Can plants think?
This week an opinion article published in the journal Trends in Plant Science, argues that plants are not conscious, in other words they are not aware of their surroundings in an intelligent way. Well so what? Surely everybody knows that plants are not conscious already? In fact the question of whether plants can think, learn, and intentionally choose their actions has been under debate in the plant science community. To try to get to the root of this, Heather Jameson went for a garden stroll with plant scientist Howard Griffiths...
Heather - We’re sitting amongst all these flowers. Are they aware of our presence; are they conscious?
Howard - Well they would be when we shaded them, or when we pulled them up.
Heather - Right, okay. But they're not conscious in the way that we think we're conscious.
Howard - Well that is debatable, but my take on it is that no they are not conscious in the way that we understand intelligent beings to be conscious and capable of making independent decisions.
Heather - And the authors of this paper they would agree with you on that.
Howard - They would, yes. They are arguing that where another paper has suggested that plants can have innate consciousness and intelligence, they argue that this is incorrect.
Heather - One of the studies referenced in the paper apparently demonstrated that plants were capable of learning the association between the occurrence of one event and the anticipation of another event, known as Pavlovian learning. This has been widely demonstrated with dogs. If you always ring a bell when you give a dog food, eventually the dog begins to associate the ringing of the bell with getting food such that the dog salivate when it hears the ringing of a bell without actually getting any food. The scientists replicated this study in pea plants by exposing them to two different stimuli during the training phase: light, and wind. The results seemed to suggest that the plants could be trained to associate the presence or absence of wind with the anticipation of light. However, the authors of this new paper argue that this study needs to be repeated with more stringent controls in place before a clear conclusion can be drawn. So, what do we know?
Howard - We know plants can remember - in inverted commas “remember” - because we have plants that track the sun. They're called heliotropic plants, and we know, for instance, that they will start the day ready to face the sun and then they will follow the sun round just like sunflowers are set to do to maximize the amount of sunlight that they get to develop their pollination and so on.
Heather - Speaking of memory, what about Venus fly traps? You might have heard that they can count up to five when an insect lands on it, the plant counts the footsteps to check that it is actually food, and not just a drop of water say. Surely this must require some sort of memory because for me to count to five I need to remember that I just counted 4 and 3 etc. Scientists believe that the planet can count by releasing a chemical for each count. Imagine you have a bottle with a small hole in the bottom if you half filled the bottle water will start to drain out and eventually empty, if you don't put any more in. But if you quickly add some more water, the water level will go up. If you do this a few times in quick succession the water bottle will overflow. The Venus flytrap uses a similar method on when the threshold is reached, the trap snaps shut. But what about pain. If I was to pluck off a leaf of one of these plants here, would the plant feel pain?
Howard - Pain is a difficult stress to define even in animal terms and we have to be very careful with the terms we use. But nonetheless I think if you went across and plucked a leaf or if you had a caterpillar walking across the surface of the leaf you can certainly see that a plant can detect that. You can see the footprints marked across the leaf surface where chemicals have been produced within the leaf, defense chemicals. And also we know that if you pull that leaf off of bite a piece out of the leaf, chemicals will be produced that will move to an adjacent plant and warn it that the plants are under attack from potential pests or pathogens. So there is a warning system, they can respond. They can send signals but I don't think they can feel pain.
Heather - So is it just a problem of metaphors? The plants seem to be able to show some sort of ability to adapt their behaviour in response to their environment in the way that we think of as learning. But actually it's very different process and it's not really appropriate to call it learning in plants.
Howard - Yeah I think so. I think that's exactly it. I mean we have a number of different definitions now of intelligence. When we talk about whether animals can show intelligent responses such as maybe shrimps or octopus or indeed humans and so on. Equally so we have artificial intelligence. You know we've all now been hearing about self-driving cars and so on. So in some respects do we define that motor vehicle as having intelligence and at what level. And I think that one could equally rationalize that plants have a form of intelligence, if you move along the spectrum of definitions. But it's not cognate intelligence as we understand it from a human perspective.