Almost all plants move their leaves and stems in some way, but usually this movement is so slow that we can’t see it. However, this is not true for a common plant called the mimosa.
Normally, the long, thin leaves of this plant extend straight out from the stem, and the stem is erect. But if a leaf of the plant is touched by a person or animal, the entire plant suddenly droops, its leaves folding downward and its stem bending, as if the plant were wilting in terror! The change from an erect mimosa to a wilted one takes just seconds!
The mimosa will also wilt when struck by a strong wind, or by sudden warmth or darkness. Shortly after the plant has wilted, though, it begins to grow erect again, like an animal carefully sneaking out of hiding. Another touch can wilt the plant. But after a few openings and closings, the plant gets “tired,” and will no longer wilt if touched.
The wilting of the “sensitive plant” is due to small sacs found at the base of the leaves. Normally, these sacs are filled with water, and help to buoy up the leaves. But when the plant is touched, an electrical impulse shoots through the plant, and the water immediately drains out of the sacs and into the stem.
Since the empty sacs can no longer support the leaves, they droop and the mimosa “wilts with fright.”