Why Plants Do That

Why Bending Makes a Plant Stem Stronger

In 1973, M. J. Jaffe, a botanist at Ohio University, rubbed the stems of young plants gently for about ten seconds, once or twice a day. Beans, cucumbers, barley, castor beans, and a vine called bryony all grew taller more slowly than plants that weren't rubbed. The change was fast. Less than three minutes after a rub, the stem stopped getting longer. When he stopped rubbing after seven days, the plants sped back up, and within three or four days they were growing at a normal rate or even faster.

Not every plant did it. Peas, wheat, and a squash plant kept growing the same whether they were rubbed or not.

Jaffe gave this response a name. When a plant changes how it grows because something touched it, bent it, or shook it, that's called thigmomorphogenesis. He suggested it helps protect plants from strong winds and from animals brushing past them. Jaffe published all of this in 1973, in the botany journal Planta.

What bending does to a sunflower stem

Thirty years later, two scientists writing from the University of Manchester, V. C. Smith and A. R. Ennos, asked a narrower question. Wind does two different things to a plant at once. It bends the stem back and forth, and it moves air past the leaves. They wanted to know which of those two things changes the plant.

So they grew sunflowers and split them into groups. Some had their stems bent, some had air blown past their leaves, some got both, and some got neither. Setting up an experiment so every combination gets tested is called a factorial design. It lets scientists see what each thing does on its own.

Bending and moving air did opposite things. Bending made the sunflowers shorter, and it made their stems stiffer and stronger. Air blowing past the leaves made the sunflowers taller, made their stems weaker and bendier, and helped their stems carry more water up the plant. Smith and Ennos published these results in 2003.

Stiff and strong sound like the same thing, but they're two different measurements. A stiff stem doesn't bend much when you push on it. A strong stem takes a lot of force before it breaks.

A stem can't be best at everything

Smith and Ennos found a trade-off. A trade-off is when getting more of one thing means getting less of another. In their sunflowers, the stems that were best at carrying water were weaker, and the stems that were strongest carried less water. As one went up, the other went down.

Wind does both at once, so a plant growing in the wind is responding to two signals, the bending and the moving air, and the stem it grows depends on the mix. Smith and Ennos suggested this could help explain why different kinds of plants, growing in different places, respond to wind in different ways.

Try it together

You can test Jaffe's idea at home with two bean seedlings. Beans were one of the plants that responded in his study.

Plant two bean seeds in two pots and keep them side by side, so they get the same light and water. Once they've sprouted, pick one to be the rubbed plant. Once or twice a day, gently stroke its stem between your fingers for about ten seconds. Leave the other one alone. Every day, measure how tall each plant is with a ruler and write both numbers down. After a week, compare them. Then stop rubbing, keep measuring, and watch whether the rubbed plant catches up.

Start a garden together

If this turns into a garden, my Field Guide to Common Medicinal Herbs has starter-garden guidance and a beginner foundations section for the plants you'd grow next: kbaco.net/downloads/field-guide

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