by Tristan Wang
On a night in 1966 interrogation specialist Cleve Backster taught how to perform lie detection to policemen. On a whim, Backster attached electrodes of a galvanometer to a nearby dracaena plant. A galvanometer is an instrument that detects minute electric currents, often used as a part of the polygraph lie detector. When Backster began to water the plant, the galvanometer did not show the same growth in electrical conductivity as he would have expected. Instead, the needle of the galvanometer started to move downward, a response often only seen with surges of human emotion. Caught completely by surprise, Backster started formulating ideas about plant conscience. Because he knew that that some of the strongest emotional stimuli came from life-threatening situations, Backster thought about burning the actual leaf the electrodes were attached to. Before he could reach for a match, the tracing pattern on the graph swept upwards as if in response to the thought of threat. These 10 short minutes changed Backster’s life and gave him the idea of plant sentience—an idea so grand that it later was coined into the term “the Backster effect.” (1)
It is no argument that humans use their senses to feel, think and act. Even the smallest baby cries when tripped and laughs when played with, but that’s not the whole story.
What about organisms that lack the complex nervous system that we share? Would animal-rights activists protect the cockroach that was stepped on or even the callous sponge and coral? Take this a step further and let us wonder about the life of plants. Plant biologists have long spurned the crazy ideas of botanical feelings and consciences such as those explored by Backster, but does that mean that these topics are not worth studying?
In the 1973 book The Secret Life of Plants by Peter Tompkins and Christopher Bird, several brazen souls decided to explore a topic that is today considered as pseudoscience: plant sentience. Some went to great lengths to see if plants could detect, understand and pinpoint pain. While this research and subsequent book made a great splash in the media, it efficiently turned away scientists from the study of senses of plant biology as a sort of the unthinkable, taboo if you will. No one was able to reproduce Backster’s projects, an overarching necessity for correct science, and plant sentience became a joke in the field of plant biology.
However, researchers recently have explored something that seems to belong in Tompkins’ book: plant communication. While incredible, there has been hard evidence that plants have been talking to each other all along through chemical signaling, but that’s not the only way that plants interact with the world. Senses like sight and smell, a domain argued to belong to animals, also apply to the plant world and attests to the significance of plant senses.
Only by learning how plants see the world through their senses can we understand how to use these interactions in the context of agriculture, ecology and human life.
Every spring narcissus flowers, better known as the daffodil, bloom as the sun’s rays arch over meadows of the north, but it’s no accident that these flowers time their flowering in spring. Although the idea that daffodils may be timing their flowers for our pleasure may be enchanting, plants have developed ways of knowing their world through sight not too differently from ours. The concept of photoperiodism conveys how the length of the night (or day as many call it) dictates seasonal responses that include flowering. (2) Plants achieve this feat through the expression of certain genes and plant hormones such as florigen (a biological flowering signal) , all of which accordingly changes to the amount of light the plant receives. (2)
Thus, a daffodil plant can “sense” the changing seasons simply by measuring how long the days and nights are. As Daniel Chamovitz puts in in his book What a Plant Knows, sight is “the physical sense by which light stimuli received by the eye are interpreted by the brain and constructed into a representation.” (3) Take out the “eye” and “brain” in this definition, and plants can see, just like us, but of course it’s difficult to project an image of the environment simply based on how much light is received.
Get this though; plants can also perceive color. Whereas humans largely rely on two types of photoreceptors (rods and cones), plants dwarf that number with at least 11 different kinds of photoreceptors that aid plants in reacting to sunlight. (3) This shouldn’t come as a surprise seeing how important light is to a plant’s survival.
Even things like fragrance can prove to be useful to plants. One study in 2006 explored the significance of plant volatiles as a method that parasitic plants use to detect host plants.
(4) In this experiment the Cuscuta pentagona (dodder) seedlings, parasitic plants known to sap life out of hosts, are germinated without contact to it’s natural host (in this case, the tomato plant). (4) The vast majority of the seedlings were able to find the nearby tomato plant indicating a possibility of volatile clues emitted by the tomato; and when a wheat plant was placed into the mix, the dodder was able to distinguish between the volatiles between the two plants and preferred the tomato. (4) Even with animal interactions, plants seem to react to insect scents. A recent study in 2012 has provided evidence that plants seem to produce defense responses in reaction to incoming herbivores. (5) Scientists exposed sex attractants of Eurosta solidaginis flies to goldenrod plants and in the field, it appeared that the female flies preferred to not lay eggs on the exposed plants. (5)
All of these senses are interesting but what of it? Surly plants can use this information to react to the world, but the curiosity arises when one asks whether plants can organize themselves to do something more than just a slight movement to the sun, or the blooming of a flower. With all of this information that plants are getting, it’s not surprising that recent research has looked into the ways that plants interact together.
Last May, several researchers looked into just how plants can react so quickly to oncoming herbivore invasions even if the plants were not in direct contact with the herbivores. Similar to how some plants can pick up the scents of fellow neighbors or notice when another plant was casting a shadow, plants were communicating. In this study, bean plants were set up in close proximity to each other and when aphids attacked one bean plant, nearby plants picked up this system of warnings and then produced plant defenses, chemicals like methyl salicylate that act to repel the herbivores and to attract the predators of aphids. (6)
The researchers set up five bean seedlings in an order in which there was a central ‘donor’ plant surrounded by four ‘receiver’ plants. (6) The central plant would be in direct contact with the aphids and in theory would send signals to the other four plants. (6) Two of the receiver plants were connected to the donor plant via mycorrhizal fungi with the other two lacked physical connection to the other plants. (6) Mycorrhizal fungi forms a symbiotic association between fungal hyphae and roots and is quite prominent in most natural soils. (7) Fungal hyphae invade roots and grow extensively often even having more than one plant host. (7) Several experiments were set up so that meshes helped to restrict plant-to-plant interaction through mycorrhizal growth and roots. (6)
After a couple of days, volatile gases were collected from each of the plants and it was demonstrated that plants connected to the central plant via underground fungal networks were able to produce protective chemicals as opposed to the two unconnected bean plants. (6) The implications of plant communication can be significant. Would it be possible to use a tribute plant far away from commercial crops as a sort of early warning for the others?
How plants react to their environment and to each other ultimately comes back to our interactions with plants. Humans are consistently changing the environment whether it’s by deforestation or agriculture and this has forced plants to adapt in different ways to cope with the stress of change. Thus, there is even more of a need to study how plants see this consistently evolving world, not through conscience thinking, but through their senses.
Backster wasn’t too far off when he thought that plants were talking, despite the ridicule his works received from the scientific world. Maybe he’s right that there is a world of plants talking and understanding each other and that we just don’t know much about yet; perhaps learning about how plants see the world may help us appreciate our world. Who knows—because after all, none of us are plants.
1.Tompkins, Peter, and Christopher Bird. The Secret Life of Plants. New York: Harper & Row, 1973.
2. Tsuji, Hiroyuki, Ken-ichiro Taoka, and Ko Shimamoto. “Regulation of Flowering in Rice: Two Florigen Genes, a Complex Gene Network, and Natural Variation.” Current Opinion in Plant Biology 14.1 (2010): 45-52.
3. Chamovitz, Daniel. What a Plant Knows: A Field Guide to the Senses. New York: Scientific American/Farrar, Straus and Giroux, 2012.
4. Runyon, J. B. “Volatile Chemical Cues Guide Host Location and Host Selection by Parasitic Plants.” Science 313.5795 (2006): 1964-967.
5. Helms, Anjel M., Consuelo M. De Moraes, John F. Tooker, and Mark C. Mescher. “Exposure of Solidago Altissima Plants to Volatile Emissions of an Insect Antagonist (Eurosta Solidaginis) Deters Subsequent Herbivory.” PNAS 110.1 (2013): 199-204.
6. Babikova, Zdenka, Lucy Gilbert, Toby J. Bruce, Michael Birkett, John C. Caulfield, Christine Woodcock, John A. Pickett, and David Johnson. “Underground Signals Carried through Common Mycelial Networks Warn Neighbouring Plants of Aphid Attack.” Ecology Letters 16.7 (2013): 835-43.
7. Bolan, N. S. “A Critical Review on the Role of Mycorrhizal Fungi in the Uptake of Phosphorus by Plants.” Plant and Soil 134.2 (1991): 189-207.