People Trade Off Speed and Accuracy. Do Plants?

The slower you do something, the more accurately you tend to do it.

Posted Oct 13, 2020

Jidanni Creative Commons 4.0
Source: Jidanni Creative Commons 4.0

You are probably aware of the speed-accuracy tradeoff. You know that, in general, the more time you spend on something, the more likely you are to do it well. You might normally clean your kitchen quickly to keep it tidy, but if someone is visiting that you really want to impress, you might spend extra time ensuring that you catch small spots you might ordinarily ignore.

Speed-accuracy tradeoffs have been studied extensively in psychology over the years. Indeed, one of the few things in psychology that is called a “law” involves the speed of physical movements people have to make. You can try this at home. Grab a small plate and a coin. Put the plate down in front of you. Then, touch your shoulder and reach out and touch the plate. Now, put a coin the same distance from you that you had put the plate. Do the same thing. Touch your shoulder and then the coin.

Chances are, you moved your hand more slowly to touch the coin than to touch the plate. Reaching out to touch the plate is easier, because it is a big surface. Touching the coin is harder, because there isn’t much room for error. Fitts law, which was first presented in the 1950s, is a specific mathematical formula that captures the degree to which movements slow down as the size of a region to be touched decreases. This relationship has been observed in many studies in humans and other animals.

So, what about plants?

At first, that seems like a strange question to ask. I certainly had never even considered whether plants exhibit a speed-accuracy tradeoff until reading a fascinating paper by Francesco Ceccarini, Silvia Guerra, Allessandro Peressotti, Francesca Peressotti, Maria Bulgheroni, Walter Baccinelli, Bianca Bonato, Umberto Castiello in the October, 2020 issue of Psychonomic Bulletin and Review

Thinking about a climbing vine. Have you ever thought about how vines actually wind themselves around a branch or trellis as they grow? If I had thought about it at all, I probably just assumed the tendrils just grew until they bumped into something and then wrapped around it.

In the study reported in this paper, 10 plants were grown in a carefully controlled environment. Half the plants were grown next to a thick rod that they could attach to and half were grown next to a thin rod. It is worth pointing out upfront that plants differ from people in that smaller targets are better. It is easier for a plant to wrap around a thin rod than a thick one, because as a rod get thicker, the plant needs to be more accurate in its trajectory to actually wrap around it.

The authors used pairs of cameras to get stereo images of the plants growing in order to be able to plot the trajectory of a growing tendril. The tendril from each plant grew toward the rod next to it and wrapped around it.

Consistent with a speed-accuracy tradeoff, tendrils growing toward a narrow rod grew substantially faster than tendrils growing toward a thick rod.

Yup. That’s right. Plants slowed their growth when they had to wrap around a rod that would be harder to do right.

Why in the world does that happen?

The authors provide several possible explanations for why this happens. At one level, for the plant to grow around a thick rod, it may have to thicken the tendril to be able to grow around a larger loop. So, that extra growth in thickness may slow the forward progress of the tendril.

At another level, though, the authors point out that there is increasing evidence for sensory systems in plants that enable them to determine what is in the environment near them. Some may actually emit sounds and use echolocation to sense what is nearby. Others may use optical information to sense things in the environment. Yes, that’s right, the plant might actually see the rod it is growing toward.

Any of you who have been reading my blog over the years know that I often try to draw some kind of lesson from a study for how we live our lives. But, not today. I want you to consider two things. First, this study is really cool. It would never have occurred to me to think of testing a psychological principle like Fitts law in plants. Second, after today, you will never look at a climbing plant the same way again.

References

Ceccarini, F., Guerra, S., Peressotti, A., Peressotti, F., Bulgheroni, M., Baccinelli, W., Bonato, B., & Castiello, U. (2020). Speed-accuracy tradeoffs in plants. Psychonomic Bulletin and Review, 27, 966-973.