The Hidden Memories of Plants
By Sarah Laskow, Atlas Obscura, September 5, 2017
Monica Gagliano began to study plant behavior because she was tired of killing animals. Now an evolutionary ecologist at the University of Western Australia in Perth, when she was a student and postdoc, she had been offing her research subjects at the end of experiments, the standard protocol for many animals studies. If she was to work on plants, she could just sample a leaf or a piece of root. When she switched her professional allegiance to plants, though, she brought with her some ideas from the animal world and soon began exploring questions few plant specialists probe--the possibilities of plant behavior, learning, and memory.
“You start a project, and as you open up the box there are lots of other questions inside it, so then you follow the trail,” Gagliano says. “Sometimes if you track the trail, you end up in places like Pavlovian plants.”
In her first experiments with plant learning, Gagliano decided to test her new subjects the same way she would animals. She started with habituation, the simplest form of learning. If the plants encountered the same innocuous stimulus over and over again, would their response to it change?
At the center of the experiment was the plant Mimosa pudica, which has a dramatic response to unfamiliar mechanical stimuli: Its leaves fold closed, perhaps to scare away eager herbivores. Using a specially designed rail, Gagliano introduced her M. pudica to a new experience. She dropped them, as if they were on a thrill ride in an amusement park for plants. The mimosa plants reacted. Their leaves shut tight. But as Gagliano repeated the stimulus--seven sets of 60 drops each, all in one day--the plants’ response changed. Soon, when they were dropped, they didn’t react at all. It wasn’t that they were worn out: When she shook them, they still shut their leaves tight. It was as if they knew that being dropped was nothing to freak out about.
Three days later, Gagliano came back to the lab and tested the same plants again. Down they went, and … nothing. The plants were just as stoic as before.
This was a surprise. In studies of animals such as bees, a memory that sticks for 24 hours is considered long-term. Gagliano wasn’t expecting the plants to keep hold of the training days later. “Then I went back six days later, and did it again, thinking surely now they forgot,” she says. “Instead, they remembered, exactly as if they had just received the training.”
She waited a month and dropped them again. Their leaves stayed open. According to the rules that scientists routinely apply to animals, the mimosa plants had demonstrated that they could learn.
In the study of the plant kingdom, a slow revolution is underway. Scientists are beginning to understand that plants have abilities, previously unnoticed and unimagined, that we’ve only ever associated with animals. In their own ways, plants can see, smell, feel, hear, and know where they are in the world. One recent study found that clusters of cells in plant embryos act a lot like brain cells and help the embryo to decide when to start growing.
Of the possible plant talents that have gone under-recognized, memory is one of the most intriguing. Some plants live their whole lives in one season, while others grow for hundreds of years. Either way, it has not been obvious to us that any of them hold on to past events in ways that change how they react to new challenges. But biologists have shown that certain plants in certain situations can store information about their experiences and use that information to guide how they grow, develop, or behave. Functionally, at least, they appear to be creating memories.
When most people look at a plant, it’s hard to imagine that it’s waiting for anything. Plants don’t seem to have long-term plans. If they lack water, they droop. If it rains, they perk up. If they sense sunlight, they grow toward it. To our human way of thinking, it doesn’t seem like plants are doing very much at all. But we don’t recognize memories in people or dogs just by looking at them, but rather by their behavior. The dog comes when called by name; the person smiles in recognition. For the mimosa plants, something from the past changed how they reacted in the future--even if we don’t notice or understand why.
Scientists first started talking about “plant memory” explicitly in the 1980s. A team in France, for example, happened upon a type of memory in which a plant recalled a history of damage to a leaf on one side of its stem and therefore dedicated its energy to growing in the other direction. Since then, scientists have found that certain plants can remember experiences of drought and dehydration, cold and heat, excess light, acidic soil, exposure to short-wave radiation, and a simulation of insects eating their leaves. Faced with the same stress again, the plants modify their responses. They might retain more water, become more sensitive to light, or improve their tolerance to salt or cold. In some cases, these memories are even passed down to the next generation. We now know that plants are capable of much more than they’re given credit for. They can “hear” vibrations, which might help them recognize insect attacks. They share information by broadcasting chemicals through the air or from their roots.
In their experiments, however, [researchers] don’t observe many plant memories being formed. What if, they ask, plant memory is rare simply because it’s better for plants to forget? “Having a memory, keeping track molecularly of signals that you’ve received in the past from your environment, does have a cost,” says Eichten. “Since we don’t see memories all that often … maybe plants don’t want to remember things all the time. Maybe it’s better to put their energies elsewhere.” Even when memories do form, they can fade. Another research group has shown, for example, that a plant might form an epigenetic memory of salt stress and pass it along across generations, but that if the stress fades, so does the memory. A plant that remembers too much might sacrifice healthy growth to be constantly on guard against drought, flood, salt, insects. Better, perhaps, to let those negative experiences go, instead of always preparing for the worst.
