Growth or movement response of a cell or an organism to gravity
Tropism is an involuntary orienting response of an organism to a stimulus. It often involves the growth rather than the movement of an organism. The response exhibited by the organism to the stimulus is involuntary, which means it cannot go either towards or away from the stimulus source as in taxis. In tropism, the response of the organism is often by its growth rather than by its movement. It may grow either towards or away from the stimulus. The growth response of a cell or an organism to gravitational field is called gravitropism.
Gravitropism is a feature exhibited by many organisms, such as most plants and fungi. For instance, the roots of the plants grow towards the gravitational field whereas the stem grows away from the gravitational field. The downward growth of roots is an example of a positive gravitropism whereas the upward growth of roots is an example of negative gravitropism. A gravitation field is thought to be sensed by sedimentation of statoliths (starch grains) in root caps. Statoliths are a type of amyloplast found in root tip cells of higher plants. They can sediment within the cell under the influence of gravity and is thought to be involved in the detection of gravity in gravitropism.
Word origin: gravi- (relating to gravity) + tropism