In the 1973 book The Secret Life of Plants, Peter Tompkins and Christopher Bird reported that plants had feelings, could read human emotions and thoughts, and had a predilection for classical music. Their claims created an uproar among members of the scientific community, with many arguing that thinking and feeling require the presence of a brain and that nothing in a plant, internally or externally, even remotely resembles one. However, those of the reading public who believed that their plants thrived when dealt with affectionately were quick to give their support.
Although much of the evidence in the Tompkins-Bird book has since been discredited, a number of articles and studies in defense of plants as being more than insentient organisms have since been published. Plants are not mere passive objects capable only of physical processes such as photosynthesis, they argued. Providing both quantitative and qualitative data, these studies noted that plants have the capacity to assess their environment and to act in accordance with the variables that exist in their surroundings. As such, the question of whether plants can think, feel, learn and recall is a legitimate inquiry for some.
An experiment conducted by Monica Gagliano’s team at the University of Western Australia is worth mentioning. In this test, Gagliano selected the mimosa plant because when disturbed, its leaves quickly fold. She subjected a large number of potted mimosa plants to the shock of being dropped from a height of about 15 centimetres. The plants were not damaged in any way since the pots were allowed to slide down an incline, but the shock was considerable enough to cause the plants’ leaves to close.
Since Gagliano was testing for intelligence, she was seeking more than a knee-jerk reaction from the plants; she wanted to determine if they would be able to recall their experience and even learn from it. The plants were dropped 60 times at intervals of a few seconds, with each series of 60 drops being repeated a total of 7 times. The team observed that toward the end of the day, the plants’ leaves stopped closing. This seemed to indicate that they had “adjusted” to their experience and no longer “thought” it threatening. The team wanted to confirm whether their conclusion was valid by giving the plants a different kind of shock—the oscillations of a plate shaker, a device used in a laboratory to stir liquids in beakers and flasks. The plants’ leaves closed shut. What was considered even more extraordinary was that after letting the plants rest from the shocks for four weeks, a repeat test showed that they had learned not to “react” even when dropped down from the same incline. The plantsseemed to have adapted to their new environment.
On the other hand, bizarre experiments abound, such as the one done on a carrot tied down to an examining table by the respected botanist and biologist Sir Jagandish Chandra Bose, who, after noting that a machine connected to the carrot registered “twitches, starts and tremors,” concluded, “Thus can science reveal the feelings of even so stolid a vegetable as the carrot.” In still another experiment, a former CIA operative connected a plant in his office to a polygraph machine, possibly for want of something to do. When he deliberately thought about setting the plant on fire, the machine registered a surge of activity, which led him to the conclusion that not only can plants think, but they can also read minds.
In attempting to understand and explain the discoveries made in legitimate experiments, the crux of the matter is whether the human experience should be used to describe what plants appear to be able to do. Words such as feel, perceive, learn, and remember are applied to human beings. When these same words are used with plants, the assertions sound absurd. Nevertheless, when Yale biologist Clifford Slayman repudiated the possibility of establishing a branch in plant neurobiology, he also acknowledged that plants are capable of intelligent behavior.
Educated persons connect intelligence to structures such as the brain, neurons and synapses, but supporters of intelligence in plants say that there may be other ways of processing stimuli and information, such as special cells and cell networks, and electric or chemical signals. Neurons, they insist, are not the only means to bring about intelligent behavior. For example, scientists have discovered that root tips can sense, among other things, gravity, moisture, light, pressure, volume, gases, salts, microbes and even chemical signals from other plants. A plant uses the information it has collected to take action. It may change the direction of its growth; it may restrain its growth if there is crowding in the area where it is rooted; it may warn other plants of a danger by releasing a chemical into the air, and these may respond by emitting a toxin to repel the attacker.
Perhaps one of the most amazing indications that plants can think is the way they are able to organize themselves. A forest ecologist at the University of British Columbia discovered how trees in a forest arrange themselves in widely distributed networks in a manner that allows the trees to share resources and protect fellow trees. They injected the trees with a radioactive carbon and followed the flow of nutrients and chemical signals through the community with a Geiger counter. A diagram they made of these movements showed that the oldest trees served as hubs and had as many as 47 connections. The diagram, they said, looked like an airline route map.
Evidence such as this has given some botanists pause for thought. Human beings see only the stillness of plants, but this is due to the nature of human perception. If time were sped up to allow us to observe the movements of plants, and if we were capable of seeing how their cell networks and signals orchestrate a myriad of functions that allow an entire forest to thrive, then and only then might we appreciate that that these seemingly quiet and immobile organisms have their own glorious intelligence.