Are Plants Intelligent?

Michael Pollan provides a fascinating look into the ways plants are capable of responding to their environments in an article in The New Yorker: The Intelligent Plant.

There seems to be considerable controversy in associating the clever things of which plants are capable with the concept of intelligence. Some members of the scientific community view intelligence as an attribute that is associated with a brain and a very human-like consciousness. Others apply a broader definition that describes intelligence as the ability to respond effectively to changes in environmental circumstances; this approach gives plants a good shot at being deemed intelligent. One could argue that the latter rendition of the term provides a quite comprehensive description of what humans do as well.

Pollan tells us two significant differences between plants and animals make it difficult for humans to appreciate the complexity developed by the earth’s flora. The first is related to the timescale involved in plant responses. Plants may be accomplishing wondrous things, but they occur on a timescale so long that to human observers they usually appear inert.

"It is only human arrogance, and the fact that the lives of plants unfold in what amounts to a much slower dimension of time, that keep us from appreciating their intelligence and consequent success."

Plant capabilities must be evaluated within an entirely different context. Evolutionary strategies for an organism that is firmly tied to a particular location are quite different from those of a mobile organism. An animal can develop an efficient brain and central nervous system because it can use its ability to move around as a defense mechanism to protect its vulnerable components from injury. An immobile plant has to assume that it will be chewed upon or otherwise injured and much of its structure will be lost. A localized brain would make little sense in that case and a more distributed form for sensing and logic would be appropriate.

"More likely, in the scientists’ view, intelligence in plants resembles that exhibited in insect colonies, where it is thought to be an emergent property of a great many mindless individuals organized in a network. Much of the research on plant intelligence has been inspired by the new science of networks, distributed computing, and swarm behavior, which has demonstrated some of the ways in which remarkably brainy behavior can emerge in the absence of actual brains."

Those who would argue that the label "intelligent" should be applied to plants have been accumulating some compelling information.

"The ‘sessile life style,’ as plant biologists term it, calls for an extensive and nuanced understanding of one’s immediate environment, since the plant has to find everything it needs, and has to defend itself, while remaining fixed in place. A highly developed sensory apparatus is required to locate food and identify threats. Plants have evolved between fifteen and twenty distinct senses, including analogues of our five: smell and taste (they sense and respond to chemicals in the air or on their bodies); sight (they react differently to various wavelengths of light as well as to shadow); touch (a vine or a root ‘knows’ when it encounters a solid object); and, it has been discovered, sound. In a recent experiment, Heidi Appel, a chemical ecologist at the University of Missouri, found that, when she played a recording of a caterpillar chomping a leaf for a plant that hadn’t been touched, the sound primed the plant’s genetic machinery to produce defense chemicals."

Much of the cleverness of plants is hidden from our view as roots search for the optimal path in an attempt to optimize nourishment.

"Scientists have since found that the tips of plant roots, in addition to sensing gravity, moisture, light, pressure, and hardness, can also sense volume, nitrogen, phosphorus, salt, various toxins, microbes, and chemical signals from neighboring plants. Roots about to encounter an impenetrable obstacle or a toxic substance change course before they make contact with it. Roots can tell whether nearby roots are self or other and, if other, kin or stranger."

One of the lesser-known attributes of plants is their ability to communicate and even to engage in collective behavior. The mechanism for communication is usually via the emission of an assortment of chemicals.

"One of the most productive areas of plant research in recent years has been plant signalling. Since the early nineteen-eighties, it has been known that when a plant’s leaves are infected or chewed by insects they emit volatile chemicals that signal other leaves to mount a defense. Sometimes this warning signal contains information about the identity of the insect, gleaned from the taste of its saliva. Depending on the plant and the attacker, the defense might involve altering the leaf’s flavor or texture, or producing toxins or other compounds that render the plant’s flesh less digestible to herbivores."

"Perhaps the cleverest instance of plant signalling involves two insect species, the first in the role of pest and the second as its exterminator. Several species, including corn and lima beans, emit a chemical distress call when attacked by caterpillars. Parasitic wasps some distance away lock in on that scent, follow it to the afflicted plant, and proceed to slowly destroy the caterpillars."

Pollan cites the work of a forest ecologist, Suzanne Simard, and her colleagues on determining the organization of trees in a forest:

"….trees in a forest organize themselves into far-flung networks, using the underground web of mycorrhizal fungi which connects their roots to exchange information and even goods. This "wood-wide web," as the title of one paper put it, allows scores of trees in a forest to convey warnings of insect attacks, and also to deliver carbon, nitrogen, and water to trees in need."

"The pattern of nutrient traffic showed how ‘mother trees’ were using the network to nourish shaded seedlings, including their offspring—which the trees can apparently recognize as kin—until they’re tall enough to reach the light. And, in a striking example of interspecies cooperation, Simard found that fir trees were using the fungal web to trade nutrients with paper-bark birch trees over the course of the season. The evergreen species will tide over the deciduous one when it has sugars to spare, and then call in the debt later in the season."

Pollan described several experiments designed to elicit behavior that would imply that plants are capable of performing tasks that would normally be assumed to require intelligence. An experiment with pole-climbing beans is illustrative.

A bean plant was observed using time-lapse photography as it grew positioned a few feet away from a pole mounted on a dolly. Pollan, like most of us, always assumed that bean plants grow in arbitrary directions until they bump into something to cling to. Not so. A video indicated that the plant "figured out" where the pole was and wasted no effort growing in other directions.

"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."

Pollan finds that time-lapse photography has taken plant activity into a timescale that is evocative of conscious intent.

"….a dimension of time in which these formerly inert beings come astonishingly to life, seemingly conscious individuals with intentions."

Whether a scientist or a layperson, the willingness to label plant behaviors "intelligent" is a matter of personal choice. However, one has to be amazed at how complex the seemingly simple organisms are turning out to be. Undoubtedly there is more yet to be revealed as research continues.

Humans tend to see themselves as the unassailable peak of evolution—as rulers of the earth. Pollan puts us in perspective:

"Plants dominate every terrestrial environment, composing ninety-nine per cent of the biomass on earth. By comparison, humans and all the other animals are, in the words of one plant neurobiologist, ‘just traces’."