All living things respond to stimuli in one way or another. Plant responses are subtle at times, but always very efficient—the solar tracking of leaves to maximize light capture is one example. Plants move, grow and change form in response to a variety of factors. Plant responses to changes in their world are regulated in many ways and on many levels, ranging from the molecular to that of the whole plant.
Tropisms are plant movements directed toward or away from a stimulus. Some familiar tropisms are:
Phototropism (movement toward light)
Gravitropism and geotropism (movement relative to a gravitational field, or toward the center of the Earth)
Thigmotropism (plant growth in response to physical contact)
Other tropisms include:
Chemotropism (movement in response to a chemical in the environment)
Hydrotropism (growth or developmental response to water)
Thermotropism (response dependent upon temperature)
It is helpful to also designate plant responses that are triggered by a definite stimulus, but not in a direction that is related to the direction of the stimulus. Such movements are called nastic movements. Examples of this are a mimosa folding its leaves in response to a touch or a Dionaea (Venus flytrap) closing in response to the presence of an insect.
A lot of tropism research has focused on how plant hormones known as auxins control plant growth by stimulating cell elongation. It is well accepted that phototropic and geotropic bending of shoots and roots results from cells on one side of a plant elongating faster than cells on the other side, thus causing the plant to bend and change the direction of its growth.
Phototropism
Photosynthesis is a popular way to make a living on Earth and plants—as well as some microscopic organisms—have finely tuned, light-controlled positioning systems. Auxins are important in regulating how plant organs move and grow toward a light source and photosynthesis is one of the most-studied topics in botany.