Cleve Backster hooked up his houseplants to a polygraph machine to read emotional responses in 1967, and six years later, Peter Tomkins and Christopher Bird wrote the book The Secret Life of Plants. The book claims that plants are sentients, yet is somewhat discredited due to its supernatural speculations. Backster was discredited soon after, when his findings were not able to be duplicated. The scientific world, for the most part, turned their backs on plant feelings and intelligence.
Plants are able to sense and optimally respond to so many environmental variables--light, water, gravity, temperature, soil structure, nutrients, toxins, microbes, herbivores, chemical signals from other plants--that there may exist some brain-like information-processing system to integrate the data and coordinate a plant’s behavioral response. Electrical and chemical signaling systems have been identified in plants which are homologous to those found in the nervous systems of animals.
Neurotransmitters such as serotonin, dopamine, and glutamate have all been found in plants. According to many plant scientists, plants perceive their circumstances and respond to environmental input in an integrated fashion. They have an intrinsic ability to process information from both abiotic and biotic stimuli that allows optimal decisions about future activities in a given environment.
The key difference between plants and animals in the Stenhouse (1974) definition is in the word ‘behaviour’. Silvertown and Gordon (1989) have defined plant behaviour as the response to internal and external signals. In plant terms these are familiar growth and development phenomena, such as de‐etiolation, flower induction, wind sway response, regeneration, induced bud break/germination, tropic bending, etc. Thus, a simple definition of plant intelligence can be coined as adaptively variable growth and development during the lifetime of the individual. To add significance to this definition, time lapse shows that virtually all plant movements are indeed the result of growth and development.
THE EVIDENCEPrevent Disease:
Touch and Sound
If bacteria can signal one another with vibrations, why not plants, said Monica Gagliano, a plant physiologist at the University of Western Australia in Crawley. Gagliano imagines that root-to-root alerts could transform a forest into an organic switchboard.
"Considering that entire forests are all interconnected by networks of fungi, maybe plants are using fungi the way we use the Internet and sending acoustic signals through this Web. From here, who knows," she said. Researchers at the University of Missouri found that plants can identify sounds nearby, such as the sound of eating, and then react to the threats in their environment.
"Previous research has investigated how plants respond to acoustic energy, including music," said Heidi Appel, senior research scientist in the Division of Plant Sciences in the College of Agriculture, Food and Natural Resources and the Bond Life Sciences Center at MU. "We found that 'feeding vibrations' signal changes in the plant cells' metabolism, creating more defensive chemicals that can repel attacks from caterpillars."
When caterpillars later fed on both sets of plants, the researchers found that the plants previously exposed to feeding vibrations produced more mustard oils, a chemical that is unappealing to many caterpillars.
"What is remarkable is that the plants exposed to different vibrations, including those made by a gentle wind or different insect sounds that share some acoustic features with caterpillar feeding vibrations did not increase their chemical defenses," Cocroft said. "This indicates that the plants are able to distinguish feeding vibrations from other common sources of environmental vibration."
Plants can hear the vibrations produced by insects, such as a bee's buzz or an aphid's wing beat, and minuscule sounds that might be created by even smaller organisms. Plants even benefit from the ability to detect certain sounds produced by other plants. For example, researchers at the Institute of Plant Sciences in Bern, Switzerland, recently recorded ultrasonic vibrations emanating from pine and oak trees during a drought, perhaps signalling to other trees to prepare for dry conditions.
Researchers in Bonn, Germany, found plants give off a gas when under "attack". Super-sensitive microphones picked up a "bubbling" sound from a healthy plant. But this rose to a piercing screech when it was under threat. "The more a plant is subjected to stress, the louder the signal," said Dr. Frank Kuhnemann.
Plants do not actually scream in pain. But different sounds are heard when the gas they emit, ethylene, is bombarded with lasers.
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.
When antelopes browse acacia trees, the leaves produce tannins that make them unappetizing and difficult to digest. When food is scarce and acacias are overbrowsed, it has been reported, the trees produce sufficient amounts of toxin to kill the animals.
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. Scientists call these insects “plant bodyguards.”
The parasitic dodder vine (Cuscuta europaea) is the sniffer dog of the vegetable world. It contains almost no chlorophyll - the pigment that most plants use to make food - so to eat it must suck the sugary sap from other plants. Dodder uses olfaction to hunt down its quarry. It can distinguish potential victims from their smell, homing in on its favorites and also using scents emitted by unhealthy specimens to avoid them.
Dodder is exceptionally sensitive to odors, but all plants have a sense of smell. In animals, sensors in the nose recognize and bind with molecules in the air. Plants also have receptors that respond to volatile chemicals. What do they smell?
Back in the 1920s, researchers with the US Department of Agriculture demonstrated that treating unripe fruit with ethylene gas would induce it to ripen. Since then, it has become apparent that all ripening fruits emit ethylene in copious amounts, can smell it, and respond by ripening. This ensures not only that a fruit ripens uniformly but also that neighboring ones ripen together, producing more ethylene and leading to a ripening cascade. Coordinated ripening is important because it attracts animals to eat the fruit and disperse the seeds.
Ethylene is a plant hormone that regulates many processes, so being able to smell it has other advantages too, such as in the coordination of leaf-colour changes in the autumn.
Above all, however, smell allows plants to communicate. Research in the 1980s showed that healthy trees in the vicinity of caterpillar-infested ones were resistant to the pests because their leaves contained chemicals that made them unpalatable. Other trees isolated from the infestation did not produce these chemicals, so it seemed that the attacked trees had sent an airborne pheromone message that primed healthy trees to prepare for imminent attack. We now know that many volatile chemicals are involved.
Learning and Memory
Monica Gagliano, who worked in Mancuso’s lab in Florence, reported that she retested her Mimosa 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. She reminded her colleagues that, in similar experiments with bees, the insects forgot what they had learned after just forty-eight hours. Gagliano concluded by suggesting that “brains and neurons are a sophisticated solution but not a necessary requirement for learning,” and that there is “some unifying mechanism across living systems that can process information and learn.”
Plants do not have a brain or neuronal network, but reactions within signalling pathways may provide a biochemical basis for learning and memory in addition to computation and problem solving. Plants produce several proteins found in the animal neuron systems such as acetylcholine esterase, glutamate receptors, GABA receptors, and endocannabinoid signaling components. They also use ATP, NO, and ROS like animals for signaling.
A plant's concomitant reactive behavior is mediated by phytochromes, kinins, hormones, antibiotic or other chemical release, changes of water and chemical transport, and other means. These responses are generally slow, taking at minimum a number of hours to accomplish, and can best be observed with time-lapse cinematography, but rapid movements can occur as well.
Plants have scientifically been show to draw alternative sources of energy from other plants. Plants influence each other in many ways and they communicate through "nanomechanical oscillations" vibrations on the tiniest atomic or molecular scale or as close as you can get to telepathic communication. However, their sense and communication are measurable in very much the same ways as humans.
Stefano Mancuso from the International Laboratory of Plant Neurobiology at the University of Florence, Italy, and his colleagues are starting to apply rigorous standards to study plant hearing. Their preliminary results indicate that corn roots grow towards specific frequencies of vibrations. What is even more surprising is that the young corn roots made clicking sounds, and that when suspended in water they would lean towards sounds made in the same frequency range (about 220 Hz).
Suzanne Simard, a forest ecologist at the University of British Columbia, tracks the flow of nutrients and chemical signals through the underground web of mycorrhizal fungi. They injected fir trees with radioactive carbon isotopes, then followed the spread of the isotopes through the forest community using a variety of sensing methods, including a Geiger counter. Within a few days, stores of radioactive carbon had been routed from tree to tree. Every tree in a plot thirty metres square was connected to the network; the oldest trees functioned as hubs, some with as many as forty-seven connections. The diagram of the forest network resembled an airline route map. 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 coöperation, 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.
For the forest community, the value of this coöperative underground economy appears to be better over-all health, more total photosynthesis, and greater resilience in the face of disturbance.
THE VERDICTThe last sentence of Darwin’s 1880 book, The Power of Movement in Plants, has assumed scriptural authority for some plant neurobiologists: “It is hardly an exaggeration to say that the tip of the radicle . . . having the power of directing the movements of the adjoining parts, acts like the brain of one of the lower animals; the brain being seated within the anterior end of the body, receiving impressions from the sense organs and directing the several movements.” Darwin was asking us to think of the plant as a kind of upside-down animal, with its main sensory organs and “brain” on the bottom, underground, and its sexual organs on top.
“Yes, plants have both short- and long-term electrical signalling, and they use some neurotransmitter-like chemicals as chemical signals,” Lincoln Taiz, an emeritus professor of plant physiology at U.C. Santa Cruz and one of the signers of the Alpi letter, told me. “But the mechanisms are quite different from those of true nervous systems.” Taiz says that the writings of the plant neurobiologists suffer from “over-interpretation of data, teleology, anthropomorphizing, philosophizing, and wild speculations.” He is confident that eventually the plant behaviors we can’t yet account for will be explained by the action of chemical or electrical pathways, without recourse to “animism.”
When most of us think of plants, to the extent that we think about plants at all, we think of them as old—holdovers from a simpler, prehuman evolutionary past. But for Mancuso plants hold the key to a future that will be organized around systems and technologies that are networked, decentralized, modular, reiterated, redundant—and green, able to nourish themselves on light. “Plants are the great symbol of modernity.” Or should be: their brainlessness turns out to be their strength, and perhaps the most valuable inspiration we can take from them.
» Zweifel and Zeugin, 2008: "Ultrasonic acoustic emissions in drought-stressed trees"
» Runyon et al., 2006: "Volatile Chemical Cues Guide Host Location and Host Selection by Parasitic Plants"
» Anderson and Cairney, 2007: "Ectomycorrhizal fungi: exploring the mycelial frontier"
» Michael Pollan in The New Yorker, 2013: "The Intelligent Plant"