Types of Tropisms

There are many types of tropisms. Each is named according to the force or stimulus that causes it. For instance, the trees and plants that grow vertically on a steep hillside are exhibiting a gravitropic response. You’ve probably observed that a plant will bend or turn toward a light or window. This movement is called phototropism. Another common tropism is movement that responds to the touch of an object. This response is known as thigmotropism.

Plants can move either toward or away from a stimulus. When they move toward a stimulus, they are exhibiting a positive tropism. When they move away from a stimulus, they are exhibiting a negative tropism. For example, most plant roots grow downward, toward Earth’s center of gravity: roots exhibit positive gravitropism. Plant stems, on the other hand, grow up from the ground, away from Earth’s gravitational pull, so stems exhibit negative gravitropism.

When a plant turns toward a light source, is it displaying a positive or negative phototropism? Click here to check your answer!