The Light Eaters: How the Unseen World of Plant Intelligence Offers a New Understanding of Life on Earth

Schlanger turns her attention to whether plants feel. Some botanists argue that plants utilize electricity to sense the world around them. Schlanger explains that plants, just like humans and other animals, have waves of electricity coursing through their anatomies. For humans, these waves head to the brain, where information is carried and stored. While plants do not have a centralized nervous system, Schlanger points to neuroscience developments that point to electrical wave patterns as the foundation for consciousness.

When a person runs a finger along the stem of the Mimosa pudica, the sensitive plant, its leaves fold inward. Slime molds, constructed of one giant cell with thousands of nuclei, move by shooting electrical waves throughout the body. Humans have long known that plants can sense touch. Early farmers advocated for whipping some crops to build a heartier plant. Some plants respond to touch differently, becoming more elastic and flexible under the pressure of outside forces.

The Secret Life of Plants offered several unsubstantiated claims and biased research about plant sensitivity and feeling. Despite its faults, the book inspired some young scientists to question how plants experience the world. Studies into electrical impulses in plants led to many interesting discoveries. One researcher, Van Volkenburgh from the University of Washington, realized that plant cell membranes act similarly to human cell membranes by overseeing the flow of electricity. Volkenburgh found a channel in the membrane that allows the flow of electrical currents.

Proof of ion channels did little to combat the stigma surrounding research into plant behavior. In his 1995 State of the Union address, President Clinton declared that he would cut wasteful spending into studies that focus on “stress in plants.” Scientists found it increasingly difficult to fund research into electrical activity in plants. Instead, the research community shifted its attention to genetics, an area of study with more financial support and opportunity. However, renewed focus on plant electricity is rapidly advancing the field. Scientists have determined that Venus flytraps, the famous insect-eating plant with jaws that snap shut, use electrical bursts in their spiky hairs to detect touch and even count how often something touches them.

Schlanger traveled to Wisconsin to visit botany professor Simon Gilroy, who offered an opportunity to witness how plant electricity works. Gilroy and a colleague discovered that plants experience and understand gravity in a similar way to animals. Humans have canals in their inner ears with trigger hairs and crystals suspended in liquid that shift as we move and interact with gravity. Similarly, plants have granules housed in their cells; how plants process information from those granules is still a mystery.

Gilroy, who knew that gravity sensing in animal ears triggered a jolt of calcium, wondered if he could study whether plants behave similarly using a jellyfish protein that would make calcium in plant cells visible. The jellyfish protein proved highly effective in revealing the electrical path of a plant. At Gilroy’s lab, Schlanger experimented with pinching plant leaves with tweezers dipped in a glutamate solution that helps boost electrical signals. The plants responded with a ripple of bioluminescence. The correlations between how humans and plants process and sense information are striking. Gilroy, a respected researcher who was hesitant to draw connections between plants and humans, suggests that plants’ electrical signaling resembles a nervous system. However, while humans electrical signaling is directed toward and around the brain, scientists still do not know what plants do with their electrical impulses.

Schlanger interrogates a traditional understanding of “hearing,” extending the mode of sensation to plants. She discusses the long-tongued bat in Cuba, a mammal that uses echolocation to find nectar. Marcgravia evenia, a ruby flower caught in an evolutionary partnership with the bat, is the prize. Scientists wondered how bats identified flowers that still had hidden pollen keels from those that did not. The flowers with intact pollen keels exhibit an appendance that acts as an echo amplifier, signaling to the bats that the flower is ready for them.

Like discussions about plant intelligence, communication, and consciousness, research into whether plants take advantage of and even participate in the world of sound is controversial. However, new research reveals a distinct correlation. Researchers Rex Cocroft and Heidi Appel, two scientists in different fields, met at a conference and began designing experiments investigating whether plants can use and make sense of insect vibrations. They discovered that plants responded to their predators’ sounds the same way they would if the insects were physically present. The sounds were enough to cause plants to release their defense mechanisms, such as bitter tannins or chemicals that cause caterpillars to eat one another.

Appel and Cocroft were suspicious of their own findings, understanding the huge ramifications of such a discovery. They scoured their notes, looking for evidence of a mistake. However, the more they interrogated their work, the more they found confirmation. Schlanger argues that Appel and Cocroft’s findings raise the question of what else plants can “hear.”

This branch of study is called phytoacoustics. For plants, the ability to sense vibrations has an evolutionary advantage. New interest in the field is driven by utilitarian gain: If scientists can determine how to use sound to prompt plants to change their chemical compounds, this could have many commercial benefits. For example, lavender oil and mustard oil are both predatory responses that could potentially be made more concentrated through phytoacoustics.

Studies emphasizing ecological relevance reveal that the relationships between plants and their surroundings are built on complex interactions between sounds and cells. Flowers respond to the vibrations of a buzzing bee by increasing the sweetness of their nectar, and desert plants respond to thunder by preparing their cells to take in as much water as possible. How plants “hear” has striking correlations to human auditory processing. Hairlike structures on leaves, called trichomes, take in sensory information much like the sensitive organs inside animal ears.

Monica Gagliano at the University of Western Australia is a polarizing figure in the scientific community who blurs the lines between science, philosophy, and spiritualism. Her research has provided profound insight into how plant roots utilize sound vibration. Her experiments utilize many of the same principles as experiments with mice, such as using mazes to understand how plants decide where to place their roots. The vibration of water through sealed pipes causes roots to change direction toward the rush of hydration. Gagliano’s methods have been scrutinized for her lack of controlled environment and her writing about her use of psychedelics. Schlanger notes that the controversy about whether plants can “hear” speaks to a larger conflict in the science community about the relationship between science and spirituality.