Intelligent Plants?

A Sensitive Plant in a garden grew,
And the young winds fed it with silver dew,
And it opened its fan-like leaves to the light.
And closed them beneath the kisses of Night.

The Sensitive Plant, a watercolour by Frank Bernard Dicksee (1853-1928) depicting Percy Bysshe Shelley’s poem (see above). Public domain, via Wikimedia Commons.

The publication of a recent paper (Khattar et al., 2022) [1] examining acceptance among scientists of various disciplines of the concept of plant intelligence provoked online enthusiasm among some herbalists, in terms such as:

“It hearkens to an emerging, emboldened approach to scientific investigation that finally dispenses with monotheism-informed, limiting belief structures and subtexts, allowing for the contemplation of intelligence and consciousness in non-human beings… (such as) …plants and dissimilar creatures in particular.”

The paper’s authors acknowledge that semantics is an issue and provide the following definition of plant intelligence:

Any type of intentional and flexible behavior that is beneficial and enables the organism to achieve its goal.”

They give some examples of behaviours which might be viewed as evidence of plant intelligence:

“the ability to problem-solve in a flexible manner, anticipate the future, store memory, learn, communicate, and ultimately be goal-oriented.”

Specific examples include:

  • Volatile chemicals being produced at the appropriate time, concentration, and amount as defence against herbivores.
  • The above behavioral responses affecting, and being affected by, neighboring plants, shaping plant communities.
  • The capability to custom-modify the quality and quantity of their nectar to attract the right species of ants to protect them, a capability that requires plants to sense the presence of different ants, and to monitor and modify their own activity accordingly.
  • The ability to anticipate the future as demonstrated, for example, in the way plants rely on light cues to remember the exact number of warm days or daylight hours (i.e., photoperiodic control) that have passed to develop their leaves and flower.
  • The ability to learn to make new associations through multiple cues and to respond adaptively. Conditioned learning of this type has been demonstrated in the pea plant (Pisum sativum L.).
  • Various types of foraging behavior for sunlight and nutrients exemplify how plants can be goal-oriented. Example: the ability of the pea plant to modify root-growth in response to varying nutrient concentrations.
  • The capability to manipulate herbivores to adopt cannabalistic behaviour by releasing certain chemicals, as demonstrated by some tomato plants (Solanum lycopersicum L.).

These capabilities are indeed fascinating, but do they demonstrate intelligence? This clearly hinges upon our definition of intelligence, and according to the study author’s definition, I would judge that they do not. The problem is with the word “intentional”. I would agree that to be “goal-oriented” is not enough for a behaviour to demonstrate intelligence, it must be intentional. And does not intention imply consciousness? None of the behaviours described above demonstrates intention. It may be there at some level (if not individual then within the community), but it has not been shown to be there.

But I am more interested in a further facet of this debate. Just say that we all agreed that plants possess some form of intelligent consciousness (remember, intention requires consciousness), would that be be anything like ours, something we as human beings could relate to? Because this is what some herbalists fancy. That plants can talk to us and we can understand them; even that they may feel well-disposed towards us.

I have little doubt it would not. I know quite a lot at an experiential level about fish, which are more closely related to us than are plants. I swim with them often and sometimes I catch them to eat. Some species are curious about people, most are not, and some will swim away very fast at the first sight of a human being. I like them. But they cannot “like” me, even the curious ones, they are merely curious in some kind of fishy way. Why on Earth should intelligent fish like human beings? If I were an intelligent fish I would have a great big prejudice against them, wouldn’t you? But fish are most probably incapable of emotion. Their behaviour is goal-oriented (even intentional) and flexible, but whatever form their “thoughts” take, they are nothing like mine. I can hardly conceive of what it is like to think like a fish. Tell me, what part of the above reasoning cannot be applied to plants?

For some herbalists there is the romantic temptation to anthropomorphise plants or to project their own thoughts and feelings onto them, but beware! Subjective realities, while valid in some terms and for some purposes, often boil down to simple flights of fancy.

[1] Khattar, J., Calvo, P., Vandebroek, I. et al. Understanding interdisciplinary perspectives of plant intelligence: Is it a matter of science, language, or subjectivity?. J Ethnobiology Ethnomedicine 18, 41 (2022). https://doi.org/10.1186/s13002-022-00539-3