Geotropic response of roots & stems.
"Geotropic" = gravity to growth response.
Sum up: Tropisms occur when plants respond to external stimuli. Tropisms are movements are movements caused by a change in a plant's growth pattern.
Negative or Positive?
Tropisms can be negative or positive. If the plant moves toward stimulus, the tropism is defined as positive. If the plant moves away from the stimulus, the tropism is considered negative.
In this image, the trunk exhibits a positive tropism, and the roots exhibit negative tropism.