Let’s say you’re thinking about painting your living room a new colour. Do you ask your cactus its opinion on the hue? Or perhaps your petunias have been acting out and need to be sent out to the yard to think about what they’ve done.

I’m exaggerating, of course; as far as we know, plants aren’t self-aware. But are plants intelligent? This was the question I discussed during the Canadian Museum of Nature’s third NatureTalks evening on March 18 (don’t worry if you missed the event; you can watch the interview).

A far cry from the 1970’s The Secret Life of Plants (which delves into plant sentience, auras and experimenting with plants hooked up to lie detectors), modern plant-intelligence research sticks to observable and quantifiable phenomena like any other hard-science discipline. That doesn’t keep it from being highly controversial, though.

An animated GIF of a sensitive plant closing its leaves after being touched.
The sensitive plant (Mimosa pudica) is one of the few plants that react quickly to touch—it closes its leaves to scare away herbivores. But as recent research shows, this plant may have the ability to learn and remember certain stimuli that it doesn’t have to close its leaves to. Image: Hrushikesh © Public domain (licence CC0 1.0 Universal)

To begin, how you define intelligence is crucial to the process of looking for it in our green and leafy friends. When we look at our own intelligence, we think of our ability to discern, be introspective, choose and reflect—we think, therefore we are. Intelligence in animals (us included) is sometimes thought of as a tool that evolved to increase our evolutionary fitness—if we can reason and think, then our chances of survival to reproduction are greater.

Plants, on the other hand, aren’t able to ponder their own plight. However, if you separate out consciousness, and define intelligence (as plant intelligence researchers often do) as the ability to process information and stimuli from the environment, adapt and change according to that stimulus, and even remember a decision for future reference, then yes, it seems that some plants behave in startlingly intelligent ways.

Some of the first plant-communication studies confirmed that a Californian sagebrush species (Artemisia douglasiana) communicates with other plants using airborne chemicals—notably one plant telling another that herbivores were munching on its leaves, thereby giving the second plant time to muster chemical defenses. Here, a Canadian Arctic sagebrush (Artemisia hyperborea) seems to sit silently on the Canadian tundra. Perhaps, unknown to us, it’s having a chemical conversation with its neighbours? Image: Roger Bull © Canadian Museum of Nature

For example, they can communicate with other plants (and even insects) using airborne chemicals, or relay messages along root systems thorough shoots or fungal associates in the soil. Plants can perceive and react to moisture, light, temperature, touch and even gravity. There is even evidence that plants can learn, form memories, and make decisions.

But does behaviour imply an intelligent driving-force in an organism? Or does it just mean that plants have evolved pre-programmed intelligent-seeming behaviours as a survival mechanism (analogous to adaptive programming in computers, for example). And at what point do neurons matter? We use our brain to think, concentrating our complex neurological connections in one place, but plants possess analogous, distributed electrical, chemical and physiological systems, minus the grey matter.

I could go on, but likely you get the idea that plant intelligence research is as philosophical as it is scientific. What does it mean to be intelligent and can we stretch the definition to include plants are semantic issues that I can’t readily answer. I do know, however, that in this contentious and rapidly developing field the answers that plant-intelligence researchers propose will be debated and analyzed for some time to come. And there can be no doubt that the more we continue to research plants, the more they will continue to amaze us in their complexity.

Upcoming NatureTalks:
Parasites: Rethinking Healthy—April 16, 2014