Plants may experience 'optical illusions' that affect their direction of growth

A new study from Tel Aviv University reveals how plants, despite having neither a brain nor a nervous system, integrate light coming from multiple directions to determine where to grow. The researchers show how this process emerges from local sensing across the stem, without the need for centralized information processing.

Light receptors scattered around the stem sense the light sources locally, convert their intensities into biological signals that "encode" the information, and only then add them together. In certain cases, this mechanism may cause the plant to lean in a direction slightly different from the most intense illumination—a phenomenon the researchers define as a kind of "optical illusion."

The study was conducted by Aharon Kempinski, Dr. Amir Porat, Dr. Mathieu Rivière and professor Yasmine Meroz of TAU's School of Plant Sciences and Food Security and Center for the Physics and Chemistry of Living Systems. The study is published in the journal PNAS.

Local signals guide the stem

The researchers explain that while many animals have dedicated sensory organs, such as eyes or ears, as well as a central nervous system that processes sensory information, plants operate as distributed systems. Their light receptors are located primarily in the outer layer of the stem, so that different regions sense light locally. The researchers developed a physical and geometric model to explain how these local signals combine into a single growth direction, without requiring a central processing system.

In the experiments, sunflower seedlings were exposed to blue light of varying intensities from different directions, and their movement was documented for more than 20 hours. The seedlings were first illuminated from a single light source and then from two opposite sources. While they did grow in the direction of the stronger light, their bending was not directly proportional to the difference between the light intensities.

The reason is that the plant's response to light is nonlinear: a light source with twice the intensity does not generate a biological signal that is twice as strong. When the plant adds together signals that have already undergone this type of processing, the resulting direction may differ from the direction that would be obtained through the simple physical addition of the light sources. The mathematical model successfully predicted how strongly and in which direction the seedlings would bend under different lighting conditions.

Weaker light and optical illusions

In the second stage, the researchers examined what happens when a plant is illuminated from several different directions. They positioned two equally strong light sources opposite one another, with a third, weaker source placed perpendicular to the first two. The two strong sources effectively canceled out each other's influence, and the seedlings grew toward the weaker light. This result confirms that plants combine directional light signals rather than simply growing toward the strongest illumination.

The model also predicts a surprising consequence: plants may experience a kind of "optical illusion." This does not mean they see images, but rather that the direction they perceive as receiving the most light can differ slightly from the actual direction of maximum illumination.

The model predicted small distortions, but due to the natural fluctuations in plant movement, the current experiment could not measure them directly.

"What is fascinating is that plants can integrate complex information about their environment without a brain or central processing system," says Meroz. "The computation emerges from the way light is sensed and processed locally across the stem. Our work shows how the physical organization of a sensory system can determine how an organism perceives and responds to its environment. This principle may extend far beyond plants, to cells, bacteria, and other living systems that rely on distributed sensing."