Sticky “hairs” on two plant species in the Saxifraga genus led famed British naturalist Charles Darwin to wonder: Could some plants in this genus be carnivorous? He couldn’t prove it then, but more than 150 years later, scientists have shown that Darwin’s suspicions were correct. Darwin proposed in 1875 that Saxifraga rotundifolia and S. umbrosa were insect-eaters; their hairlike structures called glandular trichomes ooze a sticky goo and resemble hairs found in some carnivorous plants. However, to be carnivorous, a plant must lure, trap and consume insects, and Darwin’s experiments with Saxifraga plants couldn’t conclusively demonstrate that the plants fit the bill. In a study published Tuesday in the journal Nature Communications, researchers tested Darwin’s hypothesis by experimenting with Saxifraga candelabrum, a flowering mountain plant that grows at high altitudes on the southern part of the Qinghai-Tibet Plateau and in the Hengduan Mountains in China’s Yunnan and Sichuan provinces. Through chemical analysis, scientists showed how the plants attracted tiny gnats with tempting odors and then digested trapped prey using an enzyme called phosphatase. Isotope labeling — a technique for tagging forms of an element with different atomic weights — highlighted the transfer of the nutrient nitrogen-15 from insect prey to the plants that dissolved them, indicating a direct path from trapping to digestion. Genetic analysis then clarified S. candelabrum’s relationship to other sticky carnivorous plants, revealing shared genes linked to traits such as prey attraction, digestion and nutrient absorption. When the scientists expanded their investigation to include other types of traps, they found dozens of genes across different plant groups related to digesting insect remains. This data could help botanists identify other plants that have been hiding their appetite for insects in plain sight, said senior study author Hang Sun, a professor and director of the Kunming Institute of Botany at the Chinese Academy of Sciences in Beijing. “Some species of the plants previously only known to bear sticky glandular hairs or trap insects incidentally may actually have undiscovered digestive and nutrient absorption capabilities,” Sun told CNN in an email. “Carnivory is likely far more widely distributed among angiosperms” — or flowering plants — “than previously believed, and new carnivorous plant lineages may be discovered in the future.” At least 350,000 species of flowering plants exist, of which more than 750 species are known to be carnivorous. Different styles of carnivory evolved independently in 14 plant families — a process called convergent evolution — and the study provides important new information about this intriguing lifestyle, said botanist Susan Myers, horticulture and conservation director at the Pittsburgh Botanic Garden. Myers was not involved in the new research. “The study design, including identifying the necessary morphology and digestive enzymes, and especially tracing the labeled nitrogen, was brilliant,” she said in an email. “What Darwin surmised from observation and inference, these researchers confirmed with modern technology.” Trapping then eating Study coauthor Douglas Soltis, a distinguished professor at the Florida Museum of Natural History and at the department of biology at the University of Florida, has investigated the Saxifragaceae family for more than four decades, and he always wondered if some of these plants might be carnivorous, he told CNN in an email. “Many members of the family to which Saxifraga belongs have sticky glandular hairs and you often see insects stuck on them,” Soltis said. But even if its hairs trap insects, the plant isn’t necessarily digesting them, he added. Hairs can also be defensive structures for protection against plant-eaters. Their work began as Darwin’s did: by observing the plants and the insects that got trapped, in the field and under controlled conditions. Most of the mature plants that the researchers observed captured dozens of insects, with up to 70 gnats collecting on their sticky branches. The researchers found that the flowers’ scent lured their prey, using compounds previously identified as attractive to insects. Larger flies that pollinated the flowers were not trapped by the hairs, suggesting that only ineffective pollinators were caught. Insect prey is full of nitrogen, and in many types of carnivorous plants, a common driver of evolution is life in habitats where soil is naturally nitrate-poor, Soltis said. “These plants can photosynthesize — they can make their own food. But they need nitrates,” he explained. “When the soil is low in nitrates this is one adaptation that has emerged.” The strategy makes sense for S. candelabrum, which grows at altitudes of 8,200 to 10,800 feet (2,500 to 3,300 metres) above sea level in rocky, dry and nutrient-poor soil. The plant’s alpine habitat is a first for carnivorous plants, Soltis added. “Most are in boggy soils — think pitcher plants and flytraps.” One advantage of living in a remote, high-altitude location is that the plants are not currently threatened by climate change or human activity, according to Sun. “Saxifraga candelabrum is growing well under its current environmental conditions, and it is not an endangered species based on its existing population size and distribution range,” he said. ‘Barely scratching the surface’ Carnivory in Saxifraga wasn’t the first prediction of Darwin’s to later be proven correct. After Darwin saw the star-shaped Madagascar orchid Angraecum sesquipedale, with a nectar tube measuring 11.8 inches (30 centimetres) long, he wrote in 1862 that it could only be pollinated by a “huge moth with a wonderfully long proboscis,” though he had never seen such an insect. When he died in 1882, the mystery pollinator was still unknown. But Darwin was vindicated in 1903 with the discovery of the Madagascar moth Xanthopan morganii praedicta and its 12-inch strawlike tongue (the insect was reclassified in 2021 as Xanthopan praedicta). By picking up where Darwin left off, the new findings serve as a reminder that scientific research “is an ongoing relay pursuit spanning different eras,” Sun said. “It allows us to truly appreciate the continuous academic inheritance of fundamental research in the field of plant evolution passed down from generation to generation.” Identifying S. candelabrum as carnivorous also reminds us that plants are full of surprises and that there is much to still be discovered about them, Myers said. “We are still barely scratching the surface of botanical knowledge,” she said. By Mindy Weisberger, CNN
