Plant Tropisms

What would happen if you suspended a plant upside down in midair? Would the stems and leaves of the plant grow toward the ground? Why or why not?

Think about plants and trees you may have seen growing on hillsides or on the banks of highways. Although their roots are anchored to the sides of these steep slopes, the stems of the plants still point straight toward the sky. What do you think causes these plants to grow vertically, rather than at an angle?

Plants and trees that grow vertically on steep slopes and hillsides are responding to an external stimulus: gravity! The movements that a plant makes in response to gravity and other environmental stimuli are known as tropisms.

When a plant exhibits a tropic response to a stimulus, something within the plant causes it to move. This is different from the movement of a plant caused by wind or rain. A tropism is a movement caused by the plant itself, not by its environment.