Tropism

Tropism is the movement of a plant in response to a stimulus (an external force). The categories of tropism include:

Chemotropism a response to chemicals, in which the plant leaves may curl up.

Geotropism or gravitropism a response to gravity. Shoots of a plant are negatively geotropic (growing upward), while roots are positively geotropic (growing downward).

Hydrotropism a response to water or moisture, in which roots grow toward the water source.

Paraheliotropism a response by the plant leaves, in which they turn away from the sun in order to avoid exposure to it.

Phototropism a response to light. A plant may be positively phototropic (bending toward the light source) or negatively phototropic (bending away from the light source). Main axes of shoots are usually positively phototropic, whereas roots are either negatively phototropic