How Do Plants Respond to Stimuli?
Plants respond to stimuli either through growth or through movement without growth. These movements or growth can either be based on the direction from which the stimulus comes or they can be independent of the direction of the stimulus. Different plants can respond to a wide range of stimuli.
Movement or growth toward or away from a stimulus is known as tropism. Tropism includes gravitropism, which is growth in response to gravity, and phototropism, which is growth or movement in response to light. Plants in general exhibit these two types of tropism. Plant stems tend to grow toward sources of light and against gravity, while roots tend to grow away from sources of light and with the force of gravity.
Nastic movements are still responses to stimuli, but their direction is independent of the direction of the stimuli. Some plants, for instance, orient their leaves horizontally during relatively cool mornings, but orient them vertically during the heat of midday to minimize sun exposure. These sorts of movements are accomplished through changes in turgor, or the water content, in certain cells, causing them to change their volume. Another example is the rapid closure of the Venus fly trap, which responds to repeated stimuli to entrap insect prey.