Light-induced de-etiolation (greening) of dark-grown potatoes.
Before exposure to light, a dark-grown potato exhibits morphological adaptations called etiolation: few leaves enable the shoots to penetrate the soil; few roots absorb little water, but little water is lost by the shoots. After a week's exposure to natural daylight, the plant now has broad green leaves and long roots. this behavior is called tropism: a directional response to a stimulus.