Phototropism, heliotropism and hydrotropism are three types of tropism. Tropism refers to the way plants react to external stimuli, such as light, soil and water.
Tropisms that move plants toward the stimuli are positive, while those that move plants away from the stimuli are negative. Phototropism, which is the plant's reaction to light, is a positive tropism since plants move towards the light. Heliotropism can be either positive or negative since it deals with the plant's reaction to the intensity and direction of light. Hydrotropism, which is the plant's growth response to water, can also be either positive or negative.