Plants responding to touch. This may sound like something out
of one of those "new age" gardening guides—"talk nicely to your
plants, play soothing music, and stroke them lovingly, etc." The ability to respond
to touch seems to belong solely to the animal world.But
consider the tendrils of your pea plants twining around their string supports. Or your
morning glories, winding around their trellis. These are examples of plants responding to
touch. How else would they know when and where to begin winding?
When a morning glory vine touches a support,
the stem cells begin to grow at different rates, causing the stem to bend and curve around
the support. This phenomenon is called thigmotropism
(Gr. thigma = touch). Again, the mechanism for this is the migration of
auxin—but just what happens to stimulate this migration and the resulting difference
in growth rates remains a fascinating mystery.
You may be familiar with two striking examples of plants
responding to touch: The Mimosa, or sensitive plant, whose delicate, fern-like leaves fold
up in response to even the lightest touch; and the Venus fly trap, a carnivorous plant
whose jaw-like leaves close when touched by an unsuspecting insect. The mechanism for
these responses to touch is thought to involve the rapid movement of water, rather than
the migration of auxin. As water moves into or out of certain cells, it causes them to
swell or shrink. These changes in cell size cause the leaves to fold up, and to reopen
when conditions are right.
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Plants can count!
On the inside surface of the "trap" leaves on a Venus fly trap plant,
there are three trigger hairs. If an unsuspecting insect lands on a trap leaf and touches
one of the hairs...nothing happens. However, if the insect touches two hairs, or touches
one hair twice, then the trap closes and traps the insect. So, the Venus fly trap can
count—at least to two! |
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