Like all living things, plants detect and respond to stimuli in their environment. Unlike animals, plants can't run, fly, or swim toward food or away from danger. They are usually rooted firmly in the soil. Instead of fleeing, a plant's primary way of responding is to change how it is growing.     One way plants respond is by tropisms. A tropism is a turning toward, or away from, a stimulus in the environment.     Examples of tropisms in plants include gravitropism and phototropism. Gravitropism is a response to gravity. Plant roots grow downward toward the center of Earth because of... Show more

Like all living things, plants detect and respond to stimuli in their environment. Unlike animals, plants can't run, fly, or swim toward food or away from danger. They are usually rooted firmly in the soil. Instead of fleeing, a plant's primary way of responding is to change how it is growing.    
One way plants respond is by tropisms. A tropism is a turning toward, or away from, a stimulus in the environment.    
Examples of tropisms in plants include gravitropism and phototropism. Gravitropism is a response to gravity. Plant roots grow downward toward the center of Earth because of Earth's gravity. Phototropism is a response to light.    
Plant stems and leaves grow toward a light source.    
Plants also detect and respond to daily and seasonal cycles. For example, some plants open their leaves during the day to collect sunlight and then close their leaves at night to prevent water loss. Many plants respond to the days growing shorter in the fall by going dormant. They suspend growth and development in order to survive the extreme coldness and dryness of winter.    
Plants don't have an immune system, but they do respond to disease. Typically, their first line of defense is the death of cells surrounding infected tissue. This prevents the infection from spreading. Many plants also produce hormones and toxins to fight pathogens.

Show less