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The intelligence of plants

Maneka Sanjay Gandhi

One day we will know for sure that all the trees we killed, the grass we pulled out, plants we mutilate and flowers we arrange in vases – all of them have the same senses and intelligence as us. They are not inanimate, passive species, but highly evolved beings.

Monica Gagliano of the University of Sydney in her book ‘Thus Spoke the Plant’ has demonstrated through scientific experimentation, the different senses of the plant. I am going to write about two of her experiments.

There is an elementary type of learning called habituation, in which a subject is taught to focus on important information, while filtering out irrelevant rubbish. Gagliano wanted to know whether the same thing can be done by a plant. As children you have played at touching the fernlike leaves of the mimosa pudica to watch them fold up immediately. This is probably a defence mechanism, or to frighten away insects. The mimosa also collapses its leaves when the plant is dropped or jostled. Gagliano potted 56 mimosa plants and made a system to drop them from a height of fifteen centimetres every five seconds. Each “session” involved sixty drops. At first, each mimosa plant folded its leaves as soon as it was dropped. But some started to reopen their leaves after just four, five, or six drops, as if they had concluded that nothing bad was going to happen. By the end, all of them were completely open, no matter how many times they were dropped. Was this just fatigue? Apparently not: when the plants were shaken, they again closed up. The plant had attuned itself to a new stimulus. Gagliano retested her plants after a week and found that they continued to disregard the drop stimulus, indicating that they “remembered” what they had learned. Even after twenty-eight days, the lesson had not been forgotten.

Humans insist that intelligence comes from an identifiable “brain”, a command centre. It could well be cells exchanging intelligent signals in a network. Memory is part of intelligence – and yet we know so little about how it works. We think that animal memory involves the laying down of new pathways in a neuron network. But there are ways to store information that don’t require neurons. Immune cells “remember” their experience of pathogens, and call on that memory in subsequent encounters. In plants, experiences such as stress can alter the molecular wrapping around the chromosomes, and this determines which genes will be silenced. Scientists now know that events, like traumas and starvation, change animal brains and can be passed on to offspring. This happens in plants too.

In another experiment, Gagiliano places a parasitic vine, Cuscuta europaea, near potential host plants. This vine coils itself around the stalk of another plant and sucks nourishment from it. The vine always chooses, assessing by scent, the host which offers the best potential nourishment. Having selected a target, the vine then performs a cost-benefit calculation before deciding exactly how many coils it should invest—the more nutrients in the victim, the more coils it deploys.

The book is full of such experiments. Gagliano also invites us to see the world at a more profound level, than what we are accustomed to. We have a constructed a simplistic narrative of plants lacking in intelligence or sentience. This version ignores their evolutionary history. Plants dominate every terrestrial environment, composing ninety-nine per cent of the biomass on earth. In comparison, humans and all animals are just traces.

Many years ago, Ashok Khosla of Development Alternatives and I visited a place in Italy called Damanhur. It is a place humming with experiments on how to live. One of the things we saw was a small machine that, when attached to a plant, brings its voice down to human hearing levels. Meaning, you can hear it speak. Not in any way that humans can understand, but it certainly converses with another plant attached to a machine too. Another plant species has a different language, but in time learns the first.

‘The Secret Life of Plants’, by Peter Tompkins and Christopher Bird, described the experiments of a CIA polygraph expert named Cleve Backster, who, in 1966, hooked up the galvanometer to the leaf of a dracaena plant. Backster found that by simply imagining the dracaena on fire he could make the needle of the polygraph machine go up and down, registering a surge of electrical activity that in human beings meant stress. “Could the plant have been reading his mind?” the authors ask.

Backster and his collaborators went on to hook up polygraph machines to dozens of plants, including lettuces, onions, oranges, and bananas. They found that plants reacted to the thoughts of humans in close proximity and, in the case of humans familiar to them, over a great distance. In one experiment, Backster found that a plant that had witnessed the murder (by stomping) of another plant could pick out the killer from a line-up of six suspects, registering a surge of electrical activity when the murderer was brought before it. Backster’s plants also displayed a strong aversion to interspecies violence. Some had a stressful response when an egg was cracked in their presence, or when live shrimp were dropped into boiling water.

Our tendency to equate behaviour with mobility, keeps us from appreciating what plants can do. In fact, many of the most impressive capabilities of plants can be traced to the fact that they are unable to move when they need something, or when conditions turn unfavourable. Imagine defending yourself and finding everything you need while being fixed in one place. Plants have evolved between fifteen and twenty distinct senses, including variations of our five: smell and taste (they sense and respond to chemicals in the air, or on their bodies); sight (they react differently to various wavelengths of light, as well as to shadow); touch (a vine, or a root, “knows” when it encounters a solid object); and sound. In an experiment, Heidi Appel, a chemical scientist at the University of Missouri, found that when she played a recording of a caterpillar chomping on a leaf, for a plant that hadn’t been touched, the sound primed the plant to produce defence chemicals. Another experiment found that plant roots would seek out a buried pipe through which water was flowing, even if the exterior of the pipe was dry, which suggests that the plants could “hear” the sound of flowing water.

Unable to run away, plants deploy a complex vocabulary to signal distress, deter or poison enemies, and recruit animals to perform various services for them. A study in Science found that the caffeine produced by plants functions not only to keep away certain insects but as an addictive drug in their nectar, which keeps bringing the same bees back, making them faithful and effective pollinators.

We now know that when a plant’s leaves are infected or chewed by insects they emit chemicals that signal other leaves to mount a defence. This warning signal contains information about the insect’s identity. The defence involves altering the leaf’s flavour or texture, or producing toxins that render the plant’s flesh less digestible. When antelopes browse acacia trees, the trees let them. When the eating increases, the leaves produce tannins that make them unappetising. When food is scarce and acacias are over-browsed, the trees produce sufficient amounts of toxins to kill the animals.

Plants signal insects as well. Corn and lima beans emit a chemical distress call when attacked by caterpillars. Parasitic wasps, some distance away, follow the scent to the afflicted plant and destroy the caterpillars.

Two years ago the Bill and Melinda Gates Foundation sponsored a think tank on plant communication in Seattle, and thirty leading scientists attended it. Open your senses. Change your view of the world.

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