Tropic movements allow plants to respond to stimuli with differential growth patterns dependent on the direction of the stimuli. Gravitropic movement occurs through three stages: perception of gravity, transduction of the signal, and growth response. This growth response may be:
positive -- vertically down
negative -- vertically up
diagravitropic -- horizontal growth
plagiotropic -- at an angle
agravitropic -- indendent of gravity

The complex mechanism for the gravitropic signal-transduction pathway is not entirely understood, but the Cholodny-Went hypothesis, which implicates the asymmetric distribution of auxin as the driving force behind gravitropic growth, is the prevailing model. Recent studies have suggested more complex variations of this hypothesis, and have indicated that both genetic and environmental components may play a role in gravitropic response.

For a closer look at the signal-transduction pathway, please click below:

The perception of gravity

The transduction of the signal

Differential growth responses

Research on gravitropism has traditionally been complicated by the ever-present force of gravity. Efforts to study plant growth without the effects of gravity began in 1873 with von Sachs' clinostat, which rotates the plant around a horizontal axis to stimulate weightlessness. Today's space program provides a much more accurate and less stressful means to studying the effect of gravity on plant growth. NASA's space biology program investigates the effects of weightlessness on crop plants and animals. In conjuction with the National Science Foundation, NASA is also sponsoring a five-year study of plant-sensory systems.