In my Science 10 class, we are currently discussing plants, and how they adapt to different situations by moving. A plants response to a non-directional stimulus is known as a nastic response. Most of the time, plants move on a slow enough time scale that we do not notice they are moving at all. Over the course of a few days, a rotated plant will shift its leaves so they are facing the sun. Sunflowers will rotate their heads to allow the sun to warm the nectar in their florets. Pea plants and creepers will twine around their supports.
Some plants move at a much quicker pace, in response to a touch stimulus (known generally as nastic response or thigmonasty) or daylight hours (known as sleep movements or photonasty).
The venus flytrap is the most famous of all moving plants, using the response to touch to allow it to "eat" insects that touch sensitive hairs on its traps.
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| Smaller than you'd think. The traps are about the size of a fingernail. |
Mimosa plants (colloquially "sensitive plants") will fold up suddenly when the leaves are touched. Another touch will cause the entire stem to droop quickly.
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| Before. |
Mimosa plants also exhibit sleep movements, closing up and drooping at night only to perk up again in the morning. Shamrock plants also display this kind of behavior.
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| After I touched the left leaves. |
Both of these responses are achieved by controlling the amount of water in specialized cells within the plants. When tiny hairs on the plant are touched (by wandering insects usually) an electrical signal is sent through the plant, and it will rapidly decrease the turgor (water) pressure in the specialized cells. Plants maintain their rigid bodies using water pressure, so the rapid loss of turgor pressure causes the plant to wilt in very specific locations, leading to the quick change in shape.
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