Carnivorous plants stir the imagination. You can find the results in science fiction novels (The Day of the Triffids), Broadway plays (Little Shop of Horrors) and in recent research that concludes that the Venus flytrap can count. Not out loud, of course. And no one is claiming that the plants are aware that they are counting. But even so, this is the first time someone has demonstrated counting in a plant, according to the researcher who led the experiments, Rainer Hedrich at the University of Wurzburg, in Germany.
Rainer, Jennifer Bohm and Sonke Scherzer, all at Wurzburg, and a team of other scientists reported their research in Current Biology. Venus flytraps are carnivorous. They live in poor soil and pull needed nutrients from the insects they trap and dissolve. Their trap is a pair of leaves that act as jaws and a stomach.
When an insect lands and bumps into trigger hairs on the surface of these leaves, the trap closes. As digestive enzymes seep into the trap, it becomes what Rainer calls a “green stomach,” and the prey is gradually turned into a nourishing soup.
Scientists knew that an insect had to bump the trigger hairs more than once to cause the trap to shut, presumably to avoid wasting energy by responding to random raindrops and windblown debris.
In the recent experiment, researchers studied how the plant was responding to movement of the trigger hairs, and determined that it was counting electrical pulses from them.
Plants don’t have a nervous system to transmit these pulses as animals do, but a spike in electricity produced by biochemical changes can travel on the surfaces of cells.
The researchers flicked the trigger hairs while they recorded electrical activity in the plant. The motor cells that close the leafy jaws on prey acted only when they received two signals within about 20 seconds. That meant that the cells somehow remembered the first signal for a short time. After 20 seconds, this first electrical pulse was forgotten, essentially resetting the process.
But closing the trap on an insect is only Step 1. The Venus flytrap must also dissolve its prey. Two flicks of a trigger hair were not enough to kick off that mechanism. More than three flicks of a trigger hair were needed to signal the cells that produce digestive enzymes to begin that process.
In nature, the trigger hairs are activated time and again as the trapped prey struggles. That frenzy gives the plant a way to judge the amount of digestive enzymes needed. Rainer and colleagues found that more electric signals from the trigger hairs translated proportionally into more enzymes for the green stomach.
David Clapham at Harvard, who studies the biochemistry of how animal cells generate electric signals as a way to transmit signals in their nervous systems, said he was intrigued by what seemed to be this “just-in-time” system of providing digestive juices.
The flytrap expends energy only to produce enzymes when they are needed and only in the amount needed, an efficient mechanism for a plant living in a poor environment.
The process is slow compared with what occurs in animals, he said, but “plants have a lot more time to react.” Rainer said that electrical signals were produced by biochemical changes and that this process evolved very early in the history of life. “A single cell can be electrically excited,” he said.
Asked about primitive animals, like the worm C. elegans, studied in many laboratories around the world by many, many scientists, he joked, “I think the Venus flytrap is much smarter than C. elegans,” quickly adding with a laugh, “Don’t quote me on that.”