proprioception, the sense of the relative position of our body parts in space that allows us to complete coordinated movements without tripping over our own feet. Do plants have something like proprioception? Certainly, says Chamovitz, but for plants, it’s about the position of their parts relative to gravity. This sense has been the subject of experimental study for more than two centuries. In the mid-1800s, Thomas Andrew Knight set out to test an observation made 50 years earlier by Henri-Louis Duhamel de Monceau: that roots had a propensity to grow down (positive gravitropism) and shoots to grow up (negative gravitropism). With an elegant experiment in artificial gravity, Knight manipulated these tendencies. He arranged bean seedlings in various positions on a circular wooden plate: some with their roots pointing toward the center of the wheel, some with their roots pointing toward the rim. The plate was attached to a water wheel turned by a stream; it spun at 150 revolutions per minute. The spinning created a local reactive centrifugal force that was stronger than gravity. No matter how the plants were initially positioned, they grew in the same way: root toward the new perceived gravitational force—the rim—and shoot away from it. Charles Darwin, intrigued by this phenomenon, turned his now-underappreciated botanical inclinations toward the problem some years later. Working with his son Francis, he sliced varying lengths off of the roots of bean, pea and cucumber seedlings and then laid the seedlings horizontally on damp soil. The Darwins noted that cutting off 0.5 mm or more of a seedling’s root tip resulted in horizontal root growth but no downward growth. In another experiment, they placed seedlings sideways on pins and cauterized their root tips with silver nitrate. This caused the roots to cease growing downward. Uncauterized seedlings’ roots reliably grew down. These and other experiments suggested that the root’s ability to sense gravity resides in the root tip. But the mystery of how shoots orient themselves away from gravity remained. Chamovitz continues to follow the study of gravitropism through history, describing the adventures of plants in space and the insights gained from mutant plants that lack the ability to sense gravity. These experiments, he writes, have revealed a surprising parallel between statoliths, structures in the root cap and endodermis of plants that allow them to sense gravity, and otoliths, subcellular structures in the human inner ear that help us maintain our balance.