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. How, when, and why they form these memories might help scientists train plants to face the challenges—poor soil, drought, extreme heat—that are happening with increasing frequency and intensity. But first they have to understand: What does a plant remember? What is better to forget?
Scientists have shied away from studying what might be called plant cognition in part because of its association with pseudoscience, like the popular 1973 book The Secret Life of Plants. Certain types of plant memories were mixed up, too, with discredited theories of evolution. One of the most well-understood forms of plant memory, for example, is vernalization, in which plants retain an impression of a long period of cold, which helps them determine the right time to produce flowers. These plants grow tall through the fall, brace themselves during winter, and bloom in the longer days of spring—but only if they have a memory of having gone through that winter. This poetic idea is closely associated with Trofim Lysenko, one of the Soviet Union’s most infamous scientists.
Lysenko discovered early in his career that by chilling seeds he could turn winter varieties of grains, normally planted in the fall and harvested in the spring, into spring varieties, planted and harvested in the same growing season. He was, in essence, implanting a false memory of winter in plants that need a cold signal to grow. Despite this insight, Lysenko was not a very good scientist. But after he published early work on vernalization in the late 1920s, the Soviet government, looking for an agricultural panacea, inundated him with money and prestige. As Lysenko gained power, he made outrageous claims about his original idea. Vernalization, he said, could transform all kinds of plants, including potatoes and cotton, and boost the bounty of Soviet lands.
The evidence for these claims was scant, but that didn’t matter. By 1936, Lysenko led a major research institute and was a member of the Central Executive Committee, the nexus of Soviet power. With the help of a government-appointed philosopher, Lysenko developed a theory of his work that mixed Marxism with the discredited ideas of French naturalist Jean-Baptiste Lamarck. The offspring of vernalized plants, he argued, could inherit that acquired characteristic, so that by changing their environment he could create new breeds of staple crops in a fraction of the time of traditional breeding techniques—just as, by changing the environment of the working class, communism could create a new breed of men.
“All the claims were based on a principle of malleability, that genes were not all that important,” says Loren Graham, an emeritus historian at Harvard who has tracked Lysenko’s career. “Lysenko was a little unclear on the existence of genes.”
In practice, Lysenko’s theory fell apart. He couldn’t breed new varieties of grains that inherited memories of winter. He had promised fields fuller than ever before, but his ideas couldn’t save the country from famine in 1946 and ’47. And when geneticists challenged his ideas, Lysenko denounced them, which led to imprisonment and death for hundreds of scientists. He is often said to be responsible for creating a missing generation of Russian geneticists, who either gave up their work, left the country, or were punished for going against him. Without them, Lysenko never could see where he was right (plants could form these memories of winter) and where he had gone wrong (this type of memory, at least, cannot be transmitted across generations). It took a generation of scientists, working in the West, to uncover the true secrets of the phenomenon Lysenko claimed as his own but never truly understood.
Even as Lysenko was making his grandiose claims, scientists on the other side of the Iron Curtain were trying to understand how vernalization works. Some of the most important investigations to examine this mystery took place in Tübingen, Germany, in the lab of Georg Melchers and Anton Lang. Melchers was a leading biologist of plant development, and Lang was a stateless, refugee Russian biologist. Together they studied vernalization in search of the biochemical secret of flowering, a hypothetical plant hormone scientists called “florigen.”
One of their study subjects was a nightshade called henbane, Hyoscyamus niger. Some plants flower after reaching a certain point in their development, like teenagers who hit puberty and start to parade their newfound sexuality immediately, regardless of the consequences. Other plants, though, behave more like teenagers who wait for summer break to go crazy: They only flower when they receive cues from their environment that it’s the ideal time to do so. Henbane is one of the latter and requires both a period of cold and the right light to bloom. Rather than growing and dying in one season, as annual plants do, certain varieties of henbane are biennials, with a life cycle that spans two growing seasons. In their first spring and summer, these plants grow as much as they can, but hold back from flowering. Only in the following spring do they burst into bloom—creamy white flowers smudged in their centers with red-wine purple that runs through the veins on their petals. For a biennial, these…
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