An interview with plant philosopher Norbert Peeters, by sanne bloemink
When the Venus flytrap was first discovered, botanists considered the carnivorous plant to be a curiosity. John Ellis, an 18th-century naturalist, described the plant in a letter to Carl Linnaeus, who was the most renowned botanist of his time. As his letter described, the inside of the plant is covered in little tentacles, and Ellis thought that insects climbing inside the plant were killed by these tentacles. Linnaeus rejected this idea immediately, however, believing it impossible for something like that to exist in nature. People had always assumed that all green, growing things on earth existed for the good of humans and animals, and definitely not the other way around.
Botanists usually suggest that this idea goes back to Aristotle, who famously made a distinction between different, cumulative souls. In the model he proposed, plants only have a vegetative soul that allows them to grow and nothing else. Animals have this vegetative soul as well, but they also have a sensitive soul that allows them to use their senses. At the top of this model are humans, who have three souls. They have the vegetative and the sensitive soul, just like animals, but they also have a rational soul. It is possible for us to lose this soul, though. When a person loses cognitive function and the ability to respond to sensory input, we often say that he or she is in a ‘persistent vegetative state’ – a term derived from plants. In Aristotle’s view, it is hard to distinguish plants from non-living nature, since rocks can also grow over time, albeit very slowly.
In fact, this notion of different souls is rooted in all of Western philosophical tradition. Just think of the biblical story of Noah’s ark: Noah’s sons and all the animals he gathered had to board the ark in pairs, but there is no mention whatsoever of plants; they played no role. Or consider the religious practices of Islam, which do not allow portraits of living beings – but floral patterns pose no problems. In the Western tradition, plants always live in the twilight. They are alive, but at the same time not really considered living.
Based on his religious principles, Linnaeus simply could not accept that the Venus flytrap eats insects; it did not fit within his hierarchical system of life and nature. The Venus flytrap challenges the hierarchical order, so Linnaeus tried to explain it away. He argued: these plants don’t catch insects, they don’t eat them, the only thing they do is shelter insects from rain. Linnaeus applied essentially the same logic to all carnivorous plants discovered in his time, for instance claiming that the vase-shaped flowers of carnivorous plants were actually drinking cups for insects.
It is possible to create a coherent hierarchy by simply disregarding any anomalies, like Linnaeus did. Others invented new categories for some type of plant-animalia, something in between a plant and an animal; these would later be called zoophytes. However, neither approach signified an actual paradigm shift.
Plant physiology really started taking off in the 19th century, when researchers noticed that plants in fact do respond to certain environmental stimuli. Plants may not have sensory organs as such, but they do sense things. Researchers also started to realise that plants do in fact exhibit some forms of movement, even though they lack muscles or ligaments. They debated hotly whether plants’ movement was truly autonomous, or if plants were only responding to its environment. Do plants have a passive or active nature? This building tension culminated in a book by Charles Darwin entitled The Power of Movement in Plants, on phototropism and related topics.
Clearly, the entire concept is diametrically opposed to Aristotle’s hierarchy of life and nature. Darwin catalogued all kinds of different plant movements. At the very end of his book, he concludes with an analogy, asserting that it could be argued that the capillary roots of a plant are similar to the brains of the lower orders of animals. He puts it carefully, all the way at the end of the book, but he is very serious about this idea.
Although most botanists do not appreciate the comparison, rejecting the analogy altogether, there is some evidence to support the supposition. In his book, Darwin covers all the things that root tips of plants can do, showing that they are not only sensitive to pressure and density, but also to gravity, to moisture, to other plants. He saw their responsiveness as so peculiar that he dared to draw comparisons to lower-order animals. Even so, his conclusion was unpopular when the book was published, and is still largely disregarded today.
By the 20th century, the movement of plants was being subjected to much more intense study. The invention of photography and time-lapse imaging opened up a whole new perspective on moving plants. However, it was not until the late 20th and early 21st century that scientific interest in the complexity of plants, including possible intelligence, really started to take off.
Bucket orchid (coryanthes): communicating with plants
Orchids represent the biggest family of flowering plants. Each orchid has its own intricate mechanism for inducing pollination. Many flowering plants do so by simply signalling the presence of their nectar through colour and/or scent. Some authors have compared walking through a flower meadow with walking on Times Square. All those flower petals are basically marketing their contents to insects; the blossoms are shouting, “Get the best, sweetest, tastiest nectar here!”
Darwin’s first book was about orchids and orchid sexuality. He was the first to popularise the idea that plants work together with pollinating insects and birds. It may sound strange now, but in his time, it was still very rare for people to be aware of the connection between pollinators and flowers. Linnaeus, for example, thought that flowers were essentially self-pollinating.
The realisation that insects played a role was a truly new insight. Before Darwin published his research on orchids, most people thought that flowers were here for us to enjoy. For instance, Linnaeus wrote that “flower petals are the curtains of a bridal bed and the scents are the perfumes that are sprayed in the bridal room”. Later, it was discovered that flowers’ colours and scents actually facilitate communication between plants and insects. Those seemingly decorative features are a direct channel between two major kingdoms in the natural world: plants and animals.
Looking at it from our current perspective, it is so strange that this insight was new. We’re talking about the two most important and ubiquitous living organisms on earth: plants and insects. A ‘shared hegemony’, as how they are sometimes described. And we overlooked it all this time! Our misjudgement cannot be solely attributed to a lack of scientific knowledge; our perspective was wrong. We had no idea how to look at a flower meadow and see what was really happening.
Even now, we only see the tip of the iceberg when it comes to plant communication. Our visible colour spectrum is comparable to that of birds, so the flowers we notice most are the ones that are pollinated by birds. As a result, the flowers we generally consider most beautiful are often red, or red and yellow. Insects respond to a broader range of scents and colours exceptionally well, though, much better than we do. Compared to insects, we humans miss a lot.
The bucket orchid has a remarkable pollinating mechanism. Inside its cupped flower lip, the bucket orchid has a kind of tap dripping with viscous liquid. The liquid clings to the wings of the orchid bee, preventing it from flying away – which forces the insect to find a way out through the drainage channel of the flower lip. Observing this behaviour, Darwin wondered why the bee would fly in there in the first place. What is in it for the bee? It wouldn’t simply jump in unless there were some benefit to doing so.
As it turns out, only male bees patronise the bucket orchid, because the cupped lip collects scent oils that attract female bees. The presence of these scents drives the male bee to dive into the liquid. Such a strange and complex relationship. One-third of all orchids do not actually provide a direct reward for their pollinators, so these flowers come up with more complex and ingenious tricks to attract the insects that pollinate them.
Some colours seem to be really ‘meant for us’. That’s because we’re frugivores – fruit eaters – and we need the vitamin C in fruits. There are only a few animals that need to consume vitamin C because they don’t produce it endogenously. It seems to be a legacy that dates back to our early origins as jungle-dwellers. For a long time in our evolutionary history, we were fruit eaters; somewhere along the way, there was no need for our bodies to produce vitamin C themselves, because we were already getting enough vitamin C from our surroundings. Researchers even say that our visible colour spectrum is based on the fact that we needed to be able to distinguish ripe from unripe fruit. On some level, in other words, communication between us and plants already takes place.
On the other hand, a lot of plant communication is lost on us. Plants have to conquer all dangers while staying put. Plants are great chemists, or alchemists, to put it more poetically. They are very good at producing chemical compounds to make themselves unappetising, sometimes even poisonous. Nicotine, for example, is a substance that weakens muscles. If a caterpillar eats tobacco leaves, its muscles weaken so much that it can’t continue eating.
The good thing about a plant is that it doesn’t have vital organs. If it did have such vulnerabilities, it would be impossible for it to stay in the same place. Plants have a modular structure. You can cut back 95% of the leaves, but they can still grow them back. Plants are actually much more adaptive to their environment than we are. A plant has to monitor its environment very closely and find solutions to respond to all outside influences.
Pea plant: a new type of intelligence
It is difficult to translate the world of plants to a human context. But I think we have to find ways to overcome our ‘plant blindness’, for example by using time-lapse video. There’s a video of a Welsh meadow that was filmed for a couple of months, and then the footage was sped up. Suddenly you can see movement everywhere. Life (episode 9, starting at 5:18), a BBC documentary series narrated by Sir David Attenborough, shows time-lapse footage of vines climbing upward; it’s striking to see the plant’s tendrils reaching out and bringing the plants up into the canopy.
Our human measure of time is a handicap for us. We see plants as inactive, but that’s only because of our own limited sense of time. If we sped things up, we would see the world of plants coming alive. Imagine a 100-year-old oak tree and how it grows; we wouldn’t even be able to see movement, because our sense of time is simply too limited. Even time-lapse photography gives us only a tiny glimpse of what’s going on in the world of plants. Since there’s so much we can’t see, we shove plants to the background, treat them like second-class citizens, maybe even third-class.
Interest for plants is growing, however, in popular culture as well as in science. Researchers are looking into plant intelligence. For example, there is a Pavlov-like experiment with a pea plant growing in a tube that has two branches, one providing air and the other providing light. A plant will always need light, so it will grow towards the light. However, researchers taught the pea plant to learn associatively. As soon as there was air, the plant would expect light and grow in that direction, awaiting the light. I consider this a very exciting experiment; people had not expected to find associative learning in plants.
Some researchers are looking for something brain-like in the plant and talking about the neurobiology of plants. I think it’s important to not go and look for the equivalent of the human brain, but to look for other ways of intelligence without structures that resemble a brain or nervous system. We are already familiar with these types of structures; just look at artificial intelligence. No nervous system or brains are needed. This is a type of networked intelligence, like a swarm or a cloud. We see this in plants as well as in technology. Isn’t that fascinating?