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.
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.
Those who would argue that the label "intelligent" should be applied to plants have been accumulating some compelling information.
Much of the cleverness of plants is hidden from our view as roots search for the optimal path in an attempt to optimize nourishment.
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.
"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:
"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.
Pollan finds that time-lapse photography has taken plant activity into a timescale that is evocative of conscious intent.
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: