1. the turning of all or part of an organism in a particular direction in response to an external stimulus.
2. Phototropism is a response to the stimulus of light, whereas geotropism (also called gravitropism) is a response to the stimulus of gravity . ... when the stem grows against the force of gravity, this is known as a negative geotropism.
3. Terrestrial plants direct their root systems downwards into the soil, mainly in response to the gravity vector; however, as the roots search for water and nutrients, they frequently change direction to avoid obstacles (thigmotropism). Root hydrotropism is a mechanism found in many species to avoid water stress.
