May 30, 2016 | David F. Coppedge

Sunflower Motion Is a Black Box

Something as commonly observable as sunflowers following the sun is difficult to explain.

Even children can notice that plants follow the sun throughout the day. This phenomenon, given the fancy name “heliotropism” (attraction to the sun) has been known for centuries, but is still a “black box,” plant scientists admit. PhysOrg reports on botanists who have experimented with sunflowers, trying to understand the mechanisms that guide their bright yellow flowers toward the light.

Experiments have shown that if sunflowers in a field are turned 180 degrees, they continue to move during the day, but now in a west-to-east direction, opposite to the sun. After several days, the sunflower corrects itself to move east-to-west again. During this time, the sunflower retunes its internal clock, using one of the most powerful entrainment cues – sunlight.

“The external cues remind the plant what time of day it is. It slightly readjusts itself so that its internal clock is matched to the environment,” says Dr Haydon. This is essential when dawn changes by three minutes or more everyday depending on your latitude.

“This experiment shows very clearly that there’s an internal clock that is driving this plant behavior.”

But an internal clock is not enough. The signal has to be translated into actual motion – and plants have no muscles. Nor do they have brains that can be reminded.

So heliotropism is regulated by an internal rhythm of gene expression. But the actual mechanics of the movement remains “very much a black box … but we suspect that the mature leaves have something to do with it since the rhythmic movement stops when mature leaves are cut off,” says Dr Haydon.

Mysteries pile up when the phenomenon is explored in detail. Why do sunflowers stop moving with the sun when they bloom? How do they know to switch off the movement?

“It could simply be mechanical: as the sunflower head grows, it simply becomes too heavy for the stem to move, ” he says. “The other possibility is that it is related to diminishing robustness of the internal clock. There is evidence that circadian rhythms weaken with age, not only in plants but also in animals.”

Here’s an example of an easily observed biological phenomenon that to this day resists complete explanation. It shows that scientific discovery and scientific explanation are two different things. “What” is not the same as “how” or “why”. The article resisted any temptation to call on evolutionary theory for understanding.

Whatever causes heliotropism, it’s marvelous. God’s designs surpass our abilities to grasp them. It’s uniquely human to try to grasp for understanding, and when understanding comes, humans are astonished, and God is glorified.


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