Grover Cleveland Backster Jr. could always spot a liar. As he liked to tell it, he served in the Navy during World War II, but his interest in deception soon led him to the Army Counter-Intelligence Corps, where he specialized in hypno- and narco-interrogation — a.k.a. “truth serums.” Then, in 1948, he joined the C.I.A., where, he claimed, he founded the agency’s polygraph program.

A decade later, Backster moved to New York and then opened the Backster School of Lie Detection, where he taught N.Y.P.D. detectives and F.B.I. agents. He testified in courtrooms and before Congress, and his famed Backster Zone Comparison Technique — a methodology for conducting polygraphs — is still widely used. Backster’s success made him a law-enforcement legend, but he was always happier proving people innocent. “I like to think of the polygraph,” he once said, “as a truth detector.”

But this was all a prelude to Backster’s real life’s work, which began in the early morning hours of Feb. 2, 1966. Backster had been up all night in his office on West 46th Street and had just poured himself a cup of coffee when he noticed a houseplant, a Dracaena fragrans his secretary bought to brighten the office. On a lark, Backster, who had a playful streak that belied his military background (he studied astrology, dabbled with LSD and supposedly spent a summer as a stunt diver in a circus), decided to hook the plant up to his lie-detection machine.

In human subjects, a polygraph measures three things: pulse, respiration rate and galvanic skin response, otherwise known as perspiration. If you’re worried about being caught in a lie, your levels will spike or dip. Backster wanted to induce a similar anxiety in the plant, so he decided to set one of its leaves on fire. But before he could even get a match, the polygraph registered an intense reaction on the part of the Dracaena. To Backster, the implication was as indisputable as it was unbelievable. Not only had the plant demonstrated fear — it had also read his mind.

Backster concluded that plants had some heretofore undiscovered sense (he called it “primary perception”) that could detect and respond to human thoughts and emotions. When he publicized his findings, the so-called Backster effect became a pop-culture hit. There was a TV program hosted by Leonard Nimoy and a best-selling book, “The Secret Life of Plants,” inspired by Backster’s research. Backster was interviewed by Johnny Carson, Art Linkletter, Merv Griffin and David Frost. Even Backster’s old employers at the C.I.A. investigated the possibility of human-plant communication.

Not only had the plant demonstrated fear — it had also read his mind.

Scientists, however, were less convinced. No one could reproduce Backster’s results — a problem Backster explained away with a variety of post-hoc qualifiers. (A lettuce leaf didn’t respond to harmful stimuli? It probably shut down to protect itself.) As a result, Backster mainly worked outside the establishment, publishing his findings in outlets like The International Journal of Parapsychology, Volume X.

And yet — publicly, at least — his faith never wavered. Backster went on experimenting until the end, expanding his theory of nonhuman consciousness to encompass chicken eggs and even sperm, forever finding more proof of what he called the “fundamental attunement between living things.” He never married, preferring the company of his Siamese cats, and he never again performed experiments that burned plants. If the pseudoscience of his second act retroactively called into question the science of his first — after all, what is lie detection but mind-reading by another name? — Backster remained unbowed. “Such high resistance to new ideas does not concern me,” he once said. “I have a truly wonderful ally: Mother Nature.”