Discover More: Plant Intelligence

Darwin can be loosely quoted saying that the roots of a plant are brain-like and can direct movement. Are plants intelligent and what is intelligence anyway? Discover more with Linnean Learning.

Published on 7th June 2021

The Discover More series is a collection of animations, blogs and interviews exploring a wide variety of topics to enrich your appreciation of the wonderful natural world that we are a part of. There are seven topics which will be released throughout 2021 and the animations are best viewed via Instagram @LinneanLearning.

Plant Intelligence

Screenshot from our animation on Instagram

Darwin can be loosely quoted saying that the roots of a plant are brain-like and can direct movement. Are plants intelligent? Of course it depends on your definition of intelligence.

As you walk down a street and pass by an array of plants, do they perceive you? Do they inwardly groan as you cast a shadow over them, or mutter amongst themselves when you leave your car engine running? These may seem like silly questions but plants do react to situations and they can communicate with each-other and with animals too.

This month we're looking at instances of plant intelligence and hearing from an expert in biological intelligence for their view on this topic. Read a couple of examples below, watch our short animations on Instagram, Twitter or TikTok and enjoy our interview with Monica Gagliano on those platforms or as a transcript (see below).

Some plants can release chemicals into the air when they are being attacked which lets other plants prepare a defence. Plants can produce toxins on their leaves that are poisonous or, at the least, taste bad to whatever is eating it. An example of this is when 3,000 South African antelope suddenly died, and it was discovered that the toxin responsible came from Acacia trees that release airborne chemicals to tell other Acacias when they are being attacked, and they also produce toxic tannins on their leaves. Interestingly, the researchers noted that the victims were domesticated antelope, while free-roaming giraffes were able to show 'intelligence' as they were seen avoiding eating from down-wind Acacia trees, knowing that they communicate via airborne chemicals.

Another interesting way that plants show intelligence is through their utilisation of animals. Various plants have been observed to find a fascinating way of defending themselves against pests. When corn is being eaten by caterpillars, it emits a chemical distress call into the air which signals to parasitoid wasps that there are caterpillars present. The wasps fly to the plants and attack the caterpillars by injecting them with venom as well as the wasp's eggs. The caterpillars are consumed from within by the hatched larvae, meaning they can no longer snack on the corn.

An interview to a 'virus ecologist'

Monica Gagliano
Monica Gagliano

We spoke with Monica Gagliano who is a Research Associate Professor in Evolutionary Ecology at the Biological Intelligence Lab at Southern Cross University in Australia.

The following text is an edited interview between Monica and our Research Officer, Zia.
Video interview coming soon.

Could we ask you to introduce yourself?

Yes, my name is Monica Gagliano. I'm a research associate professor at the University here at Southern Cross University and I work on biological intelligence of all kinds.

People might have seen my work on plants, but I work on all sorts of critters too.

Could you define for us what plant intelligence is? What is intelligence, and what does it mean, in relation to plants?M

Intelligence is one of those loaded words in scinece because it seems everyone has a very different definition. And based on that definition, we decide who qualifies and who doesn't. For me, I take the definition from the root of the word itself, and intelligence come from 'intelligere' in Latin, and it simply means 'choose between'.

So it's all about decision-making, choices, behaviour and I would say it applies to anything living in this planet, because without being able to make choices and decide what is good for you, you simply can't survive. So it's not a trait that someone has or hasn't, it's more a part of the process of being alive. And that's why intelligence in plants to me... it doesn't really feel like it's a big deal. Of course plants are intelligent, as much as bacteria and amoebas, and fish and birds and humans.

So the idea of intelligence not being about asking questions in a particular language, but more about responding to different environments and being reactive?

Yeah, decision-making is about sensing your environment to see what's available. So the information that is available will tell you whether the environment is friendly for you, or if it's a favourable space, and if it's not, then you better get out of there. If you don't move, like plants, if you don't move fast enough to move away then you better engage with the strategies that you have evolved to deal with changes in the environment that are not friendly. And so, whether you do it by moving like we do, or most animals do, or whether you do it by activating your chemistry so that you repel the potential danger, or nobody can touch you, the result is the same. There was a problem or something that you perceived as a potential threat, and you did something, you chose to enact something, that will protect you or will keep your life safe.

And similarly, if something is really good... if there is a beautiful fruit that we want, we will stretch our hands and try to reach it to eat it... and plants do the same. They invite others to come and pollinate them. They invite more sunlight in by projecting their leaves in a particular way.

How did you end up working on plant intelligence in particular?

I was already working in terms of behavioural responses in animals, particularly in fish, and so I was kind of asking the same question that I am now. But plants became a very interesting question for me, because we are not used to asking those kinds of questions regarding plants.

Would plants remember different experiences and then, based on that memory or past experience, choose something differently the next time? This process often is referred to as the learning process. So you have an experience and you remember whether it's good or bad, and then the next time that same experience is presented to you, you have a choice to make - whether you're going to do the same as you did before, or do something else.

So I was interested in seeing whether the plants could do that and intuitively, because of the definition that I use of intelligence, I felt like that 'of course they do, of course, they can make the choice'. There was already plenty of literature before my work showing that there are lots of behavioural responses that plants make in different contexts.

I guess what I focused on more specifically was 'Can they actually remember what happened? And then, based on that specific memory, enact a certain behaviour, or a certain behavioural choice?'. And so, one of the experiments I did was with the Mimosa pudica. I looked at, what we call the most basic form of learning, which is habituation. And in that case, the Mimosa had an experience, which is supposed to feel threatening at first when the plant doesn't yet know what's going on. And then after a while, if the plant realises 'Oh, actually, this threat, this thing that looks so scary, is really not scary, nothing happens after this', then the plants should, in theory, relax. And when that experience is presented again, the plants will not waste time reacting, because it already knows, 'Oh, this is... this is nothing, I remember'. My experiment was testing this concept of habituation.

And what we found is exactly that; first the plant reacts by closing its leaves because that's the correct thing to do if you don't know what's happening to you is a dangerous thing or not - you should respond as to protect. And then, they realise that, actually, nothing happens, and if you keep your leaves closed, you can't feed on light. So there is a trade-off between protecting from a potential threat or feeding on sunlight. And if you realise that the threat is not real, then you should open the leaves and continue on feeding on sunlight, which is exactly what the plants do.

We left him in peace for a month, and then we re-presented the same experience, and when the same experience returned, the plant didn't even bother closing the leaves in the first instance, because it already knew, its like 'Oh I know this experience, I know nothing happens, and so I just carry on business as usual when I keep eating sunlight'.

And that was the first experiment to really show there is a relatively long term memory in the plant. And this is not epigenetic or generational memory, this is the individual in its own lifetime, collecting memories as we do and as everything else does, and then enacting a choice the next time around when that experience comes back.

In that sense, this was an interesting and important study for the field because it was opening really to the question of 'What is memory?' and 'What is learning?'. We are very anthropocentric about everything and so we think of memory and learning as primarily relevant to us because of course, we know that we learn and we know that we have memories, as do other animals and fundamentally anything that has a nervous system and a brain, and of course, plants have neither. So it becomes very tricky for, at least some people, to conceive of that.

So, beyond this first study into plant memory and intelligence, have you been able to study it further?

Yes, I escalated the story because I did another experiment and this time, it wasn't the most basic form of learning, but it was associative learning. And this is basically a version of the famous study of Pavlov with the dog. In the case of Pavlov, the dog needed to associate something that had no meaning, in that case a little bell, with the arrival of dinner, and of course, dinner is something that the dog really wants. The bell doesn't have any meaning until it's associated to the arrival of dinner. So what Pavlov did, was ring the bell and then present dinner, ring the bell, present dinner... and after a while ring the bell and present no dinner arrives, but the dog is salivating because he's expecting the arrival of dinner.

So in a way the dog is projecting into the future for something that is not in the present, but based on the past experience. The dog knows this should be happening and so we ran our experiment that is basically another version of the Pavlovian dog but in this case with a pea plant.

The pea does exactly the same as the dog. It's expecting the arrival of light based on the experience of a gentle breeze that, when presented on his own, doesn't mean anything to the plant, it doesn't do anything, but when it's repeatedly presented just before the light comes, then, just like the bell with the dog, it becomes meaningful to the plant. The plant is following that signal as a piece of information that is going to allow it to predict where dinner is coming next, and there are behavioural responses that suggest that it knows what's coming next, based on their past experience.

I the dog, we assume that the dog can do that, because it has a brain and a nervous system, and I'm sure that, in a dog, that's how it works. But to disqualify the idea that a plant can also do it, just because it doesn't have a brain and our nervous system is silly.

What the data showing is that you do not need a nervous system or a brain to embody this kind of behaviour and decisions. And so the plant both plants, Mimosa and the pea, are both showing choice and decision-making, and so, by definition, they are intelligent.

What was the kind of response that was equivalent to the dog saliva that you were kind of looking at?

Okay so the pea plant was placed inside a two way maze, and there was this little fan, which blew a little bit of air, and then there was a blue light, which was dinner. So within this maze, the fan and the light would move from one side to the other randomly, and for three days the plants would experience these two signals coming together - the fan was always first, the light following. And the plant cannot guess which side it's going to come from because the side keeps changing all the time randomly.

On the last day, when we tested them we only presented the fan and we basically asked the plants to make the decision of, where are you going to go within this maze? If the plant understands what the fan represents, that it proceeds the light, then the plant should follow it and should grow towards the fan, and if it doesn't understand that, it should grow randomly.

What happened is that a vast majority actually grow towards the fan. This is against their own natural instinct because in the last session, the light is presented from a different side. So if it was just a phototropic response, then you would expect the plants to go to that side because that's where they saw the light last time and it would be logical to expect them to go there.

But we put the fan on the opposite side and the plant choses betwen the phototropic response (where it last saw light) and 'trust' in the fan preceding light. And that's exactly what they do.

Can I change tack now and ask how you first got interested in scientific research?

Well, I didn't know that I was interested in scientific research because I didn't know that's what it was called. I remember being in primary school and we did this little experiment with growing beans in cotton wool. And many years later I actually found a little booklet where I had recorded the growth of my beans in the cotton wool. And so, I suspect the scientist inside me was always there, but that would be true, I would say, of most people, it's just a matter of curiosity, and if you're curious about things, you want to know, how it is that they are the way they are. And so for me, science became a way to express that, but musicians and artists of all kinds are expressing this in a similar way, the same spirit. it's just in a different form.

So, for me, I followed the science way of doing things and then I followed the academic path to find myself at a point where, especially at the moment, I'm in the process of being very curious of how we deconstruct the way in which science is done. I believe in science but I am struggling very much in believing in the structure by which science is conducted these days. I find it a very colonial science and there are many ways of approaching knowledge and being informed by wisdom, which are two different things for me. And so my science has changed over time because I'm deconstructing my own science first.

In terms of becoming a scientist, or being a scientist, were there any struggles that you faced?

Hoo! Well, I guess I am a woman in science, and I didn't really realise that women do not have the same space and are not necessarily given the same, I would say, opportunity within the science space. And I just assumed that, you know, this is how it is, until I experienced some very subtle behaviours. I spoke to a colleague of mine; I was just telling him about 'Oh, and this happened to me, it's no big deal', and he replied with 'yeah, I've seen that happening a lot, and I've seen many women going through exactly the same that you're describing, and its a form of bullying'... and I'm like, "Oh!". And suddenly, I started recognising it for what it was, and I started seeing that there actually is a very prevalent behaviour that is very subtle.

Sometimes it's just about the kinds of jobs that women are given within science. There is still an expectation that a man would do a better job. And that doesn't just affect me, it affects all of the women and people in science.

Could you name one of the best things for you about getting into science, or kind of doing scientific work?

I am very grateful for the opportunity to have, as a job, the luxury of 'thinking'. That's my job! My job is 'to think' and then come up with crazy ways of testing the things that I see around me and the processes that I see around me. And I think science loses this flavour when we just start doing science that doesn't really mean anything.

You sometimes talk about plant blindness and the importance of young people's awareness of plants. What do you the think the future holds in this regard?

I wrote a book about 10 years ago about plant blindness because I had to realise and confront my own blindness of plants as I started working with them. It's not a shame thing, it's more like, lets face it together. We have some little faults here that we need to address because our blindness can be very dangerous. I think the younger people are less 'blind' than my generation and the one before where, especially in urban environments, we are detached from the natural world and the ecological, natural cycle of things. I think we are in a situation, environmentally... ecologically-speaking, in which, without increased awareness, I don't think there is any way out and I think that the younger generations are very aware of that, and, in that sense, they are much more mature than I was. I'm very impressed. I'm surrounded by quite a few younger people, and I just find them so inspiring. So yeah, it's it's a good sign. I think for all of us.

Thank you for your time today Monica, I certainly will be looking at plants a little differently.