For details on the transduction of the signal, click here.
For details on differential growth responses, click here.

STRUCTURAL PERCEPTION:
Gravity perception has long been linked to the presence and distribution of dense intracellular statoliths. In 1901, Haberlandt and Nemec suggested that starch-containing amyloplasts served as these statoliths; although this hypothesis has been alternately supported and rejected ever since, abundant evidence makes it generally accepted.

• Amyloplasts accumulate on the underside of the marine alga Caulerpa prolifera rhizome, migrating in response to gravitational shifts. (Jacobs, 1993)
  
• The number of statoliths in Chara rhizoids is strongly correlated with the gravitropic response. (Kiss, 1994)
  Click here for a closer look at the Kiss study.
  
• A starchless mutant of Arabidopsis and a low-starch mutant of Nicotiana grow at the wild-type rate, but with greatly reduced responses to gravity. (Kiss et al, 1989; Kiss and Slack, 1989,1990)
  
• If starch is depleted, the gravitropic response decreases; gravitropic sensitivity reappears along with starch. (Salisbury and Ross, 1992)
  

SITES OF PERCEPTION:
In the roots, long-standing evidence suggests that the root cap is the site of gravity perception. In 1871, Ciesielski showed that graviresponsiveness could be suspended by cutting off the root cap. More recent and meticulous studies have supported this finding, and "gravity detection is thought to be confined to the cap" (Evans, 1991).
In the stems and coleoptiles, gravity is perceived at the site which will respond with differential growth.

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