purpose, Suetsugu observed the flower out in the wild, set up pollination experiments in the laboratory and tracked changes in the orchid’s flower structure with fluorescence microscopy.

He noticed that if no insects visited the orchid to pollinate it, the flower started wilting. As it drooped, the finger-like appendage gradually moved towards the stigma, bringing pollen into contact with the sticky receptor.

The appendage thus acts “like a bridge”, says Suetsugu, transferring the orchid’s pollen in a self-pollination trick, but only as a last resort. The wilting mechanism allows a plant to hold out for a pollinator but acts as a fail-safe, ensuring it can still reproduce even if an insect never arrives. The discovery “underscores how nature can come up with really creative solutions to common problems”, says Suetsugu.

The next step would be removing the appendage completely to see how much of a difference it makes in pollination timing and efficiency, says Katharina Nargar at the Australian Tropical Herbarium.

While this appears to be the first time such a self-pollinating trick has been formally documented, Nargar notes that observations from the early 1990s suggest two other closely related orchid species also use their unusual appendages to self-pollinate.