Terrestrial plants, anchored, by necessity, to their substrate, have limited mobility and few ways in which they can respond to environmental stimuli. The primary way in which plants do respond is through changes in growth. As we have seen in Essential Processes, Plant Hormones , hormones are often responsible for changing the growth patterns of plants. This section will explore tropisms and turgor movements, two forms of plant movement that allow the plant to react to stimuli.
Tropisms are responses to stimuli that result in the long-term growth of the plant toward or away from the stimulus. This growth results from cell elongation occurring at different rates on different sides of the plant, so that the plant bends in one direction. Phototropism, a reaction to light, causes the plant to bend toward the light source (see Essential Processes, Auxins). Thigmotropism, a reaction to touch, causes parts of the plant to thicken or coil as they touch or are touched by environmental entities. Tree trunks, for instance, grow thicker when exposed to strong winds and vines tend to grow straight until they encounter a substrate to wrap around. Gravitotropism, a response to gravity, causes parts of a plant to grow either upward or downward. If a plant is placed on its side, its shoot will begin to grow upward (against gravity) and roots will follow the pull of gravity to grow downward.
Turgor movements occur more rapidly than tropisms and are easily reversible. They rely on changes in turgor pressure (exerted by water on cell walls) within certain plant cells instead of on differential cell growth. Turgor movements are responsible for many plant responses, such as when leaves or flowers droop and fold up at certain times of the day or night or in response to an external touch. A Venus flytrap, for instance, depends on changes in turgor pressure to close its "jaws" around insects when they land on the plant.