When I was in fifth grade, I took care of a light pink flower that sat by the windowsill in my classroom. Every time I noticed my plant leaning towards the window, I turned it around so I could watch how, after a couple days, it had tilted towards the window again. I later learned in class that plants grow towards light, but I wondered, was there more to it? Did the flower learn to do this?
Plants are everywhere, but scientists have only just begun to uncover evidence for complex traits in them, like the ability for plants to defend themselves or to learn. One recent study, as I will describe, showed that plants can learn the same way some animals do. This study opened doors to future research on plant cognition and shows how one’s imagination can inspire scientists to ask seemingly outrageous questions.
A famous behavioral experiment you may have heard of involved a scientist named Pavlov, his dogs, and a bell. Initially, Pavlov noticed that his dogs drooled every time they heard him prepare their food. So, in his experiments, he rang a bell at the same time as he fed meat to his dogs to see if they’d associate the sound of the bell with eating food. After this training, the dogs drooled at the sound of the bell, even when Pavlov had no food. His experiment showed that the dogs had learned to associate the sound of the bell – which, for dogs has nothing to do with food – with when they actually ate the food. This behavior is known as associative learning.
More than a hundred years later, evolutionary biologist Monica Gagliano asked whether plants could learn like the dogs did. Instead of using food as a reward and a bell as a neutral cue, however, Gagliano used a blue LED light as the reward and a breeze from a small fan as the cue.
To begin her experiments, she first planted garden pea seedlings at the bottom tip of a Y-shaped track. During a three-day training course, Gagliano split the seedlings into two groups. In one group, she sent a light breeze from the fan and a one-hour dose of blue light to the plants along the same arm on the Y. In the other group, she sent the breeze on the arm opposite to the blue light. This way, she could see whether the first group would learn to grow towards the breeze because it predicts light, while the other group would learn to avoid the breeze, because it predicts darkness.
After training – but just before test day – all the seedlings were divided into a test group and a control group. The control group was left in the dark and represented the ordinary direction the plants would grow without any cues they may have learned from training. By comparing her results to the control group, Gagliano would be able to show that her conclusions are reliable.
For the test group, Gagliano grew the plants in darkness and turned on the fan along one arm on the Y. For the plants that learned that the light and breeze go together, Gagliano placed the breeze opposite to arm the light was last, to see if they’d chose the breeze cue over where they previously found light.
For the plants that learned to avoid the breeze, she placed the breeze on the same arm as where the light had last been to see if they continue to avoid the breeze, even if it meant also avoiding where they previously found light.
If the plant didn’t care about the breeze, it would continue to grow towards the last place it experienced light. But Gagliano’s experiment showed that 62% of the seedlings that received light and breeze in the same arm during the first phase of the experiment still grew towards the arm with the breeze, even though it found light on the other side before. As for the other group, 69% of the seedlings that learned to avoid the breeze grew in the arm opposite the fan, even when this meant growing opposite to the last location of light (remember, Gagliano switched which direction the fan was coming from for the testing phase). Her experiment showed that the training overcame the plants’ original attraction to light.
When Gagliano presented her findings at a science meeting, many biologists were unconvinced, dismissing it outright and yelling at one another. Lincoln Taiz, a professor at the University of California, Santa Cruz, argued that Gagliano’s study did not have enough plants in her control group. If she didn’t have enough controls, she couldn’t use them for a convincing comparison.
Others took issue with her use of words like “learning” and “choice” that are usually reserved for animals. Most biologists agree that brains are required for these types of traits. For them, plants — which lack brains — could not possibly learn.
On the other hand, some argued that Gagliano’s use of metaphor and humanlike words show that her vivid imagination is what makes her so curious about the world. No experiment like hers had ever been published maybe because no one used their imagination to ask the same questions.
Science depends on people raising questions, even strange ones. Gagliano’s experiment shows how imagination can help us ask more questions and deepen our knowledge about the world. To join the conversation, read about Gagliano’s experiment here and try it at home.