Brilliant Green by Stefano Mancuso and Alessandra Viola

In science “Have you ever changed your mind about anything?” should be an easy question to answer. As new information comes in, you’d hope the answer would be Yes. But pinpointing when that happens can be more difficult. Often you’re swayed by a build-up of evidence. It’s not just one thing, it’s the support of multiple pieces of evidence. For that reason I wouldn’t say it was just reading this book has converted me to the idea that Neurobotany is a sensible label for researching plant behaviour. But Mancuso and Viola’s Brilliant Green: The Surprising History and Science of Plant Intelligence has had a big effect.

You might think this is a bit of a dated book to review, it was out in 2015. There’s a reason for that. The first thing I got wrong is that I was expecting a densely written and technical book. So it was always something to read later. In fact, it’s highly accessible. This is a book you could give to your friend whose only interest in biology is David Attenborough documentaries.

The reason for this is the book is well-planned. As the subtitle says, this isn’t just about the science. Mancuso and Viola take time to put the study of plant intelligence into a historical context. They start looking at our relationship with plants, starting with the Monotheistic Religions. Are plants alive in the same way that you, I or animals are? They find that plants are kind of alive, but also not really in the same way that an animal is. The note that Islamic art follows the idea of not representing living beings in its own way, with there being no problem with floral designs. This approach to plants reminds me of an example going the other way. In classical times, the silver mines at Laurion near Athens would be closed – to allow the silver to grow back.

The rest of the opening chapter looks at the relevant parts of the history of plant research, with an ever-present theme of ambivalence. The authors point out that this ambivalence has had effects in academia, with many discoveries in plants ignored until they’re also found in animals. This means that research in plants has been valued for how it correlates with animal biology (to an extent). This is why I’ve been wary of the label neurobotany, as I’ve interpreted it as highlighting the similarities between plants and animals when one of the interesting features of plants is that they’re so different. Mancuso and Viola acknowledge this problem and it comes up again later on in the book.

Chapter Two opens with a discussion of Paramecium, a single cell organism that’s been called a swimming neuron, due to electrical signalling in its body. If you’re more familiar with Paramecium than me, you’ll know it’s a protozoan, which used to be on the animal side of the plant/animal divide. Mancuso and Viola compare it to Euglena, which does almost the same stuff but has never been called a swimming neuron, or anything special. Why? The authors leave open the suggestion that it’s because that while Euglena can do what Paramecium does, it can photosynthesise too. That would put it on the plant side of the divide.

This chapter is about the strangeness of plants. This is things like the physiology that means there isn’t the dependence on organs that animals have. They also talk about the timescale that plants live and move in. The authors also highlight the independence of plants. If plants disappeared tomorrow, then animals would follow shortly after. The reverse would be bad news for many plants, but not for all.

Mancuso and Viola move on to senses. You could write a whole book about this. If you’ve read What a Plant Knows, then this still holds a few surprises. One is the section on hearing. There’s been plenty of discussion of on hearing, and quite a few people have been sceptical. Mancuso and Viola help make some sense of the need to ability to hear by moving the discussion from above ground to below ground. Soil works to transmit sound and recent research suggests there might be something worth hearing for a plant. They also refer to research that suggests roots themselves make sounds that – if other roots can hear – would be a useful source for spatial orientation. They then note that roots appear to exhibit swarm behaviour. That suggests some method for coordination.

That moves on to the next chapter where Mancuso and Viola discuss communication, between plants, between plants and animals and also an internal conversation that the various parts of a plant have with each other. The sections on internal and plant-to-plant communication reiterate something the authors brought up earlier in the book. Plants a distributed beings. The example they give is that a signal between a foot and an arm has to go through the brain in humans. In plants, it can go direct from a root to another root, or through other signals to leaves and so on.

The chapter that will cause most problems for some readers is held back to late in the book. Chapter five is Plant Intelligence. It opens with Mancuso and Viola being deliberately provocative.
For instance: what would we say if we discovered that a faraway planet is 99 percent inhabited by a certain lifeform? We’d say the planet is dominated by that life form. Now let’s come back to Earth. What do we say about our planet? That it is dominated by humans. Now, are we really sure that this thought, so reassuring in many ways, corresponds to reality? On Earth, 99.7 percent of the biomass (estimates range from 99.5 to 99.9 percent, so we’ve averaged them), or the total mass of everything that is alive isn’t composed of humans, but plants! The human species together with all other animals represents a mere 0.3 percent

They argue this dominance means we can’t think of plants as stupid. “There can only be one explanation: plants are much more advanced, adaptable and intelligent beings than we’re inclined to think.”

This section really bothers me as it compares one species with an entire kingdom. Plants as a kingdom are adaptable, but there are no coconut palms in the Arctic, whereas there are Homo sapiens in the Arctic and the Tropics. It’s the one part where I think they overstate their case. It’s a pity as the rest of this chapter is excellent. It even includes one of my own hobby-horses, plants as a model for extra-terrestrial intelligence.

After finishing the book, I was reminded of a law class where we were asked what a car was. A few of us said it was a metal box with four wheels, an engine and so on. Others said it was a device for carrying small groups of people to places. The divide was between those of us who defined something by its physical properties and those who defined something by what it did. In the past, I’ve been wary of Neurobotany or Plant Neurobiology as I’ve been thinking of the physical element, the presence of a nervous system. The term Neuro-, I felt emphasised too much similarity between plants and animals. Mancuso and Viola are clear throughout the book, this is certainly something they’re not aiming for. Instead, Mancuso and Viola concentrate on what a nervous system does and really that’s a far more interesting approach. It’s why people are interested in nervous systems at all.