Plants Respond to Touch and Gravity

November 1, 2015 lawanda Newspaper Columns

The outdoor gardening season is over in Wisconsin, so it’s time to learn something interesting for which the average gardener will probably find no practical application.  You can always throw your new knowledge and vocabulary words around to impress your friends though.

Have you ever wondered how plants like clematis, grapes, peas, cucumbers, squash, ivies and other plants with tendrils know how to feel their way up a trellis or support wire and coil so tightly around it?

When a plant grows upward from the soil, it moves back and forth as it lengthens due to naturally unequal rates of growth on either side of the plant.  This movement back and forth is called nutation, which comes from the Latin word for nodding or swaying.  Think of it like when you walk into a dark room and wave your hands from side to side to try to avoid running into something, but on a vertical basis rather than horizontal.

So the plant is swaying back and forth on its upward journey and finally, it touches something – part of a trellis, a support wire or string, or another plant stem.  When it does, the cells on that side of the stem contract and produce a hormone called auxin, which it sends across the stem to the untouched part.  The auxin causes the cells in the untouched side to elongate faster than those on the touched side.  As a result, the stem or tendril curves inward around the support object.  The process is called thigmotropism.  “Thigmo” comes from the Greek word for touch and “tropos” means to turn.

This vining tendency is positive thigmotropism, as the plant parts move toward the object of touch.  Roots are different.  They tend to move away from any object they encounter in the soil, following the path of least resistance as they search for water and nutrients.  Thus, roots are negatively thigmotropic.  If you’ve grown carrots in clay or stony soil, you’ll easily understand this concept as they often end up twisted into some very unusual shapes.

Gravitropism is the principle of turning downward as the result of gravity.  Charles Darwin performed experiments in which he found that when he placed an obstacle in the path of a downward-growing bean root, it would grow horizontally around the obstruction.  Thus, thigmotropism trumps gravitropism.  However, when horizontally growing roots reach an obstacle, the roots always grow downward around the impediment, bowing to gravity, or exhibiting gravitropism.

Here’s an interesting fact.  Twining plants that are native to the northern hemisphere coil in a counter-clockwise direction, the same rotation we see when water drains from a bath tub.  In the southern hemisphere, water flows clockwise down the drain, and plants native to the southern hemisphere likewise twine clockwise.  Note that the direction of coiling is dependent upon a plant’s origin, not where it is currently planted.

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