Planta Sapiens by Paco Calvo review – extraterrestrials in the garden

·5 min read

In Denis Villeneuve’s 2016 movie Arrival the US army asks an expert in linguistics to decipher the complex language of the seven-limbed aliens (“heptapods”) who have landed on Earth. It’s a memorable and indeed moving attempt to portray the immense challenges involved in bridging the gulf of mutual incomprehension between two completely different species.

I thought of Arrival while reading Paco Calvo’s remarkable book, the result of “two decades of passionate exploration into a rich and alternate world that exists alongside our own” – the world of plants. The subject of his exploration is startlingly radical: the question of whether plants can be regarded as possessing intelligence.

Calvo is a professor of the philosophy of science in the Minimal Intelligence Laboratory at the University of Murcia, Spain. Although he presents detailed scientific evidence to support his case, he also draws on philosophical arguments about the nature of consciousness. We humans have a tendency to believe that the world revolves around us, but Calvo writes that intelligence is “not quite as special as we like to think”. He argues that it’s time to accept that other organisms, even drastically different ones, may be capable of it.

“I am getting very much amused by my tendrils,” Darwin wrote to a friend in 1863. Confined to his sickbed for weeks, the author of On the Origin of Species occupied himself by studying the movements of cucumber plants on his windowsill. As he convalesced, he was forced to live slowly – “to become more plant-like”, Calvo writes – and this stillness opened his mind to the wonder of his vegetable companions: “It allowed him to see them more on their terms, to experience plant life at plant pace.”

Darwin has clearly been a guiding presence in Calvo’s attempt to open up a new frontier in science: “He learned to think differently and clearly outside the frameworks in which most of his contemporaries happily confined themselves.” The result of his confinement with the cucumbers was a 118-page monograph on The Movements and Habits of Climbing Plants. Darwin realised before anyone else that these movements were in fact “behaviour”, comparable to that of animals. And observing behaviour is the route to understanding intelligence. In plants, it reveals a range of faculties “from learning and memory to competitive, risk-sensitive behaviours, and even numerical abilities”.

In the course of his book, Calvo describes many experiments that reveal plants’ remarkable range, including the way they communicate with others nearby using “chemical talk”, a language encoded in about 1,700 volatile organic compounds. He also shows how, like animals, they can be anaesthetised. In lectures, he places a Venus flytrap under a glass bell jar with a cotton pad soaked in anaesthetic. After an hour the plant no longer responds to touch by closing its traps. Tests show the plant’s electrical activity has stopped. It is effectively asleep, just as a cat would be. He also notes that the process of germination in seeds can be halted under anaesthetic. If plants can be put to sleep, does that imply they also have a waking state? Calvo thinks it does, for he argues that plants are not just “photosynthetic machines” and that it’s quite possible that they have an individual experience of the world: “They may be aware.”

Other studies show that some plants retain a memory of where the sun will rise, in order to turn their leaves towards the first rays. They store this knowledge – an internal model of what the sun is going to do – for several days, even when kept in total darkness. The conclusion must be that they constantly collect information, processing and retaining it in order to “make predictions, learn, and even plan ahead”.

Of course, these are revolutionary ideas and, as Calvo admits, contested by many scientists who study the physiology of plants. But he guides us patiently through the latest research and builds a compelling case that, unlikely as it may seem, deserves to be taken seriously.

Clearly plants don’t have brains in any familiar sense of the word. But although they lack our grey matter, Calvo believes they have a unique “green matter”. In the absence of nerves, plants use networked cells to regulate themselves. Their vascular system consists of tubes arranged in layers, like the mammalian cortex, that run from root to shoot. It transmits electrical signals, like a “green cable that carries news throughout the plant”. This, he suggests, is where we need to look for a “phytonervous” system. Calvo admits, however, that it is too early to say whether, even in the most complex plants, it amounts to “the functional equivalent of a hierarchically organised but diffused brain”.

Fundamentally, Calvo’s important book is about changing our perception of plants. He points out that without them “human life would be untenable”. As we confront the reality of the climate crisis, we need to accept plants as “co-inhabitants of the planet”. Grasping this reality could lead to a fundamental shift in our view of our own role in the biosphere, and help us to work to rebalance our destructive effects on it. As in the movie Arrival, embracing the otherness of a fundamentally different form of life could transform both our understanding of ourselves and our role on the planet.

Calvo has a wonderfully infectious enthusiasm for his subject that makes this book, for all its complex science, a joy to read. He challenges us to set aside our “zoocentric” perspective and to change our view of plants radically: from mechanisms akin to robots to complex organisms with a range of behaviours, responding to and anticipating their environments. In doing so, he has written a genuinely mind‑expanding book.

• Planta Sapiens: Unmasking Plant Intelligence by Paco Calvo with Natalie Lawrence is published by Little, Brown (£14.99) on 25 August. To support the Guardian and Observer, order your copy at guardianbookshop.com. Delivery charges may apply.