Nature

Plants, like animals and other organisms, must adapt to their constantly-changing environments. While animals are able to relocate from one place to another when environmental conditions become unfavorable, plants are unable to do the same. Being sessile (unable to move), plants must find other ways of handling unfavorable environmental conditions. Plant tropisms are mechanisms by which plants adapt to environmental changes. A tropism is a growth toward or away from a stimulus. Common stimuli that influence plant growth include light, gravity, water, and touch. Plant tropisms differ from other stimulus generated movements, such as nastic movements, in that the direction of the response depends on the direction of the stimulus. Nastic movements, such as leaf movement in carnivorous plants, are initiated by a stimulus, but the direction of the stimulus is not a factor in the response.
Plant tropisms are the result of differential growth. This type of growth occurs when the cells in one area of a plant organ, such as a stem or root, grow more quickly than the cells in the opposite area. The differential growth of the cells directs the growth of the organ (stem, root, etc.) and determines the directional growth of the entire plant. Plant hormones, like auxins, are thought to help regulate the differential growth of a plant organ, causing the plant to curve or bend in response to a stimulus. Growth in the direction of a stimulus is known as positive tropism, while growth away from a stimulus is known as a negative tropism. Common tropic responses in plants include phototropism, gravitropism, thigmotropism, hydrotropism, thermotropism, and chemotropism.