Decades after his journey aboard the HMS Beagle, Charles Darwin developed a fascination with the way plants grow and move, twisting and spinning into corkscrew shapes. Now, more than 150 years later, researchers may finally have unraveled this mystery.
In a recent study, physicists from the United States and Israel have potentially uncovered the reasons behind an intriguing behavior in growing plants, a mystery that captivated Charles Darwin in his later years.
To many people, plants appear to be fixed in place and somewhat uninteresting. However, they are actually quite active. For instance, if you observe a timelapse video of a sunflower seedling emerging from the ground, you’ll notice that it doesn’t merely shoot upwards. Instead, as it grows, the top of the sunflower rotates, twists into corkscrews, and generally moves around, albeit slowly.
The research team, led by Orit Peleg from CU Boulder and Yasmine Meroz from Tel Aviv University, found a significant purpose behind these seemingly chaotic movements, referred to as “circumnutations.” Through greenhouse experiments and computer simulations, they demonstrated that sunflowers utilize these movements to explore their surroundings in search of sunlight.
“Many people overlook plant movement because, as humans, we’re often observing them at an incorrect frame rate,” noted Peleg, a co-author and an associate professor at the BioFrontiers Institute and the Department of Computer Science.
The results of this research were published on August 15 in the journal Physical Review X.
These discoveries may eventually aid farmers in devising innovative techniques for optimizing crop arrangements.
“Our team frequently investigates social behavior in insect swarms and other animal groups,” said lead author Chantal Nguyen, a postdoctoral researcher at BioFrontiers.
“However, this study is particularly thrilling as we observe similar dynamics in plants, which remain fixed in place.”
Darwin’s observations of cucumbers
Nguyen further explained that plants do not tend to move around like animals. Instead, they grow in various directions over time. This behavior intrigued Darwin long after his expedition on the HMS Beagle, as noted in historical records.
In the 1860s, while dealing with various health issues that constrained his own movements, Darwin dedicated time to studying plants at his residence. He grew seeds from cucumbers and other varieties, meticulously tracking the movements of their tops day by day—resulting in maps that appeared chaotic and random.
“I find a great deal of amusement in observing my tendrils—it’s precisely the kind of absorbing work that suits me,” he wrote to a friend in 1863.
Despite his fascination, Darwin could not determine why some tendrils would twist.
This perplexity also intrigued Meroz, a physicist. A study from 2017 pointed her in the right direction. Researchers from the University of Buenos Aires discovered that sunflowers, when grown in tight spaces, arranged themselves in a zig-zag pattern, resembling the teeth of a zipper. This formation likely enables the plants to collectively maximize their exposure to sunlight.
Meroz began to ponder whether plant movements could be a key factor influencing such growth patterns.
“For climbing plants, it’s evident that they need to search for supports to twine around,” said Meroz, an expert in plant sciences and food security. “But for other types, the reason for this movement isn’t obvious.”
Following the sun
To further investigate, Meroz and her colleagues cultivated five one-week-old sunflowers in rows and, like Darwin, tracked their movements over the week.
Subsequently, Nguyen and Peleg created a computer program to decode the growth patterns of the sunflowers. They also used the simulations to explore various movement scenarios—whether the sunflowers wiggled randomly or with a consistent pattern.
The results were intriguing: if the digital plants did not move at all, they all leaned away from each other in a straight line. In contrast, if they wiggled excessively, they grew in an erratic manner. However, with just the right level of randomness, they formed a zig-zag pattern, which in real sunflowers, significantly improves their sunlight access. Nguyen noted that plants appear to circumnutate to locate the best light sources, subsequently growing toward them.
“Introducing a bit of randomness allows the plant to explore its environment and adopt configurations that enhance its light exposure,” she explained. “This naturally results in the distinctive zig-zag pattern we observe.”
Looking ahead, the researchers plan to investigate how sunflowers behave in more complex arrangements. Meroz expressed her satisfaction in seeing plants recognized for their dynamic nature.
“If we lived at a pace similar to that of plants, you could wander down a street and witness them moving,” she remarked. “Perhaps we would even consider plants as pets.”
