All plants have some ability to move, but most of these movements are on a scale too slow for us to see. Tropisms are the simplest plant movements. They are controlled by growth hormones and are a growth response to an external stimulus. A houseplant bending toward the light shining through a window is an example of phototropism. Roots growing from dry soil towards moist soil demonstrate hydrotropism. Other stimuli that trigger tropic responses are heat, gravity, and soil chemicals.
Nastic movements, or nasties, can be extremely fast. Although not fully understood, they often involve a rapid change in water pressure inside the plant that may be controlled by chemical and electrical mechanisms. For example, when a leaf of the sensitive plant (Mimosa pudica) is touched, all of the leaves fold up and the plant wilts to the ground. If the stimulus is strong enough, this can happen within the blink of an eye. The plant may have made this adaptation to make it less visible to grazing animals or to scare away insect pests.
Many carnivorous plants employ nastic movements to trap their prey. Probably the best known is the Venus flytrap (Dionaea muscipula). Its specialized leaves, resembling clamshells, are the traps. When insects, lured by a sweet-smelling nectar, enter the chamber, they brush against trigger hairs that snap the trap shut. Digestive juices are then released to dissolve the hapless victim.
Another insect-trapping plant is the sundew (Drosera sp.). Charles Darwin described them as being “animals in disguise.” Tentacles with bright-color tips dot the surface of the leaves and secrete a seductive nectar and sticky adhesive. Landing insects stick fast, their movements causing nearby tentacles to move toward and further entangle the prey. In some species, the leaves close around the insect, entombing it in a vat of digestive juices.
Plants capable of rapid movement and trapping their meals have long been the basis of science fiction novels and horror films depicting devious man-eating plants. While there is no proof that such botanical monsters exist, we would caution vegetarians to be watchful. After all, turnabout is fair play.