Do Plants Have Goals?

The topic this time is plants, specifically, whether plants have goals, as sentient agents do. Contemporary philosopher Scott Sehon, echoing the intuitions of many, says they don’t. I’m not so sure.

Sehon’s concern is the concept of teleology, the attempt to explain things in terms of goals or purposes. (The term comes from the Greek telos, which means an end, purpose or goal.) In trying to untangle the nuances of the concept he asks whether and to what extent any of the following can reasonably be said to have goals:⁽¹⁾

A rock remains motionless on the ground.
A marble rolls down the inside of a bowl.
A heat-seeking missile turns toward the north.
A plant turns toward the sun.
A spider runs across its web.
A cat climbs up a tree.
A person, Jackie, goes to the kitchen.

We can explain Jackie’s action by saying that she goes to the kitchen to get a drink. Getting a drink is her goal, or intention. We do not explain the rock’s remaining motionless by saying that it does so in order to maintain a constant velocity. The rock has no goal, it just responds to external forces, which at the moment are in equilibrium. The other cases are in between. The marble does not roll down in order to get to the bottom; it just responds to gravity. The heat-seeking missile acts as if it has a goal, but its goal is not its own; rather, someone has programmed the goal into it. The spider runs across its web to get to the prey ensnared there, and the cat runs up the tree to get away from a dog. These two seem to be clear-cut cases of having a goal, much as Jackie has the goal of getting a drink. But what about the plant?

We can explain the plant’s action by saying it turns toward the sun to get the most sunlight. But Sehon objects, saying "we are not comfortable with [the] apparent suggestion that we view the plant as an agent aiming for a particular goal."⁽²⁾ He views the plant’s movement as a mere tropism, mechanical and not agential.

Now, the first thing to note is that while Sehon himself is uncomfortable, it is not at all clear that everyone else is. My wife, a gardener and landscape designer, would have no problem at all with saying that the plant moves in order to get more sunlight. Philosophers often appeal to their intuitions about what words mean or what one would say in a certain situation, but their intuitions may well be biased.

Sehon is a philosopher, not a scientist, but his appeal to intuition is a type of informal research using himself and his peers as subjects. As such a researcher he is susceptible to a criticism of contemporary behavioral science, that it uses research subjects who are not representative of the human population world wide. A recent review of behavioral science research finds that "subjects are taken largely from Western, Educated, Industrialized, Rich, and Democratic (WEIRD) societies" and that "members of WEIRD societies, including young children, are among the least representative populations one could find for generalizing about humans."⁽³⁾ Sehon himself, a philosophy professor in Maine, is firmly among the WEIRD,⁽⁴⁾ so the fact that he has an intuition that plants don’t have goals does not carry much weight as evidence. What would an indigenous hunter-gather in Brazil have to say? How about a pastoral nomad in Mongolia? A farmer in Uzbekistan?

Be that as it may, it is undeniable that plants differ from animals in significant ways. (Or so it seems to me, also a WEIRD person.) Plants are less like us, who clearly have goals, than animals. To the untrained eye, it is clear that animals can do a number of things that plants can’t, most notably move around freely and perceive things at a distance.

But the untrained eye can be deceived. Aristotle, for instance, characterized plants as being able to grow and reproduce, but not to perceive, going so far as to say that they have no sense of touch.⁽⁵⁾ Nowadays we know that plants do perceive, and some have quite an obvious sense of touch. For an example, watch this video of a Mimosa Pudica, aka Sensitive Plant, whose compound leaves fold inward and droop when touched:⁽⁶⁾

Video Player00:0001:00

There are many other parallels between plants and animals. It has been known for over 25 years that plants, even though they lack an animal nervous system, send nerve-like messages through their bodies via electrical signals.⁽⁷⁾ Newer research finds much more evidence that plants have features analogous to nervous systems and brains:⁽⁸⁾

Plants have genes that are similar to those that specify components of animal nervous systems.
These genes specify proteins that behave in ways very similar to neural molecules.
Some plants have synapse-like regions between cells, across which neurotransmitter molecules facilitate cell-to-cell communication.
Many plants have vascular systems that look like they could act as conduits for impulses transmitted throughout the plant body.
Some plant cells display action potentials, events in which the electrical polarity across the cell membrane does a quick, temporary reversal, as occurs in animal neural cells. The behaviors of the Sensitive Plant and the Venus Flytrap are examples.

So plants send nerve-like messages within themselves. Does that mean they are intelligent enough to have goals? Are they, in other words, agents? Consider some additional evidence. Monica Gagliano, an animal ecologist at the University of Western Australia, did an experiment with the aforementioned Mimosa Pudica, here recounted by science journalist Michael Pollan.

Gagliano potted fifty-six mimosa plants and rigged a system to drop them from a height of fifteen centimeters every five seconds. Each "training session" involved sixty drops. She reported that some of the mimosas started to reopen their leaves after just four, five, or six drops, as if they had concluded that the stimulus could be safely ignored. "By the end, they were completely open," Gagliano said to the audience. "They couldn’t care less anymore."

Was it just fatigue? Apparently not: when the plants were shaken, they again closed up. "’Oh, this is something new,’" Gagliano said, imagining these events from the plants’ point of view. "You see, you want to be attuned to something new coming in. Then we went back to the drops, and they didn’t respond." Gagliano reported that she retested her plants after a week and found that they continued to disregard the drop stimulus, indicating that they "remembered" what they had learned. Even after twenty-eight days, the lesson had not been forgotten.⁽⁹⁾

This experiment certainly suggests that plants learn and remember. But are they really agents, with intentions, goals, desires and the like? We think they just stand in place like so much green furniture, but that’s because they move too slowly for us to notice. Consider this video of a bean plant shot with time-lapse photography provided by researcher Stefano Mancuso:⁽¹⁰⁾

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Mimosa Pudica — Sensitive Plant

I wonder if you will agree that the plant’s activity seems to be directed rather than flailing around aimlessly. To me (admittedly a WEIRD observer) it certainly seems to have a goal and to make efforts toward that goal. Pollan says

Mancuso’s video seems to show that this bean plant "knows" exactly where the metal pole is long before it makes contact with it. Mancuso speculates that the plant could be employing a form of echolocation. There is some evidence that plants make low clicking sounds as their cells elongate; it’s possible that they can sense the reflection of those sound waves bouncing off the metal pole.

The bean plant wastes no time or energy "looking"—that is, growing—anywhere but in the direction of the pole. And it is striving (there is no other word for it) to get there: reaching, stretching, throwing itself over and over like a fly rod, extending itself a few more inches with every cast, as it attempts to wrap its curling tip around the pole. As soon as contact is made, the plant appears to relax; its clenched leaves begin to flutter mildly.⁽¹¹⁾

In addition to cultural biases, we humans have a generic bias: we see things easily in our time scale but not at all or only with difficulty in other time scales. Our invention of time-lapse photography enables us to see features of the world that we normally overlook entirely. One of these features is the agential, goal-directed nature of plants.

There is quite a bit of controversy among botanists about what all this means. That’s why, out of caution, Pollan puts words such as "knows" and "looking" in scare quotes. Some have called for the creation of a whole new field, to be called "plant neurobiology" because plant signalling is so much like animal neural activity and because plant behaviour is too sophisticated to be explained by genetic and biochemical mechanisms.⁽¹²⁾ Some, less confrontationally, call the field "plant signalling and behaviour."⁽¹³⁾ Others strongly disagree, going so far as to say that plant neurobiologists are from "the nuthouse."(14) The issue is largely semantic, since nobody questions the data, but it strikes at the core of our concept of ourselves. Are humans a special category of the living, different enough to be considered distinct from other animals and especially from plants? Or are we one end of a continuum of life that ranges without sharp demarcations from tiny, single-celled bacteria to extraordinarily complex human beings?

My own preference is the latter. The view that we are part of a continuum of life seems to fit the data better than the opposite view. And if widely adopted, it might prompt us to have more empathy for our fellow living creatures and to stop the ecological devastation that threatens our survival.

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Notes
(1) Sehon, p. 160.

(2) Sehon, p. 161

(3) Henrich, et. al., p. 61.

(4) https://www.bowdoin.edu/faculty/s/ssehon/

(5) Aristotle, On The Soul, 2, 413a 26 – 413b 13. To be fair, he does not explicitly say that no plants have a sense of touch, but implies that assertion by contrasting them with animals, all of which do.

(6) Íñiguez, “Mimosa pudica – Sensitive Plant.”

(7) Yoon, “Plants Found to Send Nerve-Like Messages.”

(8) DeSalle, “Do Plants Have Brains?”

(9) Pollan, “The Intelligent Plant.”

(10) Pollan. “Plant Neurobiology.”

(11) Pollan, “The Intelligent Plant.”

(12) Brenner, et. al., “Plant neurobiology.”

(13) Baluska, et. al., Plant Signaling and Behavior.

(14) Pollan, “The Intelligent Plant.”

References
Aristotle. On the Soul, tr. Terence Irwin and Gail Fine. In Readings in Ancient Greek Philosophy: From Thales to Aristotle, Fourth Edition ed. S. Marc Cohen, et. al. Indianapolis: Hackett Publishing Company, 2011. Another translation is available online at http://classics.mit.edu/Aristotle/soul.html.

Baluska, Frantisek, et. al., eds. Plant Signaling and Behavior. Online at http://www.tandfonline.com/loi/kpsb20 as of 24 February 2018.

Brenner, Eric D., et. al. “Plant neurobiology: an integrated view of plant signaling.” Trends in Plant Science Vol. 11 No. 8 (2006), pp. 423-418. Online publication http://www.linv.org/images/papers_pdf/trends%20plant%20sci%202006%20brenner.pdf as of 24 February 2018.

DeSalle, Rob, and Ian Tattersall. “Do Plants Have Brains?” Natural History. Online publication http://www.naturalhistorymag.com/features/152208/do-plants-have-brains as of 20 February 2018.

Henrich, Joseph, et. al. “The weirdest people in the world?” Behavioral and Brain Sciences, Vol. 33 (2010), pp. 61 – 135. Online publication http://hci.ucsd.edu/102b/readings/WeirdestPeople.pdf as of 23 February 2018.

Íñiguez, Ángel Daniel Alfaro, videographer. “Mimosa pudica – Sensitive Plant.” (video) Online publication https://www.youtube.com/watch?v=Zq3UuHlPLQU as of 20 February 2018.

Pollan, Michael. “Plant Neurobiology.” (video) Online publication https://www.youtube.com/watch?v=MPql1VHbYl4&t=9s as of 20 February 2018.

Pollan, Michael. “The Intelligent Plant: Scientists debate a new way of understanding flora.” The New Yorker, 23 December 2013. Online publication https://www.newyorker.com/magazine/2013/12/23/the-intelligent-plant as of 15 February 2018. I highly recommend this piece. It has far more to say than what I have quoted.

Sehon, Scott. Teleological Realism: Mind, Agency, and Explanation, Cambridge, MA: MIT Press, 2005.

Yoon, Carol Kaesuk. “Plants Found to Send Nerve-Like Messages.” New York Times, 17 November 1992. Online publication http://www.nytimes.com/1992/11/17/science/plants-found-to-send-nerve-like-messages.html as of 20 February 2018.