Most plants do not move from place to place. This is one of the main ways one can tell a plant from an animal. A few of the simple one-celled plants are able to swim or crawl. Nevertheless they are still called plants. Plants are able to bend or turn in different directions. This helps the plant get the things it needs to live.
There are two kinds of movements that stationary plants exhibit. Some movements are due to unequal growth rates in cells and are called tropisms. Plants respond in the direction of growth to the direction of the stimulus. If the parts of the plant turn toward the stimulus this is a positive response, while moving away is a negative response. Stems and leaves turn toward the light but roots grow away from it. Roots grow with the pull of gravity while stems grow up and away from the earth.
Nastic movements are independent of the direction of the stimulus and are characteristically due to changes in turgor pressure (water content of the cells) rather than to growth. These are usually temporary or cyclical movements. Flowers open and close periodically. These are called sleep movements. If an object touches the leaves of the mimosa plant, they fold up in seconds. When an insect starts to crawl through the Venus' flytrap or sundew the trap folds up, catching the insect inside.
Time-lapse photography reveals the constant motion of plants. Tropic and nastic movements, not always visible to the naked eye, when recorded on film show the plant dancing in tune under the direction of nature's environmental factors.
