The movement of a plant in the direction of a stimulus or away trom it is called tropic movement or tropism It is said to be positive if it is directed towards the source of stimulus and negative if directed away from the source stimulus.
e.g. geotropism, the tropic respose towards gravity. When a growing portion of a plant is placed horizontally, the stem tip grows away from the pull of gravity. While the root tip grows towards it. Thus, the stem is said to be negatively geotropic and the root positively geotropic.
