The basic mechanism is still not resolved. Sufficiently large particles in cells apparently migrate due to gravity in key cells in the growing tips of roots and stems. The distribution of these particles triggers a number of physiological events mediated by hormones that influence cell expansion. The overall effect on the "gravity side" of the tissue is inhibition of cell elongation in the roots and a stimulation of cell elongation in the shoots. The nonsymmetrical distribution of cell elongation in each of these tissues causes the growing root to bend towards gravity and the stem to bend away from gravity.
Answered on November 19, 2013
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