Plants Can Be Tricked by Optical Illusions Without Eyes or a Brain

Eyes and a brain are not required to be fooled by an optical illusion. Botanists have found that a sunflower can be made to bend toward a dim light while two brighter lights, aimed at it from opposite sides, cancel each other out.

A dim light beats two bright ones

The team, from Tel Aviv University, tested sunflowers because they are the best-known light-seekers in the plant world. In one experiment the researchers lit a plant from two opposite sides with sources of identical brightness, then added a third, weaker source at right angles to the other two. The sunflower bent toward the weaker light. The two brighter lights appeared to cancel each other out.

Earlier steps in the same series were easier to predict. A single light source, made brighter and brighter, pulled the plant toward it, though each increase in brightness produced less extra bending than the one before. When two lights of different brightness faced each other, the plant bent toward the brighter one, but the angle of the bend did not grow in step with the difference in brightness.

The plant adds up directions, not brightness

The researchers explain the result through the way light signals are handled inside the plant. Doubling the brightness of a light does not double the signal the plant receives. When the plant works out which way to lean, it is adding up the direction of those signals rather than the strength of the light that produced them. Two equally bright lights on opposite sides send signals pointing in opposite directions, and the two cancel. The weaker light, arriving from a third direction, is then the only signal still pointing anywhere, so the plant leans that way. That is what leads to the apparent optical illusion. The study was published in Proceedings of the National Academy of Sciences.

Sensors spread over the stem

Where a plant keeps its light sensors matters too. In people, light-sensing cells sit at the front of the face, in two eyes. In a plant they are spread over the body, mainly along the stem, so light arriving from several directions is picked up by different patches of tissue at the same time. The team describes this as a distributed sensory system, spread across the body rather than gathered in one place, and it is what allows the mix-up to happen.

Yasmine Meroz, one of the study authors, said the work shows how the physical organisation of a sensory system can determine how an organism perceives and responds to its environment. She added that the principle may extend far beyond plants, to cells, bacteria and other living systems that rely on sensing spread across their bodies rather than gathered in one spot.