Tropisms | Introduction
Introduction
Why do plants grow toward light? How far will plants stretch to reach light? These questions fascinated the famous British biologist Charles Darwin (1809–1882), who is best known for formulating the theory of natural selection. Also called survival of the fittest, natural selection is the process by which plants and animals best adapted to their environment to survive and pass their traits on to their offspring. Darwin studied tropismThe growth or movement of a plant toward or away from a stimulus., which includes the bending of plants toward light, because he believed that this trait helped plants reach the light they needed to survive.
In 1880, Darwin performed experiments showing how the growing tip of a plant bends toward a light source. This behavior is called phototropismThe tendency of a plant to grow toward a source of light.. Photo means "light," and tropism means "the growth or movement of a plant toward or away from a stimulus." Thus, phototropism means "the tendency of a plant to grow toward a source of light." At the same time, Darwin noticed that some shade-loving plants turn away from light, a behavior called negative phototropism.
Darwin also discovered another kind of tropism: geotropismThe tendency of roots to bend toward Earth., meaning "a bending toward Earth." He found that the roots of plants are sensitive to gravityThe attraction of Earth's mass on objects., the attraction of Earth's mass on objects, and grow toward the center of gravity, which is the planet's core.