Gravity
Animals grapple with gravity. To grow vertically, plants must accurately gauge gravity. They do so with aplomb.
The sensor of gravity in plants consists of tiny starch-rich grains called statoliths that sediment and form miniature granular piles at the bottom of the gravisensing cells. Despite their granular nature, statoliths move and respond to the weakest angle, as a liquid clinometer would do. This liquid-like behavior comes from cell activity, which agitates statoliths with an apparent temperature one order of magnitude larger than actual temperature. Active fluctuations of statoliths explains the remarkable sensitivity of plants to inclination. ~ French biophysicist Yoël Forterre et al
