Plants, according to Jack C Schultz, “are just very slow animals”. This is not a misunderstanding of basic biology. Schultz is a professor in the Division of Plant Sciences at the University of Missouri in Columbia, and has spent four decades investigating the interactions between plants and insects. He knows his stuff.
Instead, he is making a point about common perceptions of our leafy cousins, which he feels are too often dismissed as part of the furniture. Plants fight for territory, seek out food, evade predators and trap prey. They are as alive as any animal, and – like animals – they exhibit behaviour.
“To see this, you just need to make a fast movie of a growing plant – then it will behave like an animal,” enthuses Olivier Hamant, a plant scientist at the University of Lyon, France. Indeed, a time-lapse camera reveals the alien world of plant behaviour in all its glory, as anyone who has seen the famous woodland sequence from David Attenborough’s Life series can attest.
These plants are moving with purpose, which means they must be aware of what is going on around them. “To respond correctly, plants also need sophisticated sensing devices tuned to varying conditions,” says Schultz.
So what is plant sense? Well, if you believe Daniel Chamovitz of Tel Aviv University in Israel, it is not quite so different from our own as you might expect.
When Chamovitz set out to write his 2012 book What a Plant Knows – in which he explores how plants experience the world by way of the most rigorous and up-to-date scientific research – he did so with some trepidation. “I was incredibly wary about what the response would be,” he says.
His worry was not unfounded. The descriptions in his book of plants seeing, smelling, feeling and, indeed, knowing have echoes of The Secret Life of Plants, a popular book published in 1973 that appealed to a generation raised on flower power, but contained little in the way of facts.
The earlier book’s most enduring claim, perhaps, is the thoroughly discredited idea that plants respond positively to the sound of classical music.
But the study of plant perception has come a long way since the 1970s, and in recent years there has been an uptick of research into plant senses. The motivation for this work has not been simply to demonstrate that “plants have feelings too”, but instead to question why, and indeed how, a plant senses its surroundings.
Enter Heidi Appel and Rex Cocroft, colleagues of Schultz at Missouri who are searching for the truth about plant hearing.
“The main contribution of our work has been to provide a reason for why plants are affected by sound,” says Appel. A Beethoven symphony is of little consequence to a plant, but the approach of a hungry caterpillar is another story.
In their experiments, Appel and Cocroft found that recordings of the munching noises produced by caterpillars caused plants to flood their leaves with chemical defences designed to ward off attackers. “We showed that plants responded to an ecologically-relevant ‘sound’ with an ecologically-relevant response,” says Cocroft.
Ecological relevance is key. Consuelo De Moraes of the Swiss Federal Institute of Technology in Zurich, along with collaborators, has shown that as well as being able to hear approaching insects, some plants can either smell them, or else smell volatile signals released by neighbouring plants in response to them.
More ominously, back in 2006 she demonstrated how a parasitic plant known as the dodder vine sniffs out a potential host. The dodder vine then wriggles through the air, before coiling itself around the luckless host and extracting its nutrients.
Conceptually, there is nothing much distinguishing these plants from us. They smell or hear something and then act accordingly, just as we do.
But, of course, there is an important difference. “We don’t really know how similar the mechanisms of odour perception in plants and animals are, because we don’t know much about those mechanisms in plants,” says De Moraes.
We have noses and ears, but what does a plant have?
The lack of obvious centres of sensory input makes it harder to understand plant senses. It is not always the case – the photoreceptors that plants use to “see”, for example, are fairly well-studied – but it is certainly an area that merits further investigation. For their part, Appel and Cocroft are hoping to track down the part or parts of a plant that respond to sound. Likely candidates are mechanoreceptor proteins found in all plant cells.