The eccentric pioneers of vegetable electricity

By Sally AdeeFeatures correspondent

In the 19th Century, a handful of scientists were gripped with a strange obsession – that electricity might be harnessed to make plants grow better. Could they have been on to something?

There's a good chance you're familiar with Frankenstein's monster. But have you heard about his garden?

Around the time the scientist who inspired Mary Shelley's novel Frankenstein was busy electrocuting live animals and dead prisoners, several of his contemporaries were doing the same to perennials and root vegetables. And just as these 18th Century forays into electrical stimulation purported to make the human body more robust (by delivering it from maladies ranging from paralysis and depression to diarrhoea and venereal disease), they were also being investigated for the betterment of plant life. Experiments on electrified gardens were alleged to produce a range of benefits, from brighter flowers to tastier fruit. Before long, this pursuit went the way of its cousin, medical electro-quackery, and by the end of the 19th Century, respectable science had largely jettisoned both.

More than a century on, better tools and new insights are reanimating the study of electricity's effects on biology. Uninformed early animal experiments have resolved over the past 200 years into real understanding – and led to promising electrical medicine. Similarly, the old vegetable experiments are being exhumed to see what modern fruit they may yield. Maybe the new understanding could even improve 21st Century gardens.

The first hints that electric shocks might have a dramatic impact on crops came not from any human intervention but from nature itself. After a lightning storm, according to longstanding Japanese farming lore, mushrooms would proliferate madly.

But you couldn't exactly call down lightning on demand to confirm this experimentally. Until, that is, the 1740s when various new devices allowed scientists to store and deploy this still-mysterious phenomena of "electricity" at will for the first time. 

Soon deploying electricity as a gardening aid became a hot topic. Pierre Bertholon de Saint-Lazare – a French physicist and philosopher who experimented widely on the still poorly understood mysteries of electricity – curated many of his contemporaries' plant experiments into a collection, De L'électricité des Végétaux.

Alongside the brighter blossoms, flowers were alleged to bloom earlier after electrification; similarly, electrifying fruit reportedly hastened the ripeness of their smell and taste. But Bertholon's main focus was on the new device he had invented: instead of zapping individual fruits and vegetables one by one, the huge contraption could infuse electricity into entire garden plots. It electrified the very soil and air that nurtured the growing plants – as if it was an electrical "manure".

The electro-vegeto-meter

The elevated system of masts and wiring Bertholon had rigged up collected atmospheric electricity, drew it down, and distributed it into his crops. According to him, it mimicked the stimulating effects of lightning. Only it did the job better than the natural variety, dispensing small, continuous amounts of electricity rather than dosing with a single, damaging strike. The "electro-vegeto-meter", he reported, increased the growth of the plants beneath its arc, accelerating "the germination, the growth, and production of leaves, flowers, fruit, and their multiplication".

Bertholon also made copious use of electricity in other forms, reportedly dispatching insect pests by using a rudimentary tool to zap an infested tree. His contemporaries had many other colourful uses for electricity in their gardens – one set out plans to irrigate his plants with a special water that he claimed, rather dubiously, had been "impregnated with electrical fluid" to replace traditional approaches to fertiliser.

Not everyone was convinced. Things went badly after Jan Ingenhousz, the Dutch-British physiologist who discovered photosynthesis, availed himself of an electro-vegeto-meter of his own to use on his garden – and it promptly shrivelled up all his plants. He concluded that Bertholon's electrical manure was, well, manure.

Interest in electroculture waned. A few private gentleman scientist types continued to run small experiments: in the 1830s, one  claimed his experiments demonstrated that plants are excellent conductors, implying that electricity was a fundamental aspect of their biology. But neither the science nor tools were sufficiently advanced to support such claims. After that, apart from a few niche projects, the idea of electroculture swiftly fell out of favour among the electrorati.

"We cannot avoid asking ourselves," wrote two critics in a plaintive 1918 paper, looking back on the fall of the events, "how it is that while the study of electricity and its many industrial applications has developed into enormous importance, electroculture in the meantime has remained practically stationary for a century and a half." They concluded: "We probably find the answer in the stagnation of the science of the living plant."

In other words, to improve electroculture you'd first have to understand how it might work, and to understand that, one would need to understand the electrical dimensions of plant biology. Luckily, by the time the duo voiced their complaint, the first slim shoots of exactly such an endeavour were already poking through the frost. Interest in vegetation and electricity had been reanimated by none other than Charles Darwin.

Darwin's carnivorous vegetables

His grandfather had been convinced that electricity could hasten the growth of plants – but Charles Darwin's contention was built on more solid scientific ground. He believed electricity to be a fundamental aspect of plant physiology, the same way the neurophysiologists of the 19th Century were starting to show how electric signals are the fundamental underpinning of the human nervous system signals that let us to think and feel and move.

Darwin's obsession had started small, with a single meat-eating plant in the genus Drosera, otherwise known as the sundews. Barely a year after the publication of On the Origin of the Species, it was all he could think about. "At the present moment, I care more about Drosera than the origin of all the species in the world," he wrote in 1860. Little wonder. Drosera did everything plants aren't supposed to – it ate meat, and it hunted. Its long, sticky tentacles trapped flies on glue-like secretions and then curled inexorably around the unfortunate prey until it was wrapped up like a macabre Swiss roll.

How could this be? "Carnivorous vegetable" was an oxymoron! But Drosera wasn't alone. Dionaea muscipula (you know it as the Venus flytrap) hunted even faster – as Darwin admiringly described, "the leaves of which catch insects just like a steel rat-trap". Their reflexes seemed animal-like. One friend, a physiologist and botanist whose expertise straddled the plant and animal kingdoms, suggested they examine these odd plants for the same kinds of "nervous" electrical changes that physiologists had recently identified animating animal muscles. 

They found them. The published results showed that when the flytrap slammed shut, it was accompanied by activity that looked awfully similar to the action potential that had defined animal electricity. These signals were not unique to the animal kingdom.

But their ideas, too, were overwhelmingly rejected by plant physiologists. You can understand why: carnivorous plants moved fast and hunted like animals – so for them, nervous signals made a kind of sense. But other plants didn't move, and they didn't hunt. They just sat there and ate sunshine. It made no sense to them to extrapolate the unique attributes of the carnivore – a taxonomic outlier – to the rest of the plant kingdom.

A couple of decades later, an Indian engineer and polymath called Jagadis Chandra Bose revisited Darwin's question. He was particularly curious about Mimosa pudica, a little fern-like perennial. It doesn't eat meat – but it does move. It folds up its little fern leaves when startled – a remarkable tic that has earned it a slew of nicknames over the years, including "sensitive plant" and touch-me-not. Bose reckoned that these fast movements should be underpinned by animal-like nervous activity too.

Sure enough, an electrometer revealed the action potentials he was looking for, spiking right before the little plant folded up its leaflets, just as they had been found preceding the snap-shut response of the Venus flytrap. Bose's curiosity was inflamed: what other plants had electric signals? In 1901, he reported strong electrical signals in a slew of ordinary plants that neither moved nor ate, including rhubarb and horse radish. Over the next decades these findings were extended to onions, trees, and pretty much every member of the plant kingdom anyone bothered to measure.

Plants are electric

This went largely unexplained until the late 20th century, when neuroscience tools revealed that plant cells use electrical charges to manage their internal communications, just as animal cells do. All living cells have pores in their outer lining which ensure that different ions stay on different sides of the membrane. Mammalian cells like to keep potassium ions inside and sodium ions outside. As a result of these imbalances, the inside of the cell carries a tiny negative charge. The nervous system uses these little batteries to send all messages about what the body is feeling and doing to and from the brain.

Plant cells have an inner voltage too, and they use them to the same effect: to communicate information about their environment. Research conducted in the late 1990s demonstrated that plants responded electrically to different stimuli, including light, temperature, touch, and injury. This aligned with insights from chemical plant communication, which suggested that plants can sense danger, communicate with other plants and call to animals for help. Corn, for example, can summon wasps to attack the kinds of caterpillars that attack corn. During those decades, concepts that had previously only been associated with neuroscience increasingly crept into plant physiology.

Such findings reignited a decades-old conversation about plant intelligence, perceived as an irrelevant wild goose chase in some circles of plant electrophysiology. Are plants intelligent? If so, what would it say about our definition of "intelligence"?  The debate continues, but it is not the only way to think about plant electrical signals.

Some botanists are not averse to the idea that plants use complicated signals to communicate with each other and the natural world. It's just that they are not like ours. In animals, electrical communication works like this: nerve cells like to keep potassium inside and sodium outside, and the electrical differences created by these ions' separation fundamentally underpins the neuron's ability to send an action potential. However, sodium plays no part in plant action potentials, because sodium is toxic to plants. In their bodies, the roles of potassium and sodium are played by potassium, chloride, and calcium. The electrical signals this enables look different, on closer inspection. For one thing they are stronger. For another they have a slightly more varied repertoire. Apart from the standard action potential, plants also enlist two further signals - the "variation potential" and the "system potential".

These signals coordinate different systems. The action potential basically acts like it does in animals: communicates quickly and over long distances, about interesting stimuli, for example someone touching it or a palpable temperature change. The variation potential is more variable (as the name suggests); it's triggered by cutting, burning, and other kinds of injury, and the size of the signal depends on the severity of the damage. The surface potential is slow and local and probably linked to nutrient status.

But plants don't just use these signals to talk to themselves about their internal state: they may also be talking to one another. Some believe they can travel through a network of fungal filaments that are ubiquitous in soil and appear to act as circuitry.

This has raised a new prospect. Could we eavesdrop on plants, and decode these electrical signals ourselves? From knowing whether the plants are sitting comfortably – are they too hot or cold? Do they need more nutrients from the soil? Or could they give us an early warning that our plants are being attacked by pathogens? 

It raises a tantalising prospect – we may be about to find out what our vegetables are "thinking".

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