In 1875, Charles Darwin suggested some plants could capture insects using sticky hairs, but he couldn't prove it. 150 years later, researchers confirmed a Chinese species attracts and traps flies with the same mechanism

Charles Darwin had a suspicion about certain plants that he could never fully prove. In 1875, the naturalist suggested that two species in the genus Saxifraga might be carnivorous, using their sticky hairs to trap insects. Now, more than 150 years later, researchers have found evidence that supports his idea. A study of Saxifraga candelabrum, a plant growing high in China's Hengduan Mountains, found that it can attract and capture insects.

The findings, published in Nature Communications on August 4, provide evidence that the Chinese species is a true carnivorous plant.

How did Darwin suspect these plants were carnivorous?

Darwin proposed that the sticky hairs found on some Saxifraga plants could serve a purpose beyond simply being a plant feature. He believed they might allow the plants to catch insects.

His experiments, however, were inconclusive, leaving the idea unconfirmed during his lifetime.

For a plant to qualify as carnivorous, it must attract, capture and digest animals. The new research examined those features in S. candelabrum through a series of experiments.

“Talk about an amazing feeling,” study co-author and botanist Doug Soltis at the University of Florida tells New Scientist’s James Woodford. “To have the opportunity to confirm a prediction of Darwin is both humbling and very exciting.”

What makes Saxifraga candelabrum a carnivorous plant?

The researchers found that mature S. candelabrum plants had an average of 71 insects stuck to the hairs on their inflorescence branches. Younger plants, which had fewer branches, had few or no insects trapped.

The pattern suggested that plants with more branches covered in sticky glandular hairs were better able to capture insects.

Researchers then investigated how the plant attracts its prey. Flowering plants were enclosed in glass or mesh, with the setups blocking either odor or vision.

The experiments showed that the plants attract insects through their odor. They also appeared to have a selective approach to what they capture.

“The plant only targets small insects and keeps large pollinators safe,” explains Hang Sun, a study co-author and plant taxonomist at the Chinese Academy of Sciences’ Kunming Institute of Botany, to Jack Tamisiea for the New York Times.

How does the plant digest insects?

Capturing insects alone was not enough to establish carnivory. The researchers also needed to determine whether S. candelabrum could digest and absorb nutrients from its prey.

Chemical analysis found that the plant uses phosphatase, an enzyme commonly found in other carnivorous plants, to digest insects.

The researchers also used fruit flies labeled with a nitrogen isotope to track whether nutrients from the insects were absorbed by the plant. Nitrogen levels increased in S. candelabrum, while they did not rise in the control plant.

Genetic evidence added another layer of support. The analysis found that S. candelabrum shares genes with other carnivorous plants.

“Together, these genomic data provide independent evidence that S. candelabrum is a true carnivorous plant,” says Sun to New Scientist.

Could other Saxifraga plants also eat insects?

The discovery could mean that carnivory is more widespread among plants than previously thought. Doug Soltis told Live Science that he would not be surprised if other members of the Saxifraga family also turned out to be carnivorous, since some have similar sticky hairs. That possibility echoes Darwin's original thinking about the plants.

The research also highlights how much remains to be learned about the plant world. Susan Myers, a botanist at the Pittsburgh Botanic Garden who was not involved in the study, described the findings as a reminder of how surprising plants can be.

“We are still barely scratching the surface of botanical knowledge,” she says.

FAQs

Did Darwin predict this?
Yes. He proposed the idea in 1875.

How does the plant catch insects?
It uses sticky hairs.