Now scientists think plants can learn, make choices, and remember things.
If that sounds pretty far-fetched, take a look at these recent experiments and see if you’re convinced.
Plants remember that it’s safe and don’t react defensively.
Ecologist Monica Gagliano of the University of Western Australia started her research career studying animal behavior, according to Atlas Obscura.
When she decided to turn her attention to studying plants, she brought a lot of her knowledge of how to study animals with her. This gave her lots of new ideas when it came to investigating how plants behave.
One experiment she decided to do was to see if plants could learn that they can safely ignore a particular stimuli after being exposed to it enough times to realize it’s not going to be harmful—a process called habituation.
She decided to use the ‘sensitive plant’ Mimosa pudica for her experiment, because it reacts very quickly to a physical disturbance.
When its fern-like leaves are touched, they fold up in seconds to give it the appearance of a thin, droopy twig—making it look less appetizing to potential predators.
Gagliano created a rig that would drop the plant a short distance, rather like a drop tower ride at an amusement park. It landed on a soft surface and was not enough of a drop to damage the plant, but enough to shock it into closing up its leaves.
After a few drops, Gagliano observed that the plants were much quicker at reopening their leaves again, almost as if they were getting used to being dropped. By the time she’d dropped them 60 times, repeating these drops seven times in all over the course of a day, they didn’t even close their leaves up in the first place.
Wondering if the lack of leaf closing was due to the plant just being physically exhausted from having to open and close its leaves so many times, she shook it gently. The leaves all closed up quickly.
Even more amazingly, the plants seemed to remember that the dropping wasn’t harmful.
When she later tried dropping the plants again, they remembered the experience from before, and didn’t close their leaves.
Surprised by this, she waited three days and tried again. They still remembered.
“Then I went back six days later, and did it again, thinking surely now they forgot,” she told Atlas Obscura. “Instead, they remembered, exactly as if they had just received the training.”
So she waited 28 days. They still remembered.
Memory can help plants make a choice.
“With the peas, I turned the dial up,” Gagliano told Atlas Obscura. “Mimosa had to follow just one experience, the drop, … while the pea had to follow two events occurring [a fan and an light].”
“Not only did the pea need to learn something, but he learned something that meant nothing, that was totally irrelevant.”
In the experiment, Gagliano trained the pea plants to associate a fan (equivalent to Pavlov’s bell) with a light source (food).
She grew the pea seedlings with a Y-shaped tube over them, so the plants had two directions they could grow in. During the first training experiment the light and fan were both turned on above one tube and then the other, three times a day for a period of three days.
When it was time to test if the pea plants actually did associate the fan with the light, she turned on just the fan above one of the tubes. More than 60 percent of the plants chose to grow towards the fan, maybe expecting that the light would turn on soon too.
In the second training, the fan was positioned above one tube, and the light above the other; then they were swapped over. This time when just the fan was turned on, 69 percent of the plants grew away from the fan, remembering that the light was in the opposite direction.
“In that context, memory is actually not the interesting bit,” Gagliano said. “Memory is part of the learning process. But—who is doing the learning? What is actually happening? Who is it that is actually making the association between fan and light?”
That is indeed an interesting question. Studies on plant memory have actually been going on for over 100 years; it’s only now that they seem to be being taken more seriously.
But now you’re probably thinking, how can a plant do that if it doesn’t have a brain?
As Gagliano’s co-author, Stefano Mancuso, director of the International Laboratory of Plant Neurobiology (LINV) in Florence, Italy, said, “Having no organs doesn’t mean that [plants] do not have the function.”
In fact, organs are a pretty dangerous set-up. If one of your organs gets seriously injured, you’re generally in big trouble. For a plant that expects to get injured a lot (i.e. eaten), having organs would probably be a bad idea.
“You can imagine that a single insect just eating a little bit of your brain would be enough to kill you,” Mancuso said. “This is why the plants have no organs.”
The process of memory formation isn’t fully understood in animals, but is thought to be related to the movement of calcium within brain cells—curiously, plants possess a similar sophisticated calcium movement system.
So basically, scientists don’t yet really understand how plants do all these things, they just know that they can. And that’s still pretty amazing.