Answer: It is fascinating, isn't it? Another example of this type of movement is the closing of a Venus fly trap on its insect prey.
Contrary perhaps to what our intuition tells us, plants do indeed respond to touch. Think of a vine twining up a support--it only begins twining when it touches something to twist around!
It might be helpful to first explain how plants move in other situations. For example, when you put a tomato seedling in a sunny window, the stem will begin to bend toward the light. This reaching or bending toward the light is called phototropism.
A group of substances within the plant cells called plant growth regulators are responsible for this change in growth. (Plant growth regulators are sometimes called hormones, as in rooting hormone.) On type, called auxin, stimulates cells to increase in length. When the sun shines on one side of our tomato seedling, the auxin collects on the dark side, causing those cells to elongate. Because these cells enlarge faster than the ones on the sunny side, the stem bends toward the light. Just why and how the auxin actually moves is still a mystery to scientists.
In the case of the sensitive plants, the movement is really accelerated. The closure of the leaves is caused by very rapid growth in response to touch--certain cells along the midrib elongate very quickly, with the result that the leaf folds up. Undoubtably this response evolved as a protection against some threat. But how the plant manages such a quick response is unknown.
You (or your children's teachers) might be interested in looking into National Gardening Association's new online botany/horticulture courses. These month-long courses will explain all sorts of interesting phenomena like that. Check out the web site at http://www.garden.org. We love to get people interested in botany!
Q&A Library Searching Tips