Are Plants Entering the Realm of the Sentient?

In 1900 the Bengali biophysicist and botanist Jagdish Chandra Bose taught that plants are not merely passive organisms lacking sense. Instead, they explore their environments and can learn and change their behavior with purpose. Plants have an electrical nervous system, he claimed, that allows them to transmit information among their roots, stems, leaves, and other parts.

Dr. Jagdish Chandra Bose

In the last couple of decades botany has begun to catch up with Bose’s ideas, leading scientists to some amazing questions: Are plants conscious? Do they have knowledge? Can they feel pain?

In 1992 researchers discovered that tomato plants will produce certain proteins throughout their bodies when they’re wounded. The speed of the response precludes the possibility of chemical signals; the plants are producing electrical signals to direct change to occur more quickly within more distant parts of the plant.

Slow yet Smart

We tend to look at plants as dumb and nearly inert. They’re anchored in place and seem to bend passively with the breeze and grow gradually to capture sunlight. With rare exceptions such as the Venus flytrap, they move only very slowly, such as when a vine seeks an object to attach to. With time-lapse photography, scientists have begun to capture plant movements that seem sensible and intelligent. Under time-lapse, the seedling of a Cuscuta (dodder) vine seems to search for a host by sniffing the air. It then lunges toward its new host when it finds one, resembling snake movements.

When plants seem to be behaving like animals, we must reconsider whether intelligence truly is an exclusively animal trait. Watch a Dodder vine sniff out its prey: http://video.pbs.org/viralplayer/2341198769

Scientists are indeed questioning whether this distinction is as clear-cut as modern science has previously assumed. In 2005 researchers founded the Society for Plant Neurobiology to advance in this debate. A founder of the organization, the Italian scientist Stefano Mancuso, argues that we should stop assuming that a brain is needed for intelligence. Even without neurons and a brain, plants can acquire, process, and integrate information to shape their behavior in a way that could be called intelligent.

Locating Intelligence

As reported in a recent article in the magazine New Scientist,2 the apparent magic of consciousness in plants seems to depend on several physiological features, particularly those of their root systems. Plant roots include various “zones,” including a “transition zone,” which is electrically active and seems analogous to the animal brain—it contains a mechanism similar to neurotransmitters. Another part of the root, the root cap, can sense various physical properties “such as gravity, humidity, light, oxygen, and nutrients.”3 Most cells in plants can make and transmit neuron-like activity. In roots every cell can do so.

Mancuso says, “If we need to find an integrative processing part of the plant, we need to look at the roots.”4

Plants also produce serotonin, GABA, and melatonin, which act as hormones and neurotransmitters in animal brains, though it’s not yet known what they do in plants. Intriguingly, drugs such as Prozac, Ritalin, and methamphetamines can disrupt these “neurotransmitters” in plants.

Vital Capacities

Plants sense light, but they also communicate with one another using chemicals. They “know” when they’re being touched. They integrate all of this information without the kind of neural system that animals have.

And they have memory—the ability to store and recall an event at a later time. A Venus flytrap, for instance, doesn’t chomp down when it receives its first sensation of a fly; it only closes if the hairs in its trap sense another contact within a half minute or so. It “remembers” the first touch.

More surprising is the result of an experiment that Mancuso carried out with Mimosa pudica, the “touch-me-not” plant. He and colleagues dropped potted mimosas repeatedly onto foam from 15 centimeters (about 6 inches) above. The plants closed their leaves in response to the fall initially, but stopped doing so after four to six drops. It seems that they “learned” that there was no danger. It’s not that they were no longer able to close their leaves—they still would do so in response to touch. They retained this ability to discriminate between the harmless fall and the potentially harmful (about to be eaten) touch after a month.

Consciousness?

Frantisek Baluska at the University of Bonn, Germany, has pushed further into the question of consciousness by suggesting that plants may even experience pain. They release the chemical ethylene when stressed—when being eaten, attacked, or cut. Nearby plants can sense the ethylene. One researcher equated this release of ethylene with a scream. Since plants also produce the chemical in large quantities when their fruit are ready to be eaten, there’s conjecture that they’re using ethylene as an anesthetic (animals can also be knocked out with ethylene, an anesthetic).

Psychologists and philosophers will likely debate the precise definition of intelligence until the end of time. It may in truth blend into the whole continuum of biological capacities—faculties of various kinds, particularly sensation and memory, that seem to exist throughout the animal world. But as we realize that plants have significant abilities in sensation, awareness, integration of information, long-term memory, and adaptive learning, we must at least leave open the possibility that intelligence is certainly not unique to humans and probably not even to animals.

What It Means for Us

Admitting the possibility that plants may be intelligent—and perhaps conscious—not only brings up many questions about our instrumental (what’s in it for me?) relationship with the rest of nature. It also gives us fodder to rethink the human place in the natural world. I wrote previously that it’s long overdue for us to stop thinking of humans as the only conscious animals. If powerful capabilities long thought unique to humans exist not only in other animals but in plants as well, we must truly begin to see greater continuity between ourselves and the rest of nature.

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1. Anil Anathaswamy, “Roots of Consciousness,” New Scientist, 6 December 2014, pp. 34–37.

2. Anil Anathaswamy, “Roots of Consciousness,” New Scientist, 6 December 2014, pp. 34–37.

3. Anil Anathaswamy, “Roots of Consciousness,” New Scientist, 6 December 2014, p. 36.

4. Anil Anathaswamy, “Roots of Consciousness,” New Scientist, 6 December 2014, p. 36.