Life as a short plant can be tough. Taller competitors hog the sunlight, leaving shrimpier species to photosynthesize from whatever scraps filter through. But at least one ground-hugger has found a solution that many of us more diminutive humans have probably at least fantasized about: shoving those rangy neighbors out of the way.
The finding, reported earlier this year in the journal Current Biology, is the first documented case of interspecies shoving in the botanical literature, said Peter Grubb, an emeritus professor of botany at Cambridge University who was not involved in the research. The study authors, Dr. Grubb said, “are the first people to have made relevant measurements on the pushing power of the leaf.”
The pushy leaf in question belongs to the evocatively named tall elephant’s foot, or Elephantopus elatus. The plant is an aster that sends out long, flat leaves from a central stalk in a circular pattern known as a rosette. The foliage can form dense mats on the forest floor of pine savannas in the Southeastern United States.
“People think it’s all grasses down there,” said Camille Sicangco, who completed the research at the University of Florida before receiving her undergraduate degree in May. “But if you take the time to look a little bit harder, you’ll see there are a lot of different growth forms.”
Ms. Sicangco, who will next study botany at Western Sydney University in Australia, and Francis “Jack” Putz, a botanist at the University of Florida, plucked a few elephant’s feet from a savanna near Dr. Putz’s house on the outskirts of Gainesville and transplanted them to his lab. Ms. Sicangco then worked with engineering professors at the university to design and 3-D-print a soil-mounted cantilever system that growing leaves could push against.
The researchers placed the device next to a growing plant and left it for 24 hours. When they returned, the leaf had pushed the lever away from its initial vertical orientation. Over a number of trials, the scientists measured an average pushing force of around .02 Newtons — roughly the force needed to lift a dime. That is, in comparison to the leaf’s tiny weight, about as strong as the force that an actual elephant can deliver. The pushing force came from hydraulic pressure generated inside plant cells, Dr. Putz suspected.
The scientists next grew the aster near some sprightly rye seedlings. As the Elephantopus leaves grew outward, their outer edges sometimes bent downward, creating surfaces the plant could use to bend up to 20 grass stalks and smother them. Collectively, a single plant’s sprawling leaves commanded as much as a square foot of soil.
Dr. Putz and Ms. Sicangco weren’t the first to speculate about pushy plants. Karl Niklas, an emeritus botanist at Cornell University, suggested the possibility years ago in a book he wrote on plant biomechanics. “But,” Dr. Niklas said, “talking about it and actually documenting it are two different things.”
The finding contradicts the common view of plants as inert and peaceful, he added. While most people may “think of plants as being kind of pretty and passive, just sitting there,” he said that plants actually “manifest a number of strategies that illustrate aggression.”
The style of aggression exhibited by elephant’s foot could be widespread. The rosette growth habit is found around the world, from the fynbos shrublands of South Africa to the dry grasslands of Australia to the prairies of the American Midwest. It’s even found in common weeds such as dandelions and plantains, the bane of suburban homeowners striving for that perfect lawn. Growing low can help these plants avoid being nibbled by grazing animals, beheaded by lawn mowers or consumed by fires, Dr. Putz said — and pushing, he suspects, is likely practiced by many.
“Once you’re aware of it, it’s pretty obvious that it’s happening all over the place,” he said. “It’s in your backyard.”
The behavior could even help ecologists study a longstanding mystery: How do so many plants coexist in natural ecosystems? In prairies and savannas, plant species often maintain an exquisite balance in which dozens of species share a few square feet of space. Ecologists debate why strong competitors such as fast-growing grasses don’t simply take over. Shoving could be part of the answer, said Ellen Damschen, an ecologist at the University of Wisconsin-Madison who studies savannas similar to those where tall elephant’s foot grows.
“This pushing behavior is probably helping it have a foothold and keep that foothold” in the larger ecosystem, Dr. Damschen said.
Even though she had never observed plant pushing, she said she wasn’t all that surprised to learn about it.
“Plants can do a lot more than we oftentimes think they can,” she said. “We just don’t give them enough credit.”