Some Plants Are Delightfully Touchy

Mimosa is both inedible and homely, but you might want to grow it anyway for entertainment purposes. Touch the leaves on this tropical shrub and they suddenly collapse.

The response to touch is the result of an electrical stimulus and a chemical stimulus which travel through the plant at about an inch per second. You can watch the leaves of a large plant collapse in a wave of motion after one leaf is touched.

Response to touch is common in the plant kingdom. This spring, watch how your peas’ tendrils grab onto a trellis, or whatever else is nearby, to pull up the vines. Your bean vines will close in tight spirals around any poles you have set out for them. And how about the Venus’ flytrap, which closes its hinged leaf around any unwitting insect?

Plants are discriminating about what they will move for. Venus’ flytrap distinguishes between living and dead prey by closing only if two of the sensing hairs within its “jaws” are touched in succession, or if one hair is touched twice. Pea tendrils respond more quickly to rough or textured surfaces than to smooth or soft surfaces.

All this movement serves a purpose. Tendrils and twining stems help get plants off the ground. Leaves that close around a fly help nourish the Venus’ flytrap. “Fainting” of sensitive plants might be useful for helping the plant conserve water in drying winds, or for making the plant less appetizing to a hungry animal or insect.

All plants, some more than others, are smaller and stockier when repeatedly touched (or shaken or bruised). This is one reason why indoor and greenhouse plants, which are not exposed to wind, grow more leggy.