My pole beans, which got a rather late start, are finally climbing their way up the strings on my bean tower. I’m always impressed that the plants know just what to do. Those reaching tendrils that come into contact with the string immediate start to coil around it, securing themselves to the support. A few plants were still free, waving in the light breeze. I tucked them between the two strands of twine, so they too could wind their way upward.
A few rows over, my pea vines have their tendrils securely wrapped around the netting I put up for them. We all know that pole beans climb and pea tendrils wrap, but I wondered how they knew to do so. After all, most plants don’t have this ability.
That’s when I learned about thigmotropism.
According to the biology dictionary, “A tropism is the innate ability of an organism to turn or move in response to a stimulus.” Note that this ability is innate—in other words, they’re born (or bud, or divide, or germinate) that way. It doesn’t have to be learned.
There are many kinds of tropism in plants, and they can be positive or negative. Roots grow toward the pull of gravity, so that’s positive geotropism (aka gravitropism). Stems (especially the initial stem rising from the sprouting seed) grows away from gravity, so that’s negative geotropism.
In the case of phototropism, plants in too much shade grow toward the light. Other tropisms include hydrotropism, in which the stimulus is water, and heliotropism, which is when plants keep their leaves or flowers oriented to the sun as it makes its way across the sky. (Again, these can be positive or negative.)
Thigmotropism is the response of plants to touch. Some plants, Sensitive Plants (Mimosa pudica) and Venus Flytraps, for example, respond to touch by closing their leaves. And other plants—such as pole beans and peas—react by coiling.
How does this happen? It’s all a matter of chemistry. Let’s take a closer look at phototropism, as it’s probably the most familiar.
When a plant stem is “growing toward the light,” what really happens is that auxin, a plant hormone, moves to the dimmer side of the stem. There, the higher auxin concentration causes the cells on that side of the stem to elongate faster (or to a greater degree) than those on the well-lit side. The uneven size of the cells on opposite sides of the stem cause the plant to bend.
This same process occurs in roots, only in the other direction. Roots are more likely to find water and minerals in the dark of the soil, so they’re negatively phototropic.
Roots are also positively geotropic. Specialized cells at the root tip contain little “sacks” (plastids) of starch. Because they’re a bit heavier than the rest of the cytoplasm, the sacks settle to the bottom of the cell, where they trigger a signal to the portion of the root tip that is actively growing. This signal results in different growth rates on the top and bottom of the root—sending the root downward into the soil.
So what about thigmotropism? There are specialized touch-sensitive cells on the outside of the plant’s stem or tendril. At first, the tendrils grow in a spiral. This increases the likelihood that they’ll touch a nearby support. Once they come in contact with something, growth is initiated on the outside of the tendril. (Again, the growth is controlled by chemicals—in this case, auxin and ethylene.)
We think of plants as placid, sedentary creatures. But a thigmotropic response is so rapid we can watch it happen. It takes a tendril only five to ten minutes to completely circle a string or wire. Try this experiment—go into the garden and stroke one side of a pea tendril. It only takes a few minutes for it to start to curl! Then, it will stay curled for several days even if there’s no further stimulation.
Having learned all this, I now have an urge to rush outside and pet my bean vines.