The Gentle Lentil: Plant intelligence?

WARNING: This post is long and has nothing to do with veganism or food!

To be honest, most of the time I find plants kind of boring. As a biologist, I’m much more interested in studying animals. They move around and do stuff, while plants just kind of sit there. Except I know that’s not true. There are plenty of biologists out there who are fascinated by plants. I first began to understand why when I watched the time-lapse opening sequence of the Plants episode of BBC’s Life a few years ago. I was captivated. The plants stretched out their leaves and opened their flowers. They seemed to taste the air with their tendrils and explore the soil with their roots. They seemed purposeful and alive in a way that I previously thought only animals could. It suddenly struck me that plants seem boring and inert only because animals and plants live on different time scales. Suddenly I could see that plants do move around and do stuff, just really slowly.

This is the time-lapse sequence that blew my mind.

Another really cool time-lapse clip.

Studying plants is like studying alien organisms. Animals and plants last shared a common ancestor about 1.5 billion years ago. Considering that life on earth is about four billion years old, 1.5 billion years is a long time for plants and animals to be following separate evolutionary paths. If you ever read science fiction, you must be familiar with the idea that we need to expect the unexpected when it comes to alien biology. Maybe these same cautionary tales can be applied to plants. Animals and plants may be distantly related, but we have evolved to adapt to some similar challenges. It’s possible we have even come up with analogous solutions to some of these challenges. When unrelated groups of organisms evolve similar solutions to a similar problem, this is known as convergent evolution. The wings of insects and birds or the fins of sharks and dolphins are two examples of convergent evolution. These adaptations are analogous solutions which evolved independently. Could it be that plants and animals have convergently evolved analogous behavioural or sensory mechanisms? Could plants be said to have anything analogous to intelligence, or pain, or memory? Maybe we’re thrown off by their form and their slow lifestyle, so alien to our own, so we have trouble recognizing when our functions converge.

Convergent evolution of Marsupial and Eutherian mammals. They may look and act similarly, but they got that way through convergent evolution, not through being related.

It’s less of an “out there” idea than you might think. In fact, “plant neurobiology” is a real, albeit contentious, field of study with real conferences and journals and labs. Plant neurobiologists recognize that plants don’t have neurons, but they argue that plants have systems analogous to animal neurology. Plants use electric signals to communicate between cells. Plants can be “knocked-out” by the same drugs as are used to anaesthetize humans. Plants can “smell” and “taste” chemical signals from other plants. They can “see” light to grow towards it. They can “feel” if their roots hit a rock or a pipe. They may even be able to “hear”. A recent study found that plants can be primed to produce defence chemicals only by being exposed to the sound of a caterpillar munching leaves. There’s some evidence that plants have memory and can learn. They even have neurotransmitters including serotonin and dopamine, although it’s not yet clear what their purpose is.

An interesting (but hyperbolically titled) video about electrical signaling and anesthetics in plants.

These findings are all very interesting, but they’ll probably never tell us in any definitive way whether plants are intelligent or can feel pain, or at least the plant version of those things. Then again, we don’t even know for sure if animals are intelligent or feel pain. Indeed, a few hundred years ago, most scientists believed that animals had no capacity to suffer and that their cries were simple physiological cause-and-effect. We now know that animal brains react analogously to human brains to painful stimulus and similar stress hormones are produced, and so we infer that they feel pain. We take a more sophisticated view on animal intelligence and awareness than we did in the early days of biology. Many agree with Peter Singer’s argument that there’s no black and white of “intelligent” and “unintelligent” or “aware” and “unaware”. Instead there are gradients along which every animal individual falls.

We can never know what it’s like to be another animal, even another human, and so we can’t know if it’s aware or if it feels pain. Instead, we assume that if it acts as though it is aware or as if it’s feeling pain, we make the assumption that it is. For all you know, every other living creature on this Earth could be a philosophical zombie, a biological automaton with no more inner life than a rock. The Turing test has gotten some coverage in the media lately, and it operates on the same principle. Alan Turing argued that if we can’t tell a computer from a human when we interact (talk) with it, then we should apply the same logic to computers as we do to humans, we should assume that the computer is self-aware and intelligent. We just can’t access the inner life (be it existent or non-existent) of another being, so we’re forced to infer based on the behaviour and physiology we observe.

xkcd

What can we infer from the behaviour and physiology of plants? Certainly it would be absurd to argue that they have the same intelligence or capacity to experience pain or memory as a human. But cognitive traits exist on spectrums. Plants are “aware” of the world to some degree in that they have at least as many senses as animals. Plants have “intelligence” to some degree in that they are phenotypically plastic – individuals can response appropriately to changes in their environment. They can “make decisions” to some degree in that individuals need to “choose” between, say, growing a root towards the left or the right. Plants have chemical “languages” they use to communicate between individuals. Plants respond to damaging stimulus in a way that’s loosely analogous to animal pain response by producing chemicals and sending electrical signals.

You might argue that it’s semantics and that plants are so unintelligent that it’s silly to even call it “intelligence”, even with quotation marks. Or to say they are “aware” or “make decisions”. But I think there’s an important point to be made here: the difference between human beings and all other living things is one of degree, not of kind. We once thought that only humans have intelligence; only humans can suffer; only humans have a whole host of traits. If you go back in time enough in the certain areas of the world, you could even replace “human” with “white human”. But every generation our understanding of the pervasiveness of traits we once thought were the sole domain of humankind grows. Maybe as science continues to progress we’ll come to realise that plants too have the capacity for certain traits we now believe only animals can possess.

I’ll admit that this post is only (very) tangentially related to veganism. Let me try to connect it back as best I can. I was just thinking about whether the animal rights argument for vegetarianism or veganism makes sense. If animals have the capacity to suffer then we shouldn’t eat them, is the argument. Then I was thinking about how we can’t know if any living being suffers or doesn’t suffer. We need to make assumptions. I wondered the science that’s out there about plant pain, and whether it could exist. I spend all morning reading about it because I thought it was fascinating! Don’t get me wrong; even if plants could be said to be “intelligent” or “suffer” I don’t think we should be launching some sort of plant rights campaign. I think we need to deal with the serious amount of human suffering on this planet before we even think of starting any questionable new project. Instead, it’s an interesting thought experiment and a possibly fruitful avenue of research.