2. Tropisms Tropisms - a biological phenomenon, indicating growth or turning movement of a biological organism, usually a plant, in response to an environmental stimulus. The word tropism comes from the Greek trope ("to turn" or "to change"). Tropisms are usually named for the stimulus involved and may be either positive (towards the stimulus) or negative (away from the stimulus). 3. 1. Phototropism is the growth response of a plant in response to light direction. 4. 2. Geotropism is the growth response of a plant in response to gravity. 5. 3. Thigmotropism is the growth response of a plant to physical contact (touch). 6. 4. Hydrotropism is the growth response of a plant to water.